2014_Coupled human and natural system dynamics as key to the sustainability of Lake Victoria’s...

Copyright copy 2014 by the author(s) Published here under license by the Resilience AllianceDowning A S E Van Nes J Balirwa J Beuving P Bwathondi L J Chapman I J M Cornelissen I G Cowx K GoudswaardR E Hecky J H Janse A Janssen L Kaufman M A Kishe-Machumu J Kolding W Ligtvoet D Mbabazi M Medard O CMkumbo E Mlaponi A T Munyaho L A J Nagelkerke R Ogutu-Ohwayo W O Ojwang H K Peter D Schindler OSeehausen D Sharpe G M Silsbe L Sitoki R Tumwebaze D Tweddle K E Van de Wolfshaar H Van Dijk E Van Donk J CVan Rijssel P A M Van Zwieten J H Wanink F Witte and W M Mooij 2014 Coupled human and natural system dynamics askey to the sustainability of Lake Victoriarsquos ecosystem services Ecology and Society 19(4) 31 httpdxdoiorg105751ES-06965-190431Research

Coupled human and natural system dynamics as key to the sustainability ofLake Victoriarsquos ecosystem servicesAndrea S Downing 123 Egbert H van Nes 2 John S Balirwa 4 Joost Beuving 5 POJ Bwathondi 6 Lauren J Chapman 7 Ilse J MCornelissen 38 Iain G Cowx 9 Kees P C Goudswaard 10 Robert E Hecky 11 Jan H Janse 312 Annette B G Janssen 23 Les Kaufman

13 Mary A Kishe-Machumu 14 Jeppe Kolding 15 Willem Ligtvoet 16 Dismas Mbabazi 4 Modesta Medard 17 Oliva C Mkumbo 18Enock Mlaponi 19 Antony T Munyaho 4 Leopold A J Nagelkerke 8 Richard Ogutu-Ohwayo 4 William O Ojwang 20 Happy K Peter

8 Daniel E Schindler 21 Ole Seehausen 22 Diana Sharpe 723 Greg M Silsbe 24 Lewis Sitoki 25 Rhoda Tumwebaze 4 Denis Tweddle 26Karen E van de Wolfshaar 27 Han van Dijk 17 Ellen van Donk 3 Jacco C van Rijssel 222829 Paul A M van Zwieten 8 Jan Wanink 2830F Witte 2829 and Wolf M Mooij 23

ABSTRACT East Africarsquos Lake Victoria provides resources and services to millions of people on the lakersquos shores and abroad Inparticular the lakersquos fisheries are an important source of protein employment and international economic connections for the wholeregion Nonetheless stock dynamics are poorly understood and currently unpredictable Furthermore fishery dynamics are intricatelyconnected to other supporting services of the lake as well as to lakeshore societies and economies Much research has been carried outpiecemeal on different aspects of Lake Victoriarsquos system eg societies biodiversity fisheries and eutrophication However todisentangle drivers and dynamics of change in this complex system we need to put these pieces together and analyze the system as awhole We did so by first building a qualitative model of the lakersquos social-ecological system We then investigated the model systemthrough a qualitative loop analysis and finally examined effects of changes on the system state and structure The model and itscontextual analysis allowed us to investigate system-wide chain reactions resulting from disturbances Importantly we built a tool thatcan be used to analyze the cascading effects of management options and establish the requirements for their success We found thathigh connectedness of the system at the exploitation level through fisheries having multiple target stocks can increase the stocksrsquovulnerability to exploitation but reduce societyrsquos vulnerability to variability in individual stocks We describe how there are multiplepathways to any change in the system which makes it difficult to identify the root cause of changes but also broadens the managementtoolkit Also we illustrate how nutrient enrichment is not a self-regulating process and that explicit management is necessary to haltor reverse eutrophication This model is simple and usable to assess system-wide effects of management policies and can serve as apaving stone for future quantitative analyses of system dynamics at local scales

Key Words eutrophication feedbacks fisheries Lake Victoria model multidisciplinary social-ecological system sustainability

INTRODUCTIONNile perch (Lates niloticus) was introduced to Lake Victoria inthe 1950s and suddenly became dominant in the system in the1980s (Goudswaard et al 2008) Prior to this dramatic event theecosystem of Lake Victoria harbored a large diversity ofhaplochromine cichlids and a few native tilapia species destinedfor local and regional markets (Pringle 2005a) The Nile perchboom co-occurred with other major transformations to LakeVictoriarsquos ecosystem including a collapse of the native diversity

of haplochromines and the eutrophication of the lakersquos waters(Witte et al 2007a Hecky et al 2010)

Despite the dramatic diversity loss and catastrophic ecosystemchanges that surrounded the Nile perch invasion managementcurrently aims to maintain the system in its Nile perch-dominatedstate not to recover the former Nile perch-free diverse ecosystemmdashshould that even be possible Indeed Nile perch has becomethe product of an international export industry worthapproximately US$250 million yearly (Pringle 2005a Kayanda et

1Stockholm Resilience Centre Stockholm University Sweden 2Aquatic Ecology and Water Quality Management group Wageningen UniversityNetherlands 3Netherlands Institute of Ecology (NIOO-KNAW) Wageningen Netherlands 4National Fisheries Resources Research Institute(NaFIRRI) Jinja Uganda 5Department of Cultural Anthropology and Development Studies Faculty of Social Sciences Radboud UniversityNijmegen Netherlands 6University of Dar es Salaam College of Natural and Applied Sciences Department of Aquatic Sciences and Fisheries Dares Salaam Tanzania 7Department of Biology McGill University Montreal Canada 8Aquaculture amp Fisheries Group Wageningen UniversityNetherlands 9Hull International Fisheries Institute University of Hull United Kingdom 10Institute for Marine Resource and Ecosystem Studies(IMARES) Wageningen University Yerseke Netherlands 11Biology Department and Large Lakes Observatory University of Minnesota-DuluthUSA 12Netherlands Environmental Assessment Agency (PBL) Bilthoven Netherlands 13Boston University Marine Program Biology DepartmentBoston University USA 14Tanzania Fisheries Research Institute (TAFIRI) Dar es Salaam Tanzania 15Department of Biology University ofBergen Norway 16Netherlands Environmental Assessment Agency (PBL) The Hague Netherlands 17Department of Sociology of Developmentand Change Social Science Group Wageningen University Netherlands 18Lake Victoria Fisheries Organisation Jinja Uganda 19Tanzania FisheriesResearch Institute (TAFIRI) Mwanza Tanzania 20Kenya Marine and Fisheries Research Institute (KMFRI) Kisumu Kenya 21Aquatic amp FisherySciencesDepartment of Biology University of Washington USA 22Eawag Kastanienbaum Switzerland 23Smithsonian Tropical ResearchInstitute Panama City Panama 24Royal Netherlands Institute for Sea Research Yerseke Netherlands 25The Technical University of KenyaNairobi Kenya 26South African Institute for Aquatic Biodiversity Grahamstown South Africa 27Institute for Marine Resource and EcosystemStudies (IMARES) Wageningen University Ijmuiden Netherlands 28Institute of Biology University of Leiden Netherlands 29NaturalisBiodiversity Center Leiden Netherlands 30Koeman en Bijkerk bv Ecological Research and Consultancy Haren Netherlands

Ecology and Society 19(4) 31httpwwwecologyandsocietyorgvol19iss4art31

al 2009) Many people migrated to the lakersquos shores to benefitfrom the fisheries a migration that has led to importanttransformations to the lakersquos societies Currently an estimated 30million people live in the lakersquos basin and are shaping and beingshaped by its ecology and economy (Awange and Ongrsquoanga2006)

Management measures on Lake Victoria have focused on fisheriesand aim to manage a certain quality of catch (size structure ofNile perch) rather than total fishing effort or stock biomass (vander Knaap et al 2002) Some of these methods are applied on thelake itself eg banning beach seines and increasing the minimummesh size of gillnets or introducing fish slot sizes In the case ofNile perch measures exist at the processing output level wherethere is a minimum length of fish that the factories are allowedto process Approaches to managing the fisheries assume that thefisher communities as well as fish stocks are homogeneous(Muhoozi 2002) Management measures are difficult toimplement and monitor given that Lake Victoria is the worldrsquossecond largest freshwater lake by area (68800 km2) has a highlyconvoluted shoreline that is shared by three countries (TanzaniaUganda and Kenya) and the fisheries are open access Also theeffectiveness of regulations is limited in part because policies canbe immediately detrimental to the filleting factories that processthe fish for export markets (Johnson 2010) Resources availableto implement or enforce policies are scarce and enforcement canbe subject to corruption

Added to difficulties in choosing and implementing effectivemanagement measures stakeholders and scientists disagree aboutthe current state of the Nile perch fisheries and what the dominantthreats to these fisheries are (Witte et al 2007a) Indeed ownersof filleting factories complain that they are operating theirfactories at decreasing capacity overlooking the fact that manyof the factories were built at structural overcapacity (Abila andJansen 1997 Johnson 2010) Some scientists read overfishing inthe fishery survey data (Mkumbo et al 2007 Mkumbo andMlaponi 2007 Kayanda et al 2009) while others see a total stockthat fluctuates around a stable average despite increased fishingeffort The stable stock under increased pressure rests on the logicthat increased productivity results from eutrophication (Koldinget al 2008)

The cumulative pressures influencing Lake Victoria are multipleintricately interconnected and threatening to all its fisheries aswell as to the lakersquos ability to carry out supporting services Indeedthe whole ecosystem is influenced by climate change (Hecky et al2010 Coacutezar et al 2012) and has been degraded througheutrophication (Dobiesz et al 2010) which impoverishes thesystem by driving an irreversible decline in biodiversity(Seehausen et al 1997a) Beyond a certain thresholdeutrophication threatens to cause a more general collapse ofstocks not only of Nile perch (Kolding et al 2008) AdditionallyNile perch is not the only exploited fish For example the nativepelagic cyprinid Rastrineobola argentea (known as dagaa inTanzania omena in Kenya and mukene in Uganda hereinreferred to as dagaa) has become an increasingly importantproduct for local societies (Wanink 1999) Furthermore since theexplosion of Nile perch numbers in the lake social and economicdrivers of fishing effort have changed Risks and opportunitiesthat face fisher communities have evolved with increasing human

population sizes and market value (Beuving 2013) Nile perch isdestined predominantly to European markets that impose stricthygiene standards and influence fish prices Market demand andprocessing costs influence factory owners fish buyers and trademiddlemen who in turn affect the organization of fishing andfisher communities and thus shape stock exploitation (Abila andJansen 1997 Ponte 2007 van der Knaap and Ligtvoet 2010)

Two lake-wide organizationsmdashthe Lake Victoria FisheriesOrganisation (LVFO) and the Lake Victoria Basin Commission(LVBC)mdashhave played the most important roles in coordinatingresearch on the lakersquos resources While originally conceived as aninstrument for holistic ecosystem-based management the LVFOhas a much stronger fisheries perspective The LVBC has thepotential to address water quality and nonfisheries managementissues through its Lake Victoria Environment ManagementProject Plan II (LVEMPII) So far coordination of field effortsand data sharing between the two organizations are poor Fisheryscience and to some extent limnology have undergone a measureof regional integration under LVEMP activities but on the wholean ecosystemrsquos view is still distinctly lacking Dissemination ofresults has also been limited at times with LVFO investing greatereffort in communications than the LVBC

In sum changes in Lake Victoria are a complex product of socialeconomic and ecological processes To comprehend the dynamicsof this complex system to visualize the scenarios likely to resultfrom alternative management measures and to manage humanactivities that influence the lake in an intelligent and farsightedmanner the lake and lake basin ecosystem must be understoodin their entirety Many analyses of their partsmdashfor example waterquality diversity or fisheriesmdashhave been conducted in isolationOur aim is to connect the dots and study the interactions amongthe parts of Lake Victoriarsquos system

An important barrier to assembling the pieces of Lake Victorialies in quantifying the ecosystemrsquos components in a spatio-temporally representative way As a first step in this direction weemployed a form of qualitative modeling called loop analysis(Levins 1974) We first built a feedback diagram of Lake Victoriarsquossocial-ecological system based on the components andinteractions that compose it matching the model to LakeVictoriarsquos past trends and changes and investigated the effects ofquantitative and structural modifications to the system on itsbehavior

SOCIAL-ECOLOGICAL SYSTEM REPRESENTATION

Qualitative modelsWe first created a feedback diagram containing three differenttypes of objects (1) the key elements of the system (2)connections between interacting key elements and (3) the sign ofthese interactions near equilibrium If an increase in the value(biomass concentration or economic value) of an element causesan increase in a connected element the sign is positive (+) If onthe other hand an increase in one causes a decrease in the otherthe sign is negative (-) We then carried out the loop analysis wherewe identified the feedback loops in the system ie the pathwayof interactions that go from any element and back to it throughother elements in the defined system We also identified whetherthese feedbacks are overall positive or negative by multiplying the


signs of the interactions along each pathway A positive feedbackloop reflects a ldquoreinforcingrdquo process where an increase in oneelement causes it to increase further positive feedback loops aretherefore destabilizing A negative feedback loop on the otherhand is self-regulating an increase in an elementrsquos value leads itto limit itself a negative feedback loop is stabilizing We obtainedseveral outputs from this qualitative analysis (a) a description ofthe system as a whole (b) all the direct and indirect pathwaysconnecting different parts of the system (c) the effectmdashpositiveor negativemdashof the change in one element on its differentconnections ie chain reactions across the system (d) themultiple positive and negative controllers of each element (e)stabilizing and destabilizing feedbacks and (f) a view of keyelements that can alter the sign of a feedback ie those for whichconnection signs can be changed eg policies

Determining which elements of a system are key is a subjectiveprocess To reduce bias caused by the oriented knowledge of anysingle Lake Victoria scientist scientists in the fields of aquaticecology fisheries sciences and social sciences as well asstakeholders all with expertise in Lake Victoriarsquos system werebrought together in a workshop in Dar es Salaam in May 2012To make the process more manageable we first built two separatediagrams one for the ecological subsystem and one for the socialsubsystem including fisheries We then combined the twodiagrams into a linked social-ecological system representationFour groups of scientists and stakeholders with mixed expertisediscussed and built the two subdiagrams These groups thusdetermined the elements and the interactions to enter the modelbased on their understanding of underlying processes (egeutrophication) and knowledge of the system (eg the types offishes landed with different nets) and discussed them until thefinal diagrams satisfactorily represented the perspectives of thedifferent topics (social sciences water quality and fisheries) Thediscussions continued online after the workshop and thenincluded even more participants who have in turn become theauthors of this paper In this way the biases that remain areinherent to research conducted on Lake Victoria thus far This isthe first time such diverse perspectives on Lake Victoriarsquos systemhave been assembled on an equal footing Our representation ofthe system as we know it can further point out where ignoranceremains

To make the diagrams tractable we excluded all self-effects andassumed density dependence to be negative reflecting a stablesystem Also where interactions are known to be nonlinear suchas the effect of eutrophication on productivity we cut the processdown into its parts the positive effect of eutrophication ongrowth and the positive effect of eutrophication on anoxia andshading which in turn is negative for growth We emphasize thatthe model is not quantitative and we do not include the weightof each interaction In this way our model can represent bothlocations or time periods where growth processes prevail overnegative effects of deoxygenation and vice versa and thusrepresent the full system irrespective of its current state To makeour final linked diagram more parsimonious we eliminated theelements flanked only by unidirectional positive interactions asthey add detail and complexity to the loops but do not alter theirsignsmdashand therefore have no effect on the overall dynamics ofthe system

Ecological subsystemThe nutrients of the system are represented in nitrogen (N) andphosphorus (P) (Fig 1) that contribute to an increase inphytoplankton biomass which is in turn consumed byzooplankton and fish (Fig 1 phytoplankton --gt zooplanktonand haplochromines) The major source of nutrient input to thelake is through atmospheric deposition resulting from intensiveland use in the basin through land clearing and burning(Tamatamah et al 2005 Hecky et al 2006) (Fig 1 from the localand regional economy and societymdashherein abbreviated toSocampeconmdashto NampP) An increase in nutrients has a nonlineareffect on the system Indeed while there is an initial phase ofincrease in phytoplankton biomass as nutrient input continuesincreasing phytoplankton biomass begins to limit lightavailability and eventually reaches an upper limit depending onlight extinction and mixing depth (Silsbe et al 2006) We try toaccount for this strong nonlinearity by adding phytoplanktonrsquosself-shading property and contribution to detritus (Fig 1phytoplankton --gt shading and detritus) Detritus also increasesshading and oxygen consumption and exists as a trophic pathwayfor the shrimp Caridina nilotica (Fig 1 detritus --gt shadinganoxia and C nilotica)

Fig 1 Ecological interactions Green arrows represent growthprocesses (also identified by + sign) blue arrows representmortality processes (associated with - signs) thick dotted linesand arrows represent complex systemsinteractions not furtherdeveloped E refers to environmental variables F to fishvariables L to limnological variables and S to societalvariables E1 = climate change F1 = large Nile perch F1 =small Nile perch F2 = Rastrineobola argentea F3 =haplochromines L1 = nitrogen and phosphorus L2 =phytoplankton L3 = zooplankton L4 = detritus L5 =Caridina nilotica L6 = shading L6 = anoxia S4 = local andregional economy and society

The shrimp C nilotica is an important detritivore and became animportant prey for juvenile Nile perch as well as for the mostlyzooplanktivorous haplochromines that remained in the 1990s(Downing et al 2012a) (Fig 1 C nilotica --gt small Nile perch


and haplochromines) Recovering haplochromines have mostlyreverted to feeding on zooplankton and other small invertebratesbut they include shrimp and larger prey in their diets when theseare abundant (Kishe-Machumu 2012 van Rijssel and Witte2013) Light is of primary importance to haplochromines becausethey rely on color vision for successful breeding and efficientfeeding (Seehausen et al 1997a Witte et al 2013) (Fig1 shading--o haplochromines) Poor light can also alter haplochrominespecies diversity by promoting hybridization (Seehausen et al1997a Seehausen 2009)

We connected anoxia negatively to all fish since reduced oxygenis ultimately detrimental to them all though usually through acomplex nonlinear process (Fig 1 anoxia --o all fish) In a firststep an anoxic deep water layer reduces the habitable part of thewater column for sensitive species and increases habitat overlapbetween species (Vonlanthen et al 2012) Habitat overlap canresult in increased predation rates of Nile perch onhaplochromines as well as hybridization rates betweenhaplochromine species In a later step the seasonal upwelling ofdeep anoxic waters can lead to large fish kills (Ochumba 1990Gophen et al 1995) Reduced oxygen concentrations also have achronic negative effect on species by influencing their metabolicprocesses and growth rates (Kolding 1993 Rutjes et al 2007)

Additionally oxygen plays a complex role in biogeochemicalcycles denitrificationmdasha process that converts nitrate todinitrogen gas (N2)mdashtakes place primarily in anoxic conditionsand these conditions also promote phosphorus release (Fig 1anoxia --gt NampP) The effect of oxygen on biogeochemical cyclesis temperature sensitive (Veraart et al 2011) and both weatherand temperature conditions influence the duration and extent ofwater stratification in the lake (Sitoki et al 2010) These processesare thus likely to be greatly affected by climate change in complexand nonlinear ways (Fig 1 dashed line from E1 --gt anoxia --gtNampP)

Besides the solid arrow indicating Nile perch predation onhaplochromines a dashed arrow was included to indicate thehypothesized negative effect of haplochromines on Nile perchrecruitment It has long been speculated that through eithercompetition or predation haplochromines have or had anegative influence on the growth of small Nile perch in a processtermed depensation (Fig 1 haplochromines --o small Nile perch)(Walters et al 1997 Walters and Kitchell 2001 Goudswaard etal 2008 Downing et al 2012b) However a recent study suggeststhat such an interaction probably did not play a significant roleat the scale of the whole lake (Downing et al 2013a) Depensationshould be included only in cases and areas where there is ademonstrated negative influence on the stockndashrecruitmentrelationship

Society and fisheriesCatches of small Nile perch dagaa and haplochromines feed aregional market (in the lakeshore and neighboring countries) (Fig2 small Nile perch dagaa and haplochromines --gt local andregional market herein abbreviated as LocMar) and catches oflarge Nile perch go to a European export market (Fig 2 Nileperch --gt international market herein abbreviated as IntMar)Markets convert catches into investment power which allows the

economy to grow fuels further investment in alternative sourcesof income (such as agriculture and farming or support businessesincluding housing transport or entertainment) and stimulateshuman population growth This growth feeds back into theregional market The local and regional markets invest in boat orcamp owners to ensure their supply of fish Owners in turn converttheir capital into effort thus further increasing total catch (Fig2 IntMar and LocMar --gt owners IntMar and LocMar --gtSocampecon)

Fig 2 Socio-economic interactions Green arrows representgrowth processes (also identified by + sign) blue arrowsrepresent mortality processes (associated with - signs) rustarrows represent investment thick dotted lines and arrowsrepresent complex systemsinteractions not further developedE2 = international economy E3 = policies F1 = large Nileperch F1 = small Nile perch F2 = Rastrineobola argenteaF3 = haplochromines F4 = Nile perch fishing effort F5 = Rargentea fishing effort S1 = boat and camp owners S2 =international market S3 = local and regional market S4 =local and regional economy S4rsquo = local society

By separating camp and boat owners from the rest of society wehave the means to reflect social political and economic disparityOwners are entrepreneurs drawing either on their own funds and


profits or on investments by industries and they themselves caninvest in further effort and gain Labor however is cheap alwaysabundant enough and plays a role in every industry we thereforeneglected its impact on owners Owners can switch between theinternational Nile perch fishery and regional fisheries or be partof both (Fig 2 owners --gt Nile perch and dagaa fishing effort)depending on the status of the owners and the benefits each fisheryyields

We added two exogenous processes to this diagram that influencethe cost-benefits that in turn regulate where owners focus theirinvestments the international economy and policies (Fig 2) Theinternational economy driven by many more products trendsand fashions than the resources of Lake Victoria can influencethe price Nile perch fetches as well as quality requirements of theNile perch destined for export (Kambewa 2007 Ponte 2007Johnson 2010) (Box 1) This international influence is in largepart independent of conditions in the lake but nonetheless definesmany of the costs in the Nile perch industry (eg installing andmaintaining storage and hygiene facilities or higher investmentin societies through eco-labeling requirements) and thereforeshapes the catch level necessary to offset costs and reap a benefit(van der Knaap et al 2002 van der Knaap and Ligtvoet 2010)

Box 1 Influence of the international market

European markets set regulations on the quality of the fish theyimported which created a need for filleting factories to invest intechnologies such as freezers and insulated trucks to obtainquality certifications (Johnson 2010) (Fig 3 internationaleconomy(+) --gt IntMar --gt owners --o Nile perch fishingefforthellip) European regulations caused import bans several timesin the late 1990s because of poor hygiene During these periodsNile perch was exported to alternative markets such as Japanthough at lower prices and fishing effort was temporarily reduced(Fig 3 international economy(-) --o IntMar --o Nile perch fishingeffort --gt Nile perchhellip) (van der Knaap et al 2002) Suchsanctions or a collapse in the international market can close upthe pathways that promote investment in fishing by owners Sucheffects could percolate down to show a positive effect on stocks(eg Fig 3 international economy(-) --gt IntMar --gt owners --gtNile perch fishing effort and dagaa fishing effort --o Nile perchdagaa and haplochromines) However this depends on theimportance or adaptability of the local market relative to theinternational one as a collapse of the international economycould simply shift dynamics toward the local and regional marketif the local market has a substantial income-source independentof fisheries

The social-ecological systemWe combined both diagrams and simplified the model to make itmanageable We summarized the positive interactions thatdominate the ecological perspective into two main processes thepositive effects of nutrient input that promote growth in fishspecies (Fig 3 NampP --gt Nile perch dagaa and haplochromines)and the negative effects that come from enrichment but arerepresented by anoxia and shading (Fig 3 NampP --gt shading --gtNile perch dagaa and haplochromines)

We removed small Nile perch from the fish groups Instead Nileperch as a whole contribute to both the regional and internationalmarkets (Fig 3 Nile perch --gt IntMar and LocMar) Thepopulation size structure of Nile perch has changed since its firstintroduction though trends in these changes vary spatially(Kolding et al 2008) and are likely a response to both theenvironmentmdashthrough diet changesmdashand to size-selectivefishing (Downing et al 2013c)

We merged society and the economy into one since they areconnected with positive interactions Here they directlycontribute to nutrient enrichment (Fig 3 Socampecon --gt NampP)

Fig 3 Social-ecological system interactions Green arrowsrepresent growth processes (also identified by + sign) bluearrows represent mortality processes (associated with - signs)rust arrows represent investment thick dotted lines and arrowsrepresent complex systemsinteractions not further developedE1 = climate change E2 = international economy E3 =policies F1 = Nile perch F2 = Rastrineobola argentea F3 =haplochromines F4 = Nile perch fishing effort F5 = Rargentea fishing effort L1 = nitrogen and phosphorus L6 =anoxia and shading S1 = boat and camp owners S2 =international market S3 = local and regional market S4 =local and regional economy and society


Loop analysisOur social-ecological system diagram has 25 loops We excludedthe direct feedback between local market and society from ouranalysis since it holds no emergent information Among theremaining 24 loops we separated nutrient-driven processesincluding nine ldquoenrichmentrdquo loops (Table 1) in which nutrientslead to stock growth increased fishing and economic andpopulation growth and thus more enrichment and nineldquoeutrophicationrdquo loops (Table 2) where stronger nutrient inputleads to anoxia and shading influences stock growth as well aseconomic and population expansion and thus controls nutrientinput Additionally we have six ldquoexploitationrdquo loops (Table 3)that describe dynamics from the different stocks through to themarkets and back

Table 1 Enrichment loops and their signs NP = nitrogen andphosphorus Np = Nile perch IntMar = international marketSocampecon = society and economy LocMar = local and regionalmarket Haplo = haplochromines Dagaa = Rastrineobolaargentea --gt = positive interaction --o = negative interaction

Loop Sign

1 NP--gtNp--gtIntMar--gtSocampecon--gtNP +2 NP--gtNp--gtLocMar--gtSocampecon--gtNP +3 NP--gtHaplo--gtLocMar--gtSocampecon--gtNP +4 NP--gtDagaa--gtLocMar--gtSocampecon--gtNP +5 NP--gtNp--oHaplo--gtLocMar--gtSocampecon--gtNP -6 NP--gtNp--gtIntMar--gtowners--gtDagaa effort--oHaplo--gt

LocMar--gtSocampecon--gtNP-

7 NP--gtNp--gtIntMar--gtowners--gtDagaa effort--oDagaa--gtLocMar--gtSocampecon--gtNP

-

8 NP--gtHaplo--gtLocMar--gtowners--gtNp effort--oNp--gtIntMar--gtSocampecon--gtNP

-

9 NP--gtDagaa--gtLocMar--gtowners--gtNp effort--oNp--gtIntMar--gtSocampecon--gtNP

-

Positive enrichment loops go from nutrients promoting thegrowth of stocks through their positive effect on markets thensociety and back to nutrient input (Table 1) Negativeeutrophication feedback loops follow the link between nutrientsand anoxia and shading Anoxia and shading are the drivers onstocks whose negative effects cascade to markets and society andthus lead to reduced nutrient input (Table 2) In the case ofnegative exploitation loops they link each stock through to themarket it trades to back to owners increased effort and thus areduction in stock (Table 3) which is ultimately self-regulating

Besides these expected loops we obtained a few inverse loopswhere enrichment is self-regulating while eutrophication andexploitation are self-reinforcing (negative loops in Tables 1 and3 positive loops in Table 2) These inverted-sign loops describelongermdashand presumably weaker through their indirectnessmdashchain reactions stemming from the presence of alternative sourcesof income for society

Exploitation loops are mostly self-regulating implying that as astock dwindles its exploitation costs increase and it invites lessfishing effort (Table 3 negative loops) However here we see thatexploitation loops can connect two fisheries either directly (Table

3 loops 3 and 5) or indirectly through owners or the local marketThese stocks are thus not independent in the eyes of exploitationFor example pelagic zooplankton-eating haplochromines canmake up a large fraction of the dagaa bycatch This couldcompensate for dwindling dagaa stocks since there is no barrierto continuing exploitation in similar ways if for a time at leastthe fish initially targeted is easily replaced (Box 2) In the sameway local and international markets provide outlets for small andlarge Nile perch respectively Therefore if a size class becomesless available fishing camp owners would not necessarily seediminishing returns immediately (Table 3 loops 1 and 2) Theexact dynamics would of course depend on the price that differentfish and fish-size classes fetch and the fact that these prices aresubject to demand-driven fluctuations In the short term thismight be perceived as evidence of functional complementarity abenefit of diversity However it also signifies a dampening of theself-regulating nature of the negative feedback

Table 2 Eutrophication loops and their signs NP = nitrogen andphosphorus AnoxampShade = anoxia and shading Np = Nileperch IntMar = international market Socampecon = society andeconomy LocMar = local and regional market Haplo =haplochromines Dagaa = Rastrineobola argentea --gt = positiveinteraction --o = negative interaction

Loop Sign

1 NP--gtAnoxampShade--oNp--gtIntMar--gtSocampecon--gtNP -2 NP--gtAnoxampShade--oNp--gtLocMar--gtSocampecon--gtNP -3 NP--gtAnoxampShade--oHaplo--gtLocMar--gtSocampecon--gtNP -4 NP--gtAnoxampShade--oDagaa--gtLocMar--gtSocampecon--gtNP -5 NP--gtAnoxampShade--oNp--oHaplo--gtLocMar--gtSocampecon--gt

NP+

6 NP--gtAnoxampShade--oNp--gtIntMar--gtowners--gtDagaaeffort--oHaplo--gtLocMar--gtSocampecon--gtNP

+

7 NP--gtAnoxampShade--oNp--gtIntMar--gtowners--gtDagaaeffort--oDagaa--gtLocMar--gtSocampecon--gtNP

+

8 NP--gtAnoxampShade--oHaplo--gtLocMar--gtowners--gtNpeffort--oNp--gtIntMar--gtSocampecon--gtNP

+

9 NP--gtAnoxampShade--oDagaa--gtLocMar--gtowners--gtNpeffort--oNp--gtIntMar--gtSocampecon--gtNP

+

Table 3 Exploitation loops and their signs Np = Nile perchIntMar = international market Socampecon = society andeconomy LocMar = local and regional market Haplo =haplochromines Dagaa = Rastrineobola argentea --gt = positiveinteraction --o = negative interaction

Loop Sign

1 Np--gtIntMar--gtowners--gtNp effort--oNp -2 Np--gtLocMar--gtowners--gtNp effort--oNp -3 Haplo--gtLocMar--gtowners--gtDagaa effort--oHaplo -4 Dagaa--gtLocMar--gtowners--gtDagaa effort--oDagaa -5 Np--oHaplo--gtLocMar--gtowners--gtNp effort--oNp +6 Np--gtIntMar--gtSocampecon--gtLocMar--gtowners--gtNp effort--

oNp-


Box 2 Connectedness of the fisheries

Haplochromine populations in the offshore and pelagic waterswere on a steep decline from the late 1970s and collapsed to lowlevels in the wake of the Nile perch boom (Witte et al 1992)Surprisingly haplochromine biomass started recovering in theearly 1990s (Seehausen et al 1997b) and quickly achievedprecollapse pelagic stock biomass (Witte et al 2007b)Haplochromines now represent up to 80 of the pelagic fishbiomass and together with dagaa they constitute more than 50of the lakersquos total fish biomass (Tumwebaze 1997 Wanink 1999Kayanda et al 2009) In Tanzania since their resurgencehaplochromines have shifted from being an important bycatch ofthe dagaa fishery (sometimes constituting 50ndash90 of the catch)(Witte et al 2000) to becoming a target that is sold to regionalmarkets or is used as bait in the Nile perch fishery (Ngupula andMlaponi 2010) Indeed since the mid-1990s the dagaa fisheryhas harvested an increasing amount of zooplanktivorous (genusYssichromis) benthivorous and detritivorous haplochromines(Haplochromis ldquoparopius-likerdquo also referred to as Haplochromis ldquobroken barrdquo in Seehausen et al [1997b]) (van Rijssel personalobservation) indicating that benthic species migrate up at night(Kishe-Machumu 2012) and that the dagaa netsmdashextending 10m deepmdashcan influence benthic populations The relativeabundance of dagaa and haplochromines in catches is site- andseason-specific suggesting that the dagaandashhaplochromineinteraction hides multiple underlying drivers While fishers andmarkets in Tanzania have adapted and now specifically targethaplochromines in Uganda and Kenya haplochromines are stillonly bycatch though they are sold on local markets The scale ofhaplochromine bycatch is clearly indicative that dagaa fishingmethods are not selective enough which is important becausehaplochromines have slower growth rates than dagaa are moresensitive to eutrophication and can therefore not sustain the samelevels of fishing mortality Furthermore since both dagaa andhaplochromines can be sold in multiple forms (bait fodderhuman consumption) the actual taxonomic content of the dagaacatch has little effect on fishers Therefore since the resurgence ofhaplochromines the dagaa and haplochromine fisheries andstocks have become tightly connected which reduces the strengthof negative feedbacks and the self-regulating ability of the system(Fig 3 dagaa fishing effort --o haplochromines versus dagaafishing effort --o dagaa haplochromines and dagaa --gt LocMar)Indeed if a reduced stock does not lead to a reduced catch (egshould dagaa replace haplochromines or vice versa) the negativefeedback of a dwindling stock on fishers and markets is reducedThis conclusion contrasts the view that selective fisheriesmdashasopposed to balanced harvesting (Garcia et al 2012)mdasharedetrimental to the ecosystem as a whole In this particularinstance the lack of selectivity does not equate to a balancedharvesting methodology since it is not adapted to the habitatslife histories and growth rates of dagaa and haplochromines

A certain level of nutrient input can increase productivity for onespecies (enrichment) but represent reduced fitness or increasedmortality (eutrophication) for another Haplochromine speciesfor example are particularly sensitive to low light (Seehausen et

al 1997a) whereas dagaa is probably relatively more sensitive todeoxygenation than is Nile perch (Wanink et al 2001) Withoutexplicit management nutrient enrichment would most likelyalways lead to further enrichmentmdashie it is not intrinsically self-limiting or regulating Enrichment would however translate onlyto increased stock growth until the negative consequences ofeutrophication start taking effect and while fishing effort does notcause mortality rates that exceed growth rates (ie stock collapse)Ultimately sustainability of stocks is a function of the relativestrength of fishing-induced mortality versus a stockrsquos response ingrowth through feeding Since enrichment and eutrophicationalter growth rates sustainable fishing mortality rates must bevariable not constant Also although eutrophication affects thewhole lake the distribution and effect of nutrients on thebiochemistry of the lake is spatially heterogeneous (Box 3 Table4) Therefore the processes that drive growth of stocksmdashand thusdictate levels of sustainable harvestingmdashare site-specific and notgeneralizable across the whole lake at any one time

Box 3 Spatial heterogeneity of the lake

In inshore areas terrestrial runoff and detritus are primarysources of light limitation whereas further offshore it is algalbiomass that induces light limitation and thus restrictsphotosynthesis to a narrower surface layer Eutrophication affectsthe whole lake as half the total phosphorus input to the lake isfrom atmospheric deposition (Tamatamah et al 2005) thoughthere is a lot of spatial heterogeneity in its effectsmdashnot only in aninshore-offshore gradient but also between bays around the lake(Silsbe et al 2006 Loiselle et al 2008 Cornelissen et al 2013)Total nitrogen concentrations have increased since the 1960sprimarily through cyanobacteria fixation and nitrogen generallyfollows a decreasing concentration pattern from the coast tooffshore waters (Hecky et al 2010) While nutrient loading hascreated the light-limited conditions of the lake seasonal andinterannual variations in primary productivity are now drivenprimarily by climate (Silsbe et al 2006 Coacutezar et al 2012) This isof particular relevance since it highlights an importantcharacteristic of qualitative models such as the one we use theydo not account for time or effect lags (Justus 2005) The synergisticand cumulative effects of warming and eutrophication onphytoplankton and the food web will probably delay the effectsof eutrophication management (Schindler 2006)

Increases in nutrients no longer lead to increased phytoplanktonprimary production in the northern Murchison Bay andNapoleon Gulf of Lake Victoria indicating that these areas arephosphorus saturated (Silsbe et al 2006) (Fig 3 NampP --gtshading) However recent experiments conducted on samplesfrom the Mwanza Gulf in southern Lake Victoria show that theincrease in primary productivity with further nutrient enrichmentcan still take place (Cornelissen et al 2013) Effects of furthernutrient enrichment are therefore likely to be area- and season-specific Over the years chlorophyll a measurements have showna great degree of variability (Table 4) Even though differences insampling frequency and measurement methods as well as inlocation depth and timing of sampling make it impossible todraw accurate trends from these data it appears that chlorophyllconcentrations in the northern parts of the lake are higher thanin the south (Table 4)


Table 4 Variability in chlorophyll a (Chl) concentrations in the lake as found in the literature Seasons DS = (dry season) JunendashAugust SR = (short rains) SeptemberndashDecember LR = (long rains) JanuaryndashMay SD = standard deviation N = number of measuresLVRO = Lake Victoria Fisheries Organisation Source Location Year Season Depth (m) Chl (μgl) SD N

Acoustic survey LVFO Murchinson Bay 2005 DS lt 20 446 1Mwanza Gulf 2005 LR lt 20 1843 1Nyanza Gulf 2005 DS lt 20 1129 819 2Nyanza Gulf 2006 LR lt 20 891 297 4Speke Gulf 2005 LR lt 20 2076 918 9

Akiyama et al (1977) Mwanza Gulf 1973ndash1974 DS 8 435 160 6Mwanza Gulf 1973ndash1974 LR 8 51 201 6Mwanza Gulf 1973ndash1974 SR 8 37 110 8

Coacutezar et al (2007) Murchinson 2003 DS lt 20 8092 1373 13Gikuma-Njuru (2008) Nyanza Gulf 2005ndash2006 All year lt 20 1757 360 7Haande et al (2011) Murchison Bay 2003ndash2004 DS lt 11 2143 663 14

Murchison Bay 2003ndash2004 LR lt 11 314 1041 20Murchison Bay 2003ndash2004 SR lt 11 2406 654 16

Lehman and Branstrator (1993) Napoleon Gulf 1992 SR lt 10 48LungrsquoAyia et al (2000) Nyanza Gulf 1994ndash1995 SR lt 105 2077 748 11

Nyanza Gulf 1994ndash1996 LR lt 105 1345 277 11Mugidde (2001) Napoleon Gulf 1994ndash1998 All year lt 20 71 1004 47

Napoleon Gulf 1994ndash1999 LR lt 20 3811 2033 9Napoleon Gulf 1994ndash2000 DS lt 20 275 1837 6Napoleon Gulf 1994ndash2001 SR lt 20 825 7994 10

Ngupula et al (2011) Tanzania 2005ndash2007 All year lt 10 16 6 10Tanzania 2005ndash2007 All year 10ndash20 225 85 11Tanzania 2005ndash2007 All year 20ndash30 11 5 9

North et al (2008) Jinja 2001ndash2002ndash2004

SR lt 26 529 1060 12

Okello et al (2009) Bunjako (Uganda) 2004 DS lt 5 9 141 2Murchinson Bay 2004 DS lt 5 18 707 2Napoleon Gulf 2004 DS lt 20 105 071 2

Shayo et al (2011) Kayenze (Speke Gulf) 2005 DS lt 20 1467 758 6Kayenze (Speke Gulf) 2005 LR lt 20 1817 376 6Kayenze (Speke Gulf) 2005 SR lt 20 1367 646 9Magu (Speke Gulf) 2005 DS lt 20 20 1012 6Magu (Speke Gulf) 2005 LR lt 20 1817 1219 6Magu (Speke Gulf) 2005 SR lt 20 1756 721 9Mwanza Gulf 2005 DS lt 20 983 279 6Mwanza Gulf 2005 LR lt 20 1367 472 6Mwanza Gulf 2005 SR lt 20 1756 617 9

Silsbe et al (2006) Fielding 2001ndash2002 DSSR lt 10 42 2306 Murchison Bay 2001ndash2002 DSSR lt 8 695 2433 Napoleon Gulf 2001ndash2003 DSSR lt 14 26 6292

Sitoki et al (2010) Whole lake 2000 DS lt 20 128 113 Whole lake 2000 LR lt 20 132 44 Whole lake 2001 DS lt 20 109 7 Whole lake 2001 LR lt 20 108 67 Whole lake 2006 DS lt 20 162 8 Whole lake 2006 LR lt 20 148 109 Whole lake 2007 DS lt 20 144 84 Whole lake 2007 LR lt 20 157 85 Whole lake 2008 DS lt 20 145 112 Whole lake 2008 LR lt 20 168 86 Whole lake 2009 DS lt 20 148 61 Whole lake 2009 LR lt 20 127 125

Yasindi and Taylor (2003) Napoleon Gulf 1998 SR lt 15 34Cornelissen et al (2013) Mwanza Gulf 2009ndash2011 DS lt 25 1324 471 68

Mwanza Gulf 2009ndash2011 LR lt 25 1496 805 73Mwanza Gulf 2009ndash2011 SR lt 25 1345 460 72


Fig 4 Species and functional diversity loss in the soft-bottomfish community of Lake Victoria The bars represent thenumber of haplochromine species per functional groupcollected in the sublittoral waters (6ndash14 m) of the northern partof the Mwanza Gulf over soft bottom in 1979ndash1982 and 2001ndash2005 Between 1979ndash1982 and 2001ndash2005 65ndash70 of speciesand 50 of functional groups were irreversibly lost Oneunclassified species is omitted here Data from Witte et al(2013) Not only does the number of functional group decreasebut the degree of specialization within trophic groups alsodecreases eg former zooplanktivores have broader diets in2001ndash2005 than in 1979ndash1982 (Kishe-Machumu 2012 vanRijssel and Witte 2013)

When using the diagrams it is important to account forunderlying assumptions and to look at different feedback loopsin concert As an example if nutrient enrichment leads to higherNile perch stocks (Table 1 row 7) this promotes investment fromthe international market which can drive owners to invest effortin alternative stocks such as dagaa which would then reducedagaa stocks This would thus negatively influence the local andregional market and lead to a decrease in human populationgrowth which would lead to a decrease in nutrient input Takenin isolation such a scenario would be based on questionableassumptions it would assume that nutrient enrichment has apositive effect only on Nile perch not on dagaa It would alsoassume a single fishery target with no alternative stimuli for theregional market and that this dynamic is the major determinantof human population growth This type of loop would representa case where international economic input does not lead to socialand economic growth in the region and thus does not lead toincreased nutrient input to the lake The diagrams presented hereare tools that can enhance understanding and map multiplesystem variables simultaneously but even though they are simpletools their interpretation should not be oversimplified

Our combined social-ecological diagram allows us to see theeffects of change in a broader context but it has a limitedpredictive value indeed it cannot account for adaptation in eitherhuman or fish populations to declines in stock or in environmentalconditions However with a limited time frame in mind thediagram can give an idea of the far-reaching effects of any singleaction on the whole system We indicated three exogenousstressors climate change the international economy and regionalpolicies More might exist and they could have broader effectsthan illustrated here These stressors are dubbed exogenousbecause they are not solely or directly influenced by LakeVictoriarsquos system Exogenous stressors are important in that theycould shift the sign of loops in the feedback diagrams or changethe relative importance of any given interaction or loop and thusalter system dynamics

SYSTEM TRENDS AND CHANGES MATCHED TO THEMODELHere we look into Lake Victoriarsquos past to contextualize the modeland identify how its elements and interactions illustrate trendsthat have shaped the dynamics of the social-ecological systemThrough this juxtaposition of model and trends we can identifythe modelrsquos scope and limitations and indicate its best usesFurthermore it allows us to explore how weights of differentinteractions might have changed over time

In the beginninghellipBefore the introduction of Nile perch Lake Victoria harbored asmall though diverse multispecies artisanal fishery Fishingalong with agriculture and livestock herding constituted theeconomic foundations of lakeshore societies The main fishingtargets were the native tilapia Oreochromis esculentus and Ovariabilis (Balirwa et al 2003 Pringle 2005b) Fish were initiallysold by number rather than weight which invited catches of smallfish (Beverton 1959 Balirwa 1998) The introduction of gillnetsin the 1920s as a new tool quadrupled fishing efficiency and thefishery became commercialmdasha process further helped by urbaninfrastructure development (including road and rail) around thelake (Balirwa 1998) (Fig 3 development of localregional marketdagaa and haplochromines --gt LocMar --gt owners andSocampeconhellip)

Since the 1960s effects of eutrophicationmdashdriven by humanpopulation growth around the lake and subsequent changes inland use (Verschuren et al 2002 Hecky et al 2010)mdashbecameapparent The phytoplankton community underwent a transitionfrom a diatom-dominated community to one increasinglydominated by nitrogen-fixing cyanobacteria and primaryproduction increased In turn the zooplankton communityshifted to be dominated by small species (Mwebaza-Ndawula1994 Gophen et al 1995 Wanink et al 2002) The dominanceshift in the phytoplankton community has been hypothesized tobe a cause in the zooplankton shift (Wanink et al 2002) (Fig 1change in relative importance of phytoplankton --gt zooplanktonversus phytoplankton --gt detritus and anoxia amp shading)

Introduced species changed ecosystemAs native stocks were declining from exploitation eutrophicationand habitat destruction Nile tilapia (Oreochromis niloticus) andother tilapiine species were introduced Simultaneously (between1954 and 1964) Nile perch were introduced to boost the


productivity of the fisheries and convert the native diversity ofhaplochromines into a more desirable fishery product(Kudhongania and Chitamwebwa 1995 Pringle 2005b) From1965 Nile tilapia gradually replaced native tilapia became amajor commercial catch (Balirwa 1998) and is currently animportant product for local and regional markets (Njiru et al2008) Tilapia does not figure explicitly in our diagrams becauseit appears to interact little with other stocks (Downing 2012Downing et al 2013b) It is however of course also influencedby enrichment eutrophication and exploitation processes andcontributes to the regional market and can therefore be read inthe system in a similar way to haplochromines or dagaa but withtilapia-specific sensitivities to the different driving processes

While the Nile perch introductions had little immediate effect inthe 1980s many changes occurred quite rapidly Nile perchsuddenly became dominant in the whole lake andmdashin concertwith increased hypoxic and anoxic conditionsmdashcontributed tothe collapse of haplochromine stocks and mass disappearance orextinction of native fish species (Witte et al 1992 Hecky et al1994 2010 Verschuren et al 2002 Goudswaard et al 2008) Thehigh abundance of haplochromines proved to be favorable forNile perch as growth estimates in both the Tanzanian and Kenyanwaters through the end of the 1980smdashcovering the time when Nileperch were still feeding heavily on haplochrominesmdashshowed thatNile perch grew at least twice as fast as in haplochromine-poorlakes (eg Lake Chad Lake Turkana Lake Albert Lake Nasser)(Ligtvoet and Mkumbo 1990) (Fig 3 Nile perch --ohaplochromines) Ecological opportunities afforded by thevanishing haplochromines were exploited in short succession bytwo native species first the detritivorous shrimp C nilotica (Goudswaard et al 2006) and then the zooplanktivorous cypriniddagaa (R argentea) (Wanink et al 2002)

Since the 1990s effects of eutrophication and fishing have becomemore apparent and Lake Victoriarsquos ecosystem is still undergoingmany changes The water hyacinth (Eichhornia crassipes) whichfirst appeared in the lake in 1989 (Chapman et al 2008) hadproduced large mats by 1995 These mats severely interfered withfishing operations with important social-economic consequences(Opande et al 2004) and altered the bio-physico-chemicalenvironment of the water under them (Masifwa et al 2001Kateregga and Sterner 2008) This event supports the notion thatwater quality influences stocks and catches and importantly thatwhen the lakersquos environment is compromised in such a way neitherinternational nor regional markets can act as backups and societyhas little (perhaps only agriculture) left to rest on (Fig 3 NampPand anoxia amp shading --gt Nile perch dagaa and haplochromines--gt IntMar and LocMar --gt Socampecon) The hyacinth invasionwas in large part managed between 1997 and 1999 through manualremoval and the introduction of a weevil (Neochetinaeichhorniae) The effects of these management procedures wereprobably enhanced by an El Nintildeo episode (1997ndash1998) duringwhich elevated water levels broke off large hyacinth stands andwhere low light conditions (clouds) and rain reduced hyacinthgrowth (Njiru et al 2002 Williams et al 2005 2007 Wilson et al2007) Even though the hyacinth bloom was probably triggeredby eutrophication its decline is not a reaction to improved waterquality there are multiple pathways to any single phenomenon

Dominance of cyanobacteria in the phytoplankton community hasled to an increase in concentrations of toxins produced bycyanobacteria (eg microcystin) Higher trophic levelsmdashincludinghumansmdashare increasingly exposed to such toxins both directlythrough water and through their accumulation in fish tissues (Posteet al 2011) Low fish quality would probably reduce fish prices on(or even access to) the international market but decreasedwellbeing or increased illness in lakeshore societies would probablyhave more complex and further reaching consequences At thispoint however the extent to which cyanotoxins affect humanhealth around Lake Victoria is unknown though it is important tokeep toxicity in mind as a process in eutrophication

Societal adaptationsThe advent of the Nile perch boom in the 1980s changed societiesdramatically (Fig 3 IntMar --gt owners and Socampecon versusLocMar --gt owners and Socampecon) People migrated to thelakeside to enter the fisheries which accelerated human populationgrowth and altered the kinship relations that had existed previouslyHuman populations around the lake continue to grow at a higherrate than the African average (Odada et al 2009)

Filleting factories were constructed around the lake and the fishwere exported to Europe (Pringle 2005a Johnson 2010) The majorfilleting factories were built to process up to 25 tons of Nile percheach per day but would break-even on costs when operating atabout 30 capacity (Johnson 2010) Nile perch catches reached apeak in 1990 but effort has continued to increase since

Changes in dagaa haplochromines and the regional marketAlthough dagaa is native to Lake Victoria a directed fishery for itfirst developed only in the 1960s The stocks of dagaa rapidlyincreased after the Nile perch boom and haplochromine collapseDagaa became the lakersquos second most important fishery by the1990s (Wanink 1999) when landing sites started organizingthemselves into camps also presenting a strong social structurefrom trader to owner through camp supervisors and laborers withwomen drying the fish Dagaa is currently a dominant commercialfishery of the lake in terms of biomass (Tumwebaze et al 2007)and it trades on domestic and regional markets (Gibbon 1997)Catch that is successfully dried and cleaned of sand is destined forhuman consumption as it serves as a cheap source of protein forthe poor and middle class (Medard 2012) and the rest sells asanimal feed chicken fodder for example Traders play an importantrole checking the quality of the product as well as establishing itsprice (Gibbon 1997) Despite no apparent decrease in its stock sizedagaa has undergone behavioral morphological and life historychanges in some regions of the lake which may reflect local adaptiveresponse to predation by Nile perch fishing pressure andoreutrophication and associated hypoxia (Wanink 1999 Wanink andWitte 2000 Manyala and Ojuok 2007 Sharpe et al 2012) Fishingeffort has been on a steady increase (Mkumbo et al 2007) at leastuntil 2006 from where data suggest a stabilization in effort (Koldinget al 2014) (Fig 3 NampP --gt dagaa versus dagga fishing effort --odagaa) Dagaa remains a product destined mostly for a local andregional market and is therefore less well monitored regulatedand studied than Nile perch (Medard 2012)


SOCIAL-ECOLOGICAL CHANGE SEEN IN MODELSTRUCTUREThe single diagram we developed to represent Lake Victoriarsquossocial-ecological system can be used to analyze and represent awhole variety of dynamics This is necessary because systemdynamics have seen both short- and long-term dynamics Seasonsinfluence light temperature weather and wind conditions whichinfluence the growth of organisms as well as access to differentareas of the lake Dynamics also have a heterogeneous spatialdistribution considering for example the different nutrientenvironments in the north and south of the lake (Box 3 Table 4)or the atmosphere of conflict in contested UgandanmdashKenyanfish camps (Box 4) or differences in fishing practices (Beuving2014) All of these factors imply that a quantitative model of LakeVictoriarsquos social-ecological dynamics ought to be set in a clearlydefined spatio-temporal setting Meanwhile this purelyqualitative model allows us to analyze three types of structuralchanges to the system and their influence on dynamics and rolein the effectiveness of policies and regulations (1) a change in therelative weight of different interactions (2) the change in the signof interactions or loops and (3) the addition or removal ofinteractions Here we analyzemdashthrough examples of pastchanges and hypothetical management policiesmdashthe effects ofthese three types of changes on the dynamics on Lake Victoriarsquossocial-ecological system

Box 4 Societal reorganization and inequity

To ensure constant supply to the factories after the trawling banbecame effective in the 1990s (Mbuga et al 1998) a new social-economical interaction emerged processors invested in buildinginfrastructure to better store Nile perch at landing sites andsponsored fishermen with nets and gear via agents who wouldthen ensure fixed prices and supply of fish (Mbuga et al 1998Geheb et al 2008 Johnson 2010) Lakeshore societies thusdeveloped a stronger economic hierarchy from processor throughagent to fisher with many fishers then entering a situation ofcredit towards agents or processors (Geheb et al 2008) Fishingeffort continues to increase and fisher communities to restructureIn the Nile perch fishery in some areas many agents (middlemen)have become boat owners and landing site or camp managers whoemploy camp supervisors and laborers to operate full fleets andwith whom they arrange credits and loans to ensure a constantfish supply The structure is likely quite diverse however and thereare reports of wealthy boat owners striking deals with factoriesand thus taking a traderrsquos role

Economic input to society is nonlinear with larger swaths ofincome distributed to big owners and much less reaching fishersand laborers Besides stocks and the international economy andregulations local social-economic settings and interactions canhave complex effects on both social stability and benefits reapableby owners through fishing Individual cost-to-benefit ratio offishing and risk-taking behavior is variable Competition forlanding sites among owners for example can influence costs andprices by leading to a local monopoly of the fisheries by fewowners Competition among owners can lead to conflict andviolence there are reports of piracy in the southern part of thelake which leads to increased costs in security and unevendistribution of fish resources Also there are chronic territorial

conflicts over fish camp islands between Kenya and Uganda(Otieno 2013) Knowledge of the distribution of poweropportunities and value of risk in society is key to the selectionand implementation of appropriate policy measures Here welack representation of the potential response diversity of societywhich is a great impediment to measuring and predicting societyrsquosreactions and adaptations to ongoing changes in the system

Changes to the relative weight of interactionsManagement policies can profoundly influence the connectednessof a system for example by creating tighter networks ofcommunication Increased economic disparity caused an increasein illegal fishing practices in the 1990s In an effort to improvecompliance and better implement fishing regulations Tanzaniaestablished a comanagement approach to the fisheries in 1997 inthe form of Beach Management Units (BMUs) (Kateka 2010)These local groups were established to increase stakeholderparticipation in surveillance and management of the fisheries andsimilar structures were later implemented in Uganda and Kenya(Mkumbo 2002 Kateka 2010) Their success has been mixedhowever BMUs have effectively suppressed poisoning anddynamiting as fishing techniques but they lack an adequate legalstatus and by establishing their own bylaws BMUs cansometimes abuse their position and authority (Geheb et al 2007Njiru et al 2007) Importantly they have also increased fishingefficiency through improving communication networks (Eggertand Lokina 2009) (Fig 3 strengthening of interactions Nile perchand dagaa fishing effort has a negative effect on Nile perch dagaaand haplochromines) helping fishers get to good fishing sitesfaster and reducing the negative feedback of declining stocks onfishers

Changes in the sign of interactions or feedbacksFertilizer subsidies which might promote agriculture andalternative sources of income would nonetheless reinforcepositive feedback loops through nutrient enrichment and thusbe overall destabilizing A fishing limiting policy such as alicencing fee could add a minus to fishing effort and change thesigns of the loops in that way though if it is not applied to allfisheries such a policy could have a backlash effect and lead tohigher exploitation of alternative fisheries or to increased landuse for agriculture In short the effect of any single policy orchange depends on its ripple effects through the broader system

Changing the number of pathways in the systemIn a bid to prevent and eliminate illegal unregulated andunreported fishing the Lake Victoria Fisheries Organisation haspushed to promote eco-labeling of Nile perch For this theyestablished codes of practice with help from the MarineStewardship Council (Kolding et al 2014) In 2009 the Germancooperative Naturland launched its eco-label for the small captureNile perch fishery certifying the sustainability of Nile perchproducts exported to Germany The label is based on ecosystemfishery social and hygiene criteria and covers eight landing sitesand 1000 fishermen in Tanzania (Bukoba) though there iscurrently no known study that evaluates the effects of eco-labelingon the stocks society or economy However seen through ourdiagram Naturlandrsquos policymdashie supporting a small-scale


fishery on the Nile perchmdashmight be translated by adding a roleto some owners whereby their connection between the regionalmarket and the Nile perch fishery is subject to regulations (Fig3 LocMar --gt owners --gt Nile perch fishing effort) and whereowners connect the regional and international market (Fig 3IntMar --gt owners --gt LocMar) One of the loops that wouldthen emerge is a negative feedback loop connecting the regionalmarket to fisheries Nile perch stocks the international marketlocal societies and the regional market (Fig 3 LocMar --gtowners --gt Nile perch fishing effort --o Nile perch --gt IntMar --gtSocampecon (or owners) --gt LocMar) This constraint on themiddleman (owner) would differ from the loops we have so faraccounted for in that the ownerrsquos role merges with that of the localand regional economy and society (Fig 3 Socampecon) Such aloop might have beneficial social-economic repercussions since itcould reduce economic disparity (note that we separated ownersfrom society to be able to reflect differences in economic benefitsand incentives) However in itself this policy still only adds apathway to harvesting Nile perch a small-scale fishery might besustainable if taken on its own but as long as it operates in parallelwith other harvesting policies the stocks might yet be vulnerableto unsustainable harvesting Therefore for a policy like that ofNaturland to actually ensure sustainable Nile perch harvestingit needs to outcompete existing harvesting strategies (effectivelyconnecting itself to other fisheries with a minus sign in ourdiagram) While not all owners are party to the policy sustainableharvesting might happen if the label succeeds in influencingconsumption patterns in the international market (and) or ifdifferent incentives to harvest sustainably are presented directlyto fishermen and fishing site owners

Quantitative versus structural changes to the systemOverall we find that policies and changes that alter theconnectedness of the system influence the self-regulatory capacityof the system Connection strength changes are probably the mostcommon and insidious changes in the system as they can be anuncalculated side effect of various other policies (as for examplethe case of BMUs) Also given the heterogeneity in conditionsand processes over the whole lake differences in connectednessacross the system influence the generalizability of policyinterventions interaction strengths are not the same across thewhole lake or even permanent in time (Boxes 2 and 3) Changesin the sign and number of interactions however as for examplethe Naturland eco-labeling policy can alter system dynamicswithout necessarily altering the integrity of regulating feedbackmechanisms though the effects of such structural changes aredependent on the configuration and dynamics of the system as awhole

DISCUSSIONHere we put together pieces of Lake Victoriarsquos social-ecologicalsystem in a qualitative conceptual model and achieve greaterunderstanding of the systemrsquos functioning from a good priorknowledge of the processes behind dynamics as well as from thekey elements and interactions that are specific to Lake Victoria

The interconnection diagram highlights the multiple pathways toany single phenomenon in the system As an example the threatto fisheries stems not only from fishing effort but also from otherenvironmental factors eutrophication hypoxia and anoxia

reduce fish growth and survival rates thereby increasing thestocksrsquo vulnerability to fishing pressure Complete overharvestingof Lake Victoriarsquos fishes be it Nile perch dagaa or tilapia wouldnot necessarily be an easy task the number of fishers and craftson the lake has increased dramatically since the early 1990s buttechnologies have not followed suit everywhere Paddling is stillthe dominant method of propulsion in a large part of the lakeand where engines are available fuel costs and in the case of theNile perch fishery costs related to the hygienic storage of fishduring longer periods as well as dangerous boating conditionsall impose limits on the distance from shore that is exploitedDagaa is also difficult to harvest efficiently despite being aproductive species fishing of dagaa occurs mostly on moonlesswindless nights using lamps that attract insects and zooplanktonand then the zooplanktivores including dagaa However even ifall stocks are not currently overfished the threat to Lake Victoriarsquosfisheries is real Indeed native tilapia (ngege [Oreochromisesculentus]) was successfully reduced to low numbers using quitesimple methods before the introduction of Nile perch and Niletilapia Even though the dagaa fishery might not be the mostimportant direct threat to dagaa stocks it could be an importantinfluence on haplochromines since the two fisheries are connectedthrough fishing methods and markets The adaptability of fishersand markets to changes in stocks might reduce the self-regulatingcharacter of exploitation (ie dampen the negative feedback ofexploitation loops) and make a stock collapse even less predictableor even avoidable In this way while overfishing is not necessarilycurrently the biggest threat to fisheries on Lake Victoria per se itbecomes an important threat in the presence of environmentalchange and eutrophication and in the context of the fullinterconnected system

Interestingly here we find that even though high connectivity offisheries at the level of owners and markets may be detrimentalto stocks (Box 2) it can also make the social-economic systemmore robust to the loss of the international market A loss of theinternational market would cause a dramatic loss of revenue butthere is no shortage of demand for fish products in general andfishing markets have diversified Dagaa haplochromines andsmall Nile perch are sold locally and regionally for humanconsumption and as animal feed Tilapia and Nile perch fetchhigher prices on the same markets Furthermore a shutdown ofall international trade is considered highly unlikely In the sameway as when the European market closed in 1997 Israel and Asiaand other African countries could perhaps serve as alternativemarkets (van der Knaap et al 2002) Nonetheless it is importantto remember that lakeshore societies have grown and developedfast and a lack of long-term investments or permanent structurescontributes to the high and increasing income disparity inlakeshore populations which in turn can influence societies andcontribute to their increased volatility (Wilkinson and Pickett2009)

The rapid Nile perch invasion created a strong dependence onfisheries (as exemplified during the water hyacinth blooms) whichprobably makes societies quite vulnerable to a loss of stocksespecially since there are few other sources of steady incomeIndeed as the Nile perch fishery grew migrant settlements thatstarted off as temporary became villagesmdashwith bars makeshiftcinema halls and brothelsmdashthough they often lack essential


permanent structures such as medical units or schools (Geheb etal 2008) Investment opportunities lie mostly in high profitmargin activities rather than in durable or saving schemes theeconomic life is deeply monetized since there is little self-sustenance and most goods must be bought (Beuving 2010)People in these boom villages easily fall into a poverty trapGeneral health status is low and there is a high HIV-AIDSinfection and prevalence rate (van der Knaap and Ligtvoet 2010)Therefore alternative sources of income are undoubtedly highlydependent on the fisheriesmdashthe services sold in villages and campsare targeted at fishers These will not provide a safety net shouldfisheries collapse Agriculture chicken farming and livestockherding are among the few sources of income that are independentof fisheries Following this analysis we suggest new metrics withwhich to bridge social and ecological changes in future studiesinvestments in infrastructure and the sources of these investmentsmight provide a usable proxy for the dependence of societies ontheir resources Such data might also inform the sustainability ordurability of development around Lake Victoria Whileinvestment in urban and transport infrastructure in the early 20thcentury and in fish processing infrastructures in the 1980sheralded priority areas of future development around the lakethere was no comparable spending on agricultural infrastructure

This model does not yet encompass the full complexity of LakeVictoriarsquos societal components (eg Box 4) We lacked thereferences and detail needed to generalize the different processesthat drive societal dynamics and how these induce individuals totake risks drive investment and frame decision-makingprocesses In short our model strongly underrepresents theresponse diversity of society (all those processes hidden in thefeedback between society and the local market) In the same wayour model lacks resolution in terms of biodiversity (Box 5) Wecan represent functional diversity to a small degreemdashwhichunderlies in part the haplochromine-dagaa interrelationmdashbut thefull biodiversity and richness of the ecosystem which sets thestage for the adaptability of the system to changes anddisturbances is not represented These two elementsmdashbiodiversity and social structuremdashare directly relevant to anymedium-term predictions on system dynamics to policy-makingand to the assessment of the social-ecological systemrsquos currentstate Also it is important to remind the reader that the ecosystemwe describe has a strong aquatic bias In truth any visitor to thelake will first encounter a large diversity of birds insects andvegetation all of which influence and are influenced by changesin the aquatic terrestrial climatic and human systems Thishowever is at least a first step in bridging the social-economic-(aquatic)-ecosystem in hopes that encouraging furtherwork on bridges to avian and terrestrial ecology might be possiblein the future

Box 5 Diversity changes

Even though the resurging offshore haplochromine communityhas largely recovered in biomass and retained a 40-strong speciesdiversity (Seehausen et al 1997b) of which 15ndash20 species arepelagic this is only a small fraction of the 1970s haplochrominediversity Indeed 65ndash70 of both benthic and pelagic speciesdwelling over soft bottoms in the sublittoral waters (6ndash14 m deep)of the Mwanza Gulf seem to have been irreversibly lost (Kishe-

Machumu 2012 Witte et al 2013) (Fig 4) In the pre-Nile perchsystem an estimated minimum of 500 different species ofhaplochromines occupied almost every function in the food webfrom detritivorous species through to piscivores Fishing pressureon Nile perch (Fig 3 Nile perch fishing effort --o Nile perch --ohaplochromines) as well as morphological adaptations tochanged environmental conditions and major habitat shifts arepossibly associated with the reappearance of haplochromines(Kitchell et al 1997 Seehausen et al 1997b Schindler et al 1998Witte et al 2000 2008 Balirwa et al 2003 Mkumbo and Mlaponi2007 van Rijssel and Witte 2013) Dominant haplochromines ofthe Mwanza Gulf in 2006ndash2008 and 2010ndash2011 were found to bepredominantly the former zooplanktivores and detritivoresphytoplanktivores (Kishe-Machumu 2012) However diets ofhaplochromine species recovering since the 1990s changed theycontained more macro-invertebrates than in the 1970s and early1980s (Kishe-Machumu et al 2008) Although in recent yearshaplochromines seem to have reverted to zooplanktivory they areless specialized than they were in the 1970s and 1980s (Kishe-Machumu 2012 van Rijssel and Witte 2013)

In our feedback diagrams we linked all fields of research at theirboundaries thus shifting the focus from each fieldrsquos central themesto the interconnections between fields The diagrams show howa change in any part of Lake Victoriarsquos system can trigger chainreactions across the system Many of these chain reactionscontain or are part of feedback loops indicating that changes inthe system can initiate and drive new dynamic regimes Byanalyzing this interconnected system we find multiple pathwaysto what may at first appear to be singular linear phenomena Thepresence of multiple pathways can be dangerous since it can leadto a misinterpretation of mechanisms driving change but it canalso be advantageous by understanding different pathways thatregulate a single process we obtain multiple tools with which tomanage change

We investigate the mechanisms of change through the social-ecological model Changes to the structure of the system whichcan be brought about through evolution and adaptation in thesocial-ecological system (for example a broadening of a speciesrsquodiet creating a new interaction a species extinction leading to aloss of an interaction or technical or cultural changes in the socialsystem) or through the implementation of policies candramatically alter systems dynamics and responses to changeThough our analysis is not quantitative we can nonetheless seethat differences in relative strengths of interactions which can beunderstood as a change in the system state can greatly influencethe vulnerability of system elements to further change Suchquantitative changes and differences can be quite pervasive inboth time and space The dominant processes shaping the LakeVictoriarsquos dynamics have changed over time and areheterogeneous in space This heterogeneity implies that multiplesystem states exist which means on the one hand that studies onany part of the system must be understood in their specific spacendashtime context and on the other hand that the scope of managementactions must be scale adapted

The systemrsquos heterogeneity can also explain some of the scientificdisagreement that prevails regarding the state of the fisheries


Efforts ought to be made to determine the spatio-temporal scalesat which different processes dominate Lake Victoriarsquos complexchanging and interconnected system calls for an adaptivemanagement approach capable of embracing this heterogeneityAdaptive management requires in large part that stakeholdershave a common understanding and knowledge of their system(Ostrom 2009) By combining knowledge from different fields andperspectives we tried to develop such common knowledge Thisknowledge should not be interpreted as absolute Rather it willwax and develop as users and scientists learn from further changesin Lake Victoriarsquos system and from the systemrsquos responses tomanagement Choices of management policies need not and willnot wait for more or better knowledge of the systemrsquos dynamicsthey are part of the learning processes

This little history of Lake Victoria highlights how nearly allregulations or policies that have been put in place in Lake Victoriahave only ever targeted fishing Almost nothing has been done tohalt or reverse eutrophication In part this is because the mainsources of eutrophicationmdashie land burningmdashhave only recentlybeen defined Though eutrophication already displayed effects inthe 1960s it has only recently been acknowledged as a potentialand in fact important driver of change in Lake Victoriarsquos systemIndeed this is to our knowledge the first interdisciplinary studyto put eutrophication and fisheries in Lake Victoria on an equalfooting and perhaps in this way signals a turning point in theunderstanding of drivers of change in Lake VictoriaEutrophication is ongoing has damaging consequences for LakeVictoriarsquos system cannot compensate for or be remediated byfishery policies and must be addressed for the general health ofthe system its users and fisheries Additionally any fishery-oriented policy is likely to be ineffective in the absence of a paralleleutrophication and broader lake basin management policyAssociated policy options must be processed and understood bysocieties to be made operational yet we still lack a lot ofknowledge about social dynamics and interactions Furthermorebiodiversity is likely to be key in helping to process excessnutrients restore better water quality and value and enjoy thehost of nonfishery-related ecosystem services that the lake alsoprovides The full role of biological diversity and its potential torecover in Lake Victoria are largely unknown Our study pointsto society and diversity as two key subjects Lake Victoria scientistsneed to unravel

Responses to this article can be read online at httpwwwecologyandsocietyorgissuesresponsesphp6965

Acknowledgments

We would like to thank Koos Vijverberg for his very constructivecomments on the manuscript Many thanks to Tijs Goldschmidt andthe Artis-bibliotheek for hosting interesting debates on LakeVictoria This work is part of the integrated project ldquoExploitationor eutrophication as threats for fisheries Disentangling social andecological drivers of ecosystem changes in Lake Victoria(SEDEC)rdquo supported by the Netherlands Organisation forScientific Research (NWOWOTRO) grant number W016530400

LITERATURE CITEDAbila R O and E G Jansen 1997 From local to global marketsThe fish exporting and fishmeal industries of Lake Victoriamdashstructure strategies and socio-economic impacts in Kenya IUCNNairobi Kenya

Akiyama T A A Kajumulo and S Olsen 1977 Seasonalvariations of plankton and physico-chemical condition inMwanza Gulf Lake Victoria Bulletin of Freshwater FisheriesResearch Laboratory 2749ndash61

Awange J L and O Ongrsquoanga 2006 Lake Victoria ecologyresources environment Springer Victoria

Balirwa J S 1998 Lake Victoria wetlands and the ecology of theNile tilapia Oreochromis niloticus Linne DissertationWageningen University Balkema Rotterdam The Netherlands

Balirwa J S C A Chapman L J Chapman I G Cowx KGeheb L Kaufman R H Lowe-McConnell O Seehausen JH Wanink R L Welcomme and F Witte 2003 Biodiversityand fishery sustainability in the Lake Victoria basin anunexpected marriage BioScience 53703ndash715 httpdxdoiorg1016410006-3568(2003)053[0703BAFSIT]20CO2

Beuving J J 2010 Playing pool along the shores of Lake VictoriaFishermen careers and capital accumulation in the Ugandan Nileperch business Journal of the International African Institute(Africa) 801ndash27

Beuving J 2013 Chequered fortunes in global exports thesociogenesis of African entrepreneurship in the Nile perchbusiness at Lake Victoria Uganda European Journal ofDevelopment Research 25501ndash517 httpdxdoiorg101057ejdr201328

Beuving J 2014 Spatial diversity in small-scale fishing a socio-cultural interpretation of the Nile perch sector on Lake VictoriaUganda Tijdschrift voor economische en sociale geografie httpdxdoiorg101111tesg12081

Beverton R J H 1959 A report on the state of Lake Victoriafisheries Mimeo Fisheries Laboratory Lowestoft

Chapman L J C A Chapman L Kaufman F Witte and JBalirwa 2008 Biodiversity conservation in African inland waterslessons of the Lake Victoria region Verhandlungen desInternationalen Verein Limnologie 3016ndash34

Cornelissen I J M G M Silsbe J A J Verreth E van Donkand L A J Nagelkerke 2013 Dynamics and limitations ofphytoplankton biomass along a gradient in Mwanza Gulfsouthern Lake Victoria (Tanzania) Freshwater Biology 59127ndash141 httpdxdoiorg101111fwb12253

Coacutezar A N Bergamino S Mazzuoli N Azza L Bracchini AM Dattilo and S A Loiselle 2007 Relationships betweenwetland ecotones and inshore water quality in the Ugandan coastof Lake Victoria Wetlands Ecology and Management 15499ndash507httpdxdoiorg101007s11273-007-9046-6

Coacutezar A M Bruno N Bergamino B Uacutebeda L Bracchini AM Dattilo and S A Loiselle 2012 Basin-scale control on thephytoplankton biomass in Lake Victoria Africa PloS ONE 7e29962 httpdxdoiorg101371journalpone0029962


Dobiesz N E R E Hecky T B Johnson J Sarvala J MDettmers M Lehtiniemi L G Rudstam C P Madenjian andF Witte 2010 Metrics of ecosystem status for large aquaticsystemsmdasha global comparison Journal of Great Lakes Research 36123ndash138 httpdxdoiorg101016jjglr200911003

Downing A S 2012 Seeing the water for the fish building onperspectives of Lake Victoria Dissertation WageningenUniversity The Netherlands

Downing A S N Galic K P C Goudswaard E H van NesM Scheffer F Witte and W M Mooij 2013a Was Lates lateA null model for the Nile perch boom in Lake Victoria PloS ONE 8e76847 httpdxdoiorg101371journalpone0076847

Downing A S E H van Nes J H Janse F Witte I J MCornelissen M Scheffer and W M Mooij 2013b Assemblingthe pieces of Lake Victoriarsquos many food webs Reply to KoldingEcological Applications 23671ndash675 httpdxdoiorg10189012-14181

Downing A S E H van Nes J H Janse F Witte I J MCornelissen M Scheffer and W M Mooij 2012a Collapse andreorganization of a food web of Mwanza Gulf Lake VictoriaEcological Applications 22229ndash239 httpdxdoiorg10189011-09411

Downing A S E H van Nes W M Mooij and M Scheffer2012b The resilience and resistance of an ecosystem to a collapseof diversity PloS ONE 7e46135 httpdxdoiorg101371journalpone0046135

Downing A S E H van Nes K E van de Wolfshaar MScheffer and W M Mooij 2013c Effects of resources andmortality on the growth and reproduction of Nile perch in LakeVictoria Freshwater Biology 58828ndash840 httpdxdoiorg101111fwb12089

Eggert H and R B Lokina 2009 Regulatory compliance inLake Victoria fisheries Environment and Development Economics 15197

Garcia S M J Kolding J Rice M-J Rochet S Zhou TArimoto J E Beyer L Borges A Bundy D Dunn E A FultonM Hall M Heino R Law M Makino A D Rijnsdorp FSimard and A D M Smith 2012 Reconsidering theconsequences of selective fisheries Science 3351045ndash1047 httpdxdoiorg101126science1214594

Geheb K S Kalloch M Medard A-T Nyapendi C Lwenyaand M Kyangwa 2008 Nile perch and the hungry of LakeVictoria gender status and food in an East African fishery FoodPolicy 3385ndash98 httpdxdoiorg101016jfoodpol200706001

Geheb K M Medard M Kyangwa and C Lwenya 2007 Thefuture of change roles dynamics and functions for fishingcommunities in the management of Lake Victoriarsquos fisheriesAquatic Ecosystem Health amp Management 10467ndash480 httpdxdoiorg10108014634980701704098

Gibbon P 1997 ldquoThe poor relation A political economy of themarketing chain for dagaa in Tanzaniardquo Pages 1ndash67 in CDRWorking Paper 972 Center for Development ResearchCopenhagen Denmark

Gikuma-Njuru P 2008 Physical and biogeochemical gradientsand exchange processes in Nyanza Gulf and main Lake Victoria

(East Africa) Dissertation University of Waterloo WaterlooOntario Canada

Gophen M P B O Ochumba and L S Kaufman 1995 Someaspects of perturbation in the structure and biodiversity of theecosystem of Lake Victoria (East Africa) Aquatic LivingResources 827ndash41 httpdxdoiorg101051alr1995003

Goudswaard K P C F Witte and E F B Katunzi 2008 Theinvasion of an introduced predator Nile perch (Lates niloticusL) in Lake Victoria (East Africa) chronology and causesEnvironmental Biology of Fishes 81127ndash139 httpdxdoiorg101007s10641-006-9180-7

Goudswaard K P C F Witte and J H Wanink 2006 Theshrimp Caridina nilotica in Lake Victoria (East Africa) beforeand after the Nile perch increase Hydrobiologia 56331ndash44 httpdxdoiorg101007s10750-005-1385-9

Haande S T Rohrlack R P Semyalo P Brettum B EdvardsenA Lyche-Solheim K Soslashrensen and P Larsson 2011Phytoplankton dynamics and cyanobacterial dominance inMurchison Bay of Lake Victoria (Uganda) in relation toenvironmental conditions Limnologica 4120ndash29 httpdxdoiorg101016jlimno201004001

Hecky R E H A Bootsma and E O Odada 2006 Africanlake management initiatives the global connection Lakes ampReservoirs Research amp Management 11203ndash213 httpdxdoiorg101111j1440-1770200600307x

Hecky R E F W B Bugenyi P Ochumba J F Talling RMugidde M Gophen and L Kaufman 1994 Deoxygenation ofthe deep water of Lake Victoria East Africa Limnology andOceanography 391476ndash1481 httpdxdoiorg104319lo19943961476

Hecky R E R Mugidde P S Ramlal M R Talbot and G WKling 2010 Multiple stressors cause rapid ecosystem change inLake Victoria Freshwater Biology 5519ndash42 httpdxdoiorg101111j1365-2427200902374x

Johnson J L 2010 From Mfangano to Madrid the globalcommodity chain for Kenyan Nile perch Aquatic EcosystemHealth amp Management 1320ndash27 httpdxdoiorg1010801463shy4980903584694

Justus J 2005 Qualitative scientific modeling and loop analysisPhilosophy of Science 721272ndash1286 httpdxdoiorg101086508099

Kambewa E V 2007 Balancing the people profit and planetdimensions in international marketing channels A study oncoordinating mechanisms in the Nile perch channel from LakeVictoria Wageningen University The Netherlands

Kateka A G 2010 Co-management challenges in the LakeVictoria fisheries Stockholm University Stockholm Sweden

Kateregga E and T Sterner 2008 Lake Victoria fish stocks andthe effects of water hyacinths on the catchability of fish Pages 1ndash28 in Environment for Development Discussion Paper Series

Katunzi E F B W L T Van Densen J H Wanink and F Witte2006 Spatial and seasonal patterns in the feeding habits ofjuvenile Lates niloticus (L) in the Mwanza Gulf of Lake VictoriaHydrobiologia 568121ndash133 httpdxdoiorg101007s10750-006-0033-3


Katunzi E F B J Zoutendijk T Goldschmidt J H Waninkand F Witte 2003 Lost zooplanktivorous cichlid from LakeVictoria reappears with a new trade Ecology of Freshwater Fish 12237ndash240 httpdxdoiorg101046j1600-0633200300023x

Kayanda R A M Taabu R Tumwebaze L Muhoozi TJembe E Mlaponi and P Nzungi 2009 Status of the majorcommercial fish stocks and proposed species-specificmanagement plans for Lake Victoria African Journal of TropicalHydrobiology and Fisheries 2115ndash21

Kishe-Machumu M A 2012 Inter-guild differences and possiblecauses of the recovery of cichlid species in Lake Victoria TanzaniaDissertation Leiden University Leiden The Netherlands

Kishe-Machumu M F Witte and J H Wanink 2008 Dietaryshift in benthivorous cichlids after the ecological changes in LakeVictoria Animal Biology 58401ndash417 httpdxdoiorg101163157075608X383700

Kishe-Machumu M A F Witte J H Wanink and E F BKatunzi 2012 The diet of Nile perch Lates niloticus (L) afterresurgence of haplochromine cichlids in the Mwanza Gulf ofLake Victoria Hydrobiologia 682111ndash119 httpdxdoiorg101007s10750-011-0822-1

Kitchell J F D E Schindler R Ogutu-Ohwayo and P NReinthal 1997 The Nile perch in Lake Victoria interactionsbetween predation and fisheries Ecological Applications 7653ndash664 httpdxdoiorg1018901051-0761(1997)007[0653TNPILV]20CO2

Kolding J 1993 Population dynamics and life-history styles ofNile tilapia Oreochromis niloticus in Fergusonrsquos Gulf LakeTurkana Kenya Environmental Biology of Fishes 3725ndash46httpdxdoiorg101007BF00000710

Kolding J M Medard O Mkumbo and P A M van Zwieten2014 Status trends and management of the Lake Victoriafisheries In R L Welcomme J Valbo-Joslashrgensen and A S Hallseditors Inland fisheries evolution and managementmdashcase studiesfrom four continents FAO Fisheries and Aquaculture TechnicalPaper 579 In press

Kolding J P van Zwieten O Mkumbo G Silsbe and R Hecky2008 Are the Lake Victoria fisheries threatened by exploitationor eutrophication Towards an ecosystem-based approach tomanagement Pages 309ndash354 in G Bianchi and H R Skjoldaleditors The ecosystem approach to fisheries httpdxdoiorg10107997818459341490309

Kudhongania A W and D B R Chitamwebwa 1995 Impactof environmental change species introductions and ecologicalinteractions on the fish stocks of Lake Victoria Pages 19ndash32 in T J Pitcher and P J B Hart editors The impact of species changesin African Lakes Chapman amp Hall London UK httpdxdoiorg101007978-94-011-0563-7_2

Lehman J T and D K Branstrator 1993 Effects of nutrientsand grazing on the phytoplankton of Lake VictoriaVerhandlungen der internationale Vereinigung fuumlr Limnologie 25850ndash855

Levins R 1974 The qualitative analysis of partially specifiedsystems Annals of the New York Academy of Sciences 231123ndash138 httpdxdoiorg101111j1749-66321974tb20562x

Ligtvoet W and O C Mkumbo 1990 Synopsis of ecologicaland fishery research on Nile perch (Lates niloticus) in LakeVictoria conducted by HESTTAFIRI in Report of the fifthsession of the sub-committee for the development andmanagement of the fisheries of Lake Victoria Pages 35ndash74 FAOFisheries Report 430 Rome Italy

Loiselle S A N Azza A Coacutezar L Bracchini A Tognazzi ADattilo and C Rossi 2008 Variability in factors causing lightattenuation in Lake Victoria Freshwater Biology 53535ndash545httpdxdoiorg101111j1365-2427200701918x

LungrsquoAyia H B O A MrsquoHarzi M Tackx J Gichuki and J JSymoens 2000 Phytoplankton community structure andenvironment in the Kenyan waters of Lake Victoria FreshwaterBiology 43529ndash543 httpdxdoiorg101046j1365-24272000t01-1-00525x

Manyala J O and J E Ojuok 2007 Survival of the LakeVictoria Rastrineobola argentea in a rapidly changingenvironment biotic and abiotic interactions Aquatic EcosystemHealth amp Management 10407ndash415 httpdxdoiorg10108014634980701704155

Masifwa W F T Twongo and P Denny 2001 The impact ofwater hyacinth Eichhornia crassipes (Mart) Solms on theabundance and diversity of aquatic macroinvertebrates along theshores of northern Lake Victoria Uganda Hydrobiologia 45279ndash88 httpdxdoiorg101023A1011923926911

Mbuga J S A Getabu A Asila M Medard and R Abila1998 Trawling in Lake Victoria its history status and effectsSocio-economics of the Lake Victoria Fisheries Project Report No3 IUCN Nairobi Kenya

Medard M 2012 Relations between people relations aboutthings gendered investment and the case of the Lake Victoriafishery Tanzania Signs 37555ndash566 httpdxdoiorg101086662704

Mkumbo O C 2002 Assessment and management of Nile perch (Lates niloticus L) stocks in the Tanzanian waters of LakeVictoria Dissertation University of Hull Hull UK

Mkumbo O C and E Mlaponi 2007 Impact of the baited hookfishery on the recovering endemic fish species in Lake VictoriaAquatic Ecosystem Health amp Management 10458ndash466 httpdxdoiorg10108014634980701704197

Mkumbo O C P Nsinda C N Ezekiel I G Cowx and MAeron 2007 Towards sustainable exploitation of Nile perchconsequential to regulated fisheries in Lake Victoria AquaticEcosystem Health amp Management 10449ndash457 httpdxdoiorg10108014634980701708057

Mugidde R 2001 Nutrient status and planktonic nitrogen fixationin Lake Victoria Dissertation University of Waterloo WaterlooOntario Canada

Muhoozi L I 2002 Exploitation and management of the artisanalfisheries in the Ugandan waters of Lake Victoria DissertationUniversity of Hull Hull UK

Mwebaza-Ndawula L 1994 Changes in relative abundance ofzooplankton in northern Lake Victoria East AfricaHydrobiologia 272259ndash264 httpdxdoiorg101007BF00006526


Ngupula G W A S E Mbonde and C N Ezekiel 2011 Spatialand temporal patterns of phytoplankton abundance andcomposition in three ecological zones in the Tanzanian waters ofLake Victoria African Journal of Aquatic Science 36197ndash206httpdxdoiorg102989160859142011589118

Ngupula G W and E Mlaponi 2010 Changes in abundanceof Nile shrimp Caridina nilotica (Roux) following the decline ofNile perch and recovery of native haplochromine fishes LakeVictoria Tanzanian waters Aquatic Ecosystem Health ampManagement 13196ndash202 httpdxdoiorg101080146349882shy010483188

Njiru M P Nzungi A Getabu E Wakwabi A Othina TJembe and S Wekesa 2007 Are fisheries management measuresin Lake Victoria successful The case of Nile perch and Nile tilapiafishery African Journal of Ecology 45315ndash323 httpdxdoiorg101111j1365-2028200600712x

Njiru M J Ojuok a Getabu T Jembe M Owili and C Ngugi2008 Increasing dominance of Nile tilapia Oreochromis niloticus (L) in Lake Victoria Kenya consequences for the Nile perchLates niloticus (L) fishery Aquatic Ecosystem Health ampManagement 1142ndash49 httpdxdoiorg10108014634980701878090

Otieno E 2013 Kenya and Uganda clash over Migingo DailyNation Monday July 1 2013 httpwwwnationcokenewsKenya+and+Uganda++clash+over+Migingo-10561901406-cljpxw-indexhtml

Njiru M A N Othina A Getabu D Tweddle and I G Cowx2002 Is the invasion of water hyacinth Eichhornia crassipes Solms( Mart ) a blessing to Lake Victoria fisheries Page 396 in I GCowx editor Management and Ecology of Lake and ReservoirFisheries Blackwell Science Oxford UK

North R L S J Guildford R E H Smith M R Twiss andH J Kling 2008 Nitrogen phosphorus and iron colimitation ofphytoplankton communities in the nearshore and offshoreregions of the African Great Lakes Verhandlungen derinternationale Vereinigung fuumlr Limnologie 30259ndash264

Ochumba P B O 1990 Massive fish kills within the Nyanza Gulfof Lake Victoria Kenya Hydrobiologia 20893ndash99 httpdxdoiorg101007BF00008448

Odada E O W O Ochola and D O Olago 2009 Drivers ofecosystem change and their impacts on human well-being in LakeVictoria basin African Journal of Ecology 4746ndash54 httpdxdoiorg101111j1365-2028200801049x

Okello W C Portmann M Erhard K Gademann and RKurmayer 2009 Occurrence of microcystin-producingcyanobacteria in Ugandan freshwater habitats EnvironmentalToxicology 25367ndash380 httpdxdoiorg101002tox20522

Opande G O J C Onyango and S O Wagai 2004 LakeVictoria the water hyacinth (Eichhornia crassipes [Mart] Solms)its socio-economic effects control measures and resurgence in theWinam gulf Limnologica 34105ndash109 httpdxdoiorg101016S0075-9511(04)80028-8

Ostrom E 2009 A general framework for analyzingsustainability of social-ecological systems Science 325419ndash422httpdxdoiorg101126science1172133

Ponte S 2007 Bans tests and alchemy food safety regulationand the Uganda fish export industry Agriculture and HumanValues 24179ndash193 httpdxdoiorg101007s10460-006-9046-9

Poste A E R E Hecky and S J Guildford 2011 Evaluatingmicrocystin exposure risk through fish consumptionEnvironmental Science amp Technology 455806ndash5811 httpdxdoiorg101021es200285c

Pringle R M 2005a The Nile perch in Lake Victoria localresponses and adaptations Africa Journal of the InternationalAfrican Institutes 75510ndash538 httpdxdoiorg103366afr2005754510

Pringle R M 2005b The origins of the Nile perch in LakeVictoria BioScience 55780ndash787 httpdxdoiorg1016410006-3568(2005)055[0780TOOTNP]20CO2

Rutjes H A M C Nieveen R E Weber F Witte and G E EJ M Van den Thillart 2007 Multiple strategies of Lake Victoriacichlids to cope with lifelong hypoxia include hemoglobinswitching American Journal of Physiology ndash RegulatoryIntegrative and Comparative Physiology 293R1376-1383 httpdxdoiorg101152ajpregu005362006

Schindler D W 2006 Recent advances in the understanding andmanagement of eutrophication Limnology and Oceanography 51356ndash363 httpdxdoiorg104319lo2006511_part_20356

Schindler D E J F Kitchell and R Ogutu-Ohwayo 1998Ecological consequences of alternative gill net fisheries for Nileperch in Lake Victoria Conservation Biology 1256ndash64

Seehausen O 2009 Speciation affects ecosystems Nature 4581122ndash1123 httpdxdoiorg1010384581122a

Seehausen O J J M van Alphen and F Witte 1997a Cichlidfish diversity threatened by eutrophication that curbs sexualselection Science 2771808ndash1811 httpdxdoiorg101126science27753331808

Seehausen O F Witte E F Katunzi J Smits and N Bouton1997b Patterns of the remnant cichlid fauna in southern LakeVictoria Conservation Biology 11890ndash904 httpdxdoiorg101046j1523-1739199795346x

Sharpe D M T S B Wandera and L J Chapman 2012 Lifehistory change in response to fishing and an introduced predatorin the East African cyprinid Rastrineobola argentea EvolutionaryApplications 5677ndash693 httpdxdoiorg101111j1752-4571201200245x

Shayo S D C Lugomela and J F Machiwa 2011 Influence ofland use patterns on some limnological characteristics in thesouth-eastern part of Lake Victoria Tanzania AquaticEcosystem Health amp Management 14246ndash251 httpdxdoiorg101080146349882011599607

Silsbe G M R E Hecky S J Guildford and R Mugidde 2006Variability of chlorophyll a and photosynthetic parameters in anutrient-saturated tropical great lake Limnology andOceanography 512052ndash2063 httpdxdoiorg104319lo20065152052

Sitoki L J Gichuki C Ezekiel F Wanda O C Mkumbo andB E Marshall 2010 The environment of Lake Victoria (EastAfrica) current status and historical changes InternationalReview of Hydrobiology 95209ndash223 httpdxdoiorg101002iroh201011226


Tamatamah R A R E Hecky and H C Duthie 2005 Theatmospheric deposition of phosphorus in Lake Victoria (EastAfrica) Biogeochemistry 73325ndash344 httpdxdoiorg101007s10533-004-0196-9

Tumwebaze R 1997 Application of hydroacoustics in fish stockassessment of Lake Victoria Thesis University of Bergen BergenNorway httpdxdoiorg10108014634980701709527

Tumwebaze R I Cowx S Ridgway A Getabu and D NMacLennan 2007 Spatial and temporal changes in thedistribution of Rastrineobola argentea in Lake Victoria AquaticEcosystem Health amp Management 10398ndash406

van der Knaap M and W Ligtvoet 2010 Is Westernconsumption of Nile perch from Lake Victoria sustainableAquatic Ecosystem Health amp Management 13429ndash436 httpdxdoiorg101080146349882010526088

van der Knaap M M J Ntiba and I G Cowx 2002 Keyelements of fisheries management on Lake Victoria AquaticEcosystem Health amp Management 5245ndash254 httpdxdoiorg10108014634980290031947

Van Rijssel J C and F Witte 2013 Adaptive responses inresurgent Lake Victoria cichlids over the past 30 yearsEvolutionary Ecology 27253ndash267 httpdxdoiorg101007s10682-012-9596-9

Veraart A J J J M de Klein and M Scheffer 2011 Warmingcan boost denitrification disproportionately due to altered oxygendynamics PloS ONE 6e18508 httpdxdoiorg101371journalpone0018508

Verschuren D T C Johnson H J Kling D N Edgington PR Leavitt E T Brown M R Talbot and R E Hecky 2002History and timing of human impact on Lake Victoria EastAfrica Proceedings of the Royal Society of London B BiologicalSciences 269289ndash294 httpdxdoiorg101098rspb20011850

Vonlanthen P D Bittner A G Hudson K A Young R MuumlllerB Lundsgaard-Hansen D Roy S Di Piazza C R Largiaderand O Seehausen 2012 Eutrophication causes speciationreversal in whitefish adaptive radiations Nature 482357ndash362httpdxdoiorg101038nature10824

Walters C V Christensen and D Pauly 1997 Structuringdynamic models of exploited ecosystems from trophic mass-balance assessments Reviews in Fish Biology and Fisheries 7139ndash172 httpdxdoiorg101023A1018479526149

Walters C and J F Kitchell 2001 Cultivationdepensationeffects on juvenile survival and recruitment implications for thetheory of fishing Canadian Journal of Fisheries and AquaticSciences 5839ndash50 httpdxdoiorg101139f00-160

Wanink J H 1999 Prospects for the fishery on the small pelagicRastrineobola argentea in Lake Victoria Hydrobiologia 407183ndash189 httpdxdoiorg101023A1003708624899

Wanink J H J J Kashindye K P C Goudswaard and F Witte2001 Dwelling at the oxycline Does increased stratificationprovide a predation refugium for the Lake Victoria sardineRastrineobola argentea Freshwater Biology 4675ndash85

Wanink J H E F B Katunzi K P C Goudswaard F Witteand W L T van Densen 2002 The shift to smaller zooplanktonin Lake Victoria cannot be attributed to the lsquosardinersquo

Rastrineobola argentea (Cyprinidae) Aquatic Living Resources 1537ndash43 httpdxdoiorg101016S0990-7440(01)01145-7

Wanink J H and F Witte 2000 Rapid morphological changesfollowing niche shift in the zooplanktivorous cyprinidRastrineobola argentea from Lake Victoria Netherlands Journalof Zoology 50365ndash372

Wilkinson R G and K Pickett 2009 The spirit level why moreequal societies almost always do better Allen Lane London UK

Williams A E H C Duthie and R E Hecky 2005 Waterhyacinth in Lake Victoria Why did it vanish so quickly and willit return Aquatic Botany 81300ndash314 httpdxdoiorg101016jaquabot200501003

Williams A E R E Hecky and H C Duthie 2007 Waterhyacinth decline across Lake VictoriamdashWas it caused by climaticperturbation or biological control A reply Aquatic Botany 8794ndash96 httpdxdoiorg101016jaquabot200703009

Wilson J R U O Ajuonu T D Center M P Hill M H JulienF F Katagira P Neuenschwander S W Njoka J Ogwang RH Reeder and T Van 2007 The decline of water hyacinth onLake Victoria was due to biological control by Neochetina sppAquatic Botany 8790ndash93 httpdxdoiorg101016jaquabot200606006

Witte F T Goldschmidt J Wanink M van Oijen KGoudswaard E Witte-Maas and N Bouton 1992 Thedestruction of an endemic species flock quantitative data on thedecline of the haplochromine cichlids of Lake VictoriaEnvironmental Biology of Fishes 341ndash28 httpdxdoiorg101007BF00004782

Witte F B S Msuku J H Wanink O Seehausen E F BKatunzi P C Goudswaard and T Goldschmidt 2000 Recoveryof cichlid species in Lake Victoria an examination of factorsleading to differential extinction Reviews in Fish Biology andFisheries 10233ndash241 httpdxdoiorg101023A1016677515930

Witte F O Seehausen J H Wanink M A Kishe-MachumuM Rensing and T Goldschmidt 2013 Cichlid species diversityin naturally and anthropogenically turbid habitats of LakeVictoria East Africa Aquatic Sciences 75169ndash183 httpdxdoiorg101007s00027-012-0265-4

Witte F J H Wanink and M Kishe-Machumu 2007a Speciesdistinction and the biodiversity crisis in Lake VictoriaTransactions of the American Fisheries Society 1361146ndash1159httpdxdoiorg101577T05-1791

Witte F J H Wanink M Kishe-Machumu O C Mkumbo PC Goudswaard and O Seehausen 2007b Differential declineand recovery of haplochromine trophic groups in the MwanzaGulf of Lake Victoria Aquatic Ecosystem Health amp Management 10416ndash433 httpdxdoiorg10108014634980701709410

Witte F M Welten M Heemskerk I Van Der Stap L HamH Rutjes and J Wanink 2008 Major morphological changes ina Lake Victoria cichlid fish within two decades Biological Journalof the Linnean Society 9441ndash52 httpdxdoiorg101111j1095-8312200800971x

Yasindi A W and W D Taylor 2003 Abundance biomass andestimated production of planktonic ciliates in Lakes Victoria andMalawi Aquatic Ecosystem Health amp Management 6289ndash297httpdxdoiorg10108014634980301496

Title

Abstract

Introduction

Social-ecological system representation

Qualitative models

Ecological subsystem

Society and fisheries

The social-ecological system

Loop analysis

System trends and changes matched to the model

In the beginning

Introduced species changed ecosystem

Societal adaptations

Changes in dagaa haplochromines and the regional market

Social-ecological change seen in model structure

Changes to the relative weight of interactions

Changes in the sign of interactions or feedbacks

Changing the number of pathways in the system

Quantitative versus structural changes to the system

Discussion

Responses to this article

Acknowledgments

Literature cited

Figure1

Figure2

Figure3

Figure4

Table1

Table2

Table3

Table4


al 2009) Many people migrated to the lakersquos shores to benefitfrom the fisheries a migration that has led to importanttransformations to the lakersquos societies Currently an estimated 30million people live in the lakersquos basin and are shaping and beingshaped by its ecology and economy (Awange and Ongrsquoanga2006)

Management measures on Lake Victoria have focused on fisheriesand aim to manage a certain quality of catch (size structure ofNile perch) rather than total fishing effort or stock biomass (vander Knaap et al 2002) Some of these methods are applied on thelake itself eg banning beach seines and increasing the minimummesh size of gillnets or introducing fish slot sizes In the case ofNile perch measures exist at the processing output level wherethere is a minimum length of fish that the factories are allowedto process Approaches to managing the fisheries assume that thefisher communities as well as fish stocks are homogeneous(Muhoozi 2002) Management measures are difficult toimplement and monitor given that Lake Victoria is the worldrsquossecond largest freshwater lake by area (68800 km2) has a highlyconvoluted shoreline that is shared by three countries (TanzaniaUganda and Kenya) and the fisheries are open access Also theeffectiveness of regulations is limited in part because policies canbe immediately detrimental to the filleting factories that processthe fish for export markets (Johnson 2010) Resources availableto implement or enforce policies are scarce and enforcement canbe subject to corruption

Added to difficulties in choosing and implementing effectivemanagement measures stakeholders and scientists disagree aboutthe current state of the Nile perch fisheries and what the dominantthreats to these fisheries are (Witte et al 2007a) Indeed ownersof filleting factories complain that they are operating theirfactories at decreasing capacity overlooking the fact that manyof the factories were built at structural overcapacity (Abila andJansen 1997 Johnson 2010) Some scientists read overfishing inthe fishery survey data (Mkumbo et al 2007 Mkumbo andMlaponi 2007 Kayanda et al 2009) while others see a total stockthat fluctuates around a stable average despite increased fishingeffort The stable stock under increased pressure rests on the logicthat increased productivity results from eutrophication (Koldinget al 2008)

The cumulative pressures influencing Lake Victoria are multipleintricately interconnected and threatening to all its fisheries aswell as to the lakersquos ability to carry out supporting services Indeedthe whole ecosystem is influenced by climate change (Hecky et al2010 Coacutezar et al 2012) and has been degraded througheutrophication (Dobiesz et al 2010) which impoverishes thesystem by driving an irreversible decline in biodiversity(Seehausen et al 1997a) Beyond a certain thresholdeutrophication threatens to cause a more general collapse ofstocks not only of Nile perch (Kolding et al 2008) AdditionallyNile perch is not the only exploited fish For example the nativepelagic cyprinid Rastrineobola argentea (known as dagaa inTanzania omena in Kenya and mukene in Uganda hereinreferred to as dagaa) has become an increasingly importantproduct for local societies (Wanink 1999) Furthermore since theexplosion of Nile perch numbers in the lake social and economicdrivers of fishing effort have changed Risks and opportunitiesthat face fisher communities have evolved with increasing human

population sizes and market value (Beuving 2013) Nile perch isdestined predominantly to European markets that impose stricthygiene standards and influence fish prices Market demand andprocessing costs influence factory owners fish buyers and trademiddlemen who in turn affect the organization of fishing andfisher communities and thus shape stock exploitation (Abila andJansen 1997 Ponte 2007 van der Knaap and Ligtvoet 2010)

Two lake-wide organizationsmdashthe Lake Victoria FisheriesOrganisation (LVFO) and the Lake Victoria Basin Commission(LVBC)mdashhave played the most important roles in coordinatingresearch on the lakersquos resources While originally conceived as aninstrument for holistic ecosystem-based management the LVFOhas a much stronger fisheries perspective The LVBC has thepotential to address water quality and nonfisheries managementissues through its Lake Victoria Environment ManagementProject Plan II (LVEMPII) So far coordination of field effortsand data sharing between the two organizations are poor Fisheryscience and to some extent limnology have undergone a measureof regional integration under LVEMP activities but on the wholean ecosystemrsquos view is still distinctly lacking Dissemination ofresults has also been limited at times with LVFO investing greatereffort in communications than the LVBC

In sum changes in Lake Victoria are a complex product of socialeconomic and ecological processes To comprehend the dynamicsof this complex system to visualize the scenarios likely to resultfrom alternative management measures and to manage humanactivities that influence the lake in an intelligent and farsightedmanner the lake and lake basin ecosystem must be understoodin their entirety Many analyses of their partsmdashfor example waterquality diversity or fisheriesmdashhave been conducted in isolationOur aim is to connect the dots and study the interactions amongthe parts of Lake Victoriarsquos system

An important barrier to assembling the pieces of Lake Victorialies in quantifying the ecosystemrsquos components in a spatio-temporally representative way As a first step in this direction weemployed a form of qualitative modeling called loop analysis(Levins 1974) We first built a feedback diagram of Lake Victoriarsquossocial-ecological system based on the components andinteractions that compose it matching the model to LakeVictoriarsquos past trends and changes and investigated the effects ofquantitative and structural modifications to the system on itsbehavior

SOCIAL-ECOLOGICAL SYSTEM REPRESENTATION

Qualitative modelsWe first created a feedback diagram containing three differenttypes of objects (1) the key elements of the system (2)connections between interacting key elements and (3) the sign ofthese interactions near equilibrium If an increase in the value(biomass concentration or economic value) of an element causesan increase in a connected element the sign is positive (+) If onthe other hand an increase in one causes a decrease in the otherthe sign is negative (-) We then carried out the loop analysis wherewe identified the feedback loops in the system ie the pathwayof interactions that go from any element and back to it throughother elements in the defined system We also identified whetherthese feedbacks are overall positive or negative by multiplying the





























Loop Sign




-


-


-






Loop Sign


NP+


+


+


+


+


Loop Sign


oNp-




















SR lt 26 529 1060 12





















































Acknowledgments




























































































































Title

Abstract

Introduction


Qualitative models




Loop analysis


In the beginning









Discussion


Acknowledgments

Literature cited

Figure1

Figure2

Figure3

Figure4

Table1

Table2

Table3

Table4





























Loop Sign




-


-


-






Loop Sign


NP+


+


+


+


+


Loop Sign


oNp-




















SR lt 26 529 1060 12





















































Acknowledgments











































































Njiru M A N Othina A Getabu D Tweddle and I G Cowx2002 Is the invasion of water hyacinth Eichhornia crassipes Solms( Mart ) a blessing to Lake Victoria fisheries 96 in I GCowx editor Management and Ecology of Lake and ReservoirFisheries Blackwell Science Oxford UK

















































Title

Abstract

Introduction


Qualitative models




Loop analysis


In the beginning









Discussion


Acknowledgments

Literature cited

Figure1

Figure2

Figure3

Figure4

Table1

Table2

Table3

Table4






















Loop Sign




-


-


-






Loop Sign


NP+


+


+


+


+


Loop Sign


oNp-




















SR lt 26 529 1060 12





















































Acknowledgments




























































































































Title

Abstract

Introduction


Qualitative models




Loop analysis


In the beginning









Discussion


Acknowledgments

Literature cited

Figure1

Figure2

Figure3

Figure4

Table1

Table2

Table3

Table4




















SR lt 26 529 1060 12





















































Acknowledgments




























































































































Title

Abstract

Introduction


Qualitative models




Loop analysis


In the beginning









Discussion


Acknowledgments

Literature cited

Figure1

Figure2

Figure3

Figure4

Table1

Table2

Table3

Table4















































Acknowledgments




























































































































Title

Abstract

Introduction


Qualitative models




Loop analysis


In the beginning









Discussion


Acknowledgments

Literature cited

Figure1

Figure2

Figure3

Figure4

Table1

Table2

Table3

Table4














































































































Title

Abstract

Introduction


Qualitative models




Loop analysis


In the beginning









Discussion


Acknowledgments

Literature cited

Figure1

Figure2

Figure3

Figure4

Table1

Table2

Table3

Table4

2014_Coupled human and natural system dynamics as key to the sustainability of Lake Victoria’s...

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Transcript of 2014_Coupled human and natural system dynamics as key to the sustainability of Lake Victoria’s...