Diel vertical distribution and life history characteristicsof Tropodiaptomus simplex and its importance in the diet

of Stolothrissa tanganicae, Kigoma, TanzaniaH. F. Mgana,1,∗ A. Herzig,2 and Y. D. Mgaya3

1Tanzania Fisheries Research Institute, Box 90, Kigoma, Tanzania2Neusiedler See Biological Station, Illmitz, Austria

3Department of Aquatic Sciences and Fisheries, University of Dar es Salaam, Box 35064, Dar es Salaam, Tanzania∗Corresponding author: mgana77@yahoo.com

Tropocyclops simplex abundance, diel distribution and the impact of the planktivore fish Stolothrissatanganicae on the T. simplex population were studied at a deep-water site in Kigoma Bay, Lake Tanganyikaduring the wet period (October 2008–February 2009). The top 40 m were examined using discrete depthsamples, filtered through a 40-μm mesh. Ovigerous and copepodids of Tropodiaptomus simplex exhibiteda very clear diel vertical migration. Ovigerous females of Tropodiaptomus were negligibly low in the0–30 m depth during the day, while at night they stayed above 20 m. Tropodiaptomus nauplii did notshow any clear diel vertical migration. The contribution of adults, copepodite and nauplii stages to thetotal numbers of Tropodiaptomus simplex was 8.2, 8.6 and 83.2%, respectively. Stolothrissa tanganicaestomachs were analysed to quantify the contribution of zooplankton in their diets, which revealed adominance of Tropodiaptomus females and the electivity indices proved that T. simplex females (withor without eggs) were highly selected by S. tanganicae, which could probably be explained by size-selective feeding. Life table analysis indicated that approximately 60% of T. simplex nauplii did notdevelop into copepodites, a loss which could not be explained by the results of the present study. 68%of the fish gut contents were contributed by female Tropodiaptomus individuals and the contribution ofshrimps and Cyclopidae increased substantially to almost 20%. S. tanganicae consumed largely ovigerousTropodiaptomus females. However, eggs found in the guts were significantly smaller than those found inthe zooplankton discrete samples, suggesting that digestion had started. The calculated electivity indicesunderline that ovigerous and non ovigerous females of T. simplex are being positively selected in the feedingprocess. It infers existence of selective predation in L. Tanganyika by S. tanganicae on females of T. simplex(with or without eggs) and as a result distinct vertical migration as a predation escape mechanism isperformed.

Keywords: zooplankton, vertical distribution, selective predation

Introduction

Lake Tanganyika is a permanently stratifiedmeromictic lake (Plisnier et al., 1999). It relies onwind driven mixing and internal regeneration of

nutrients for primary productivity (Coulter, 1991;Plisnier et al., 1999). The stratification of the lake,however, is very sensitive to small changes in tem-perature and wind stress (Lewis, 1996). Accordingto Coulter (1991), the zooplankton community is

14

Aquatic Ecosystem Health & Management, 17(1):14–24, 2014. Copyright C© 2014 AEHMS. ISSN: 1463-4988 print / 1539-4077 onlineDOI: 10.1080/14634988.2014.877334

Mgana et al. / Aquatic Ecosystem Health and Management 17 (2014) 14–24 15

dominated by protozoans, crustaceans, coelenter-ates and fish larvae; however, each of these taxa isrepresented by relatively few species.

In Lake Tanganyika, the pelagic crustaceancommunity dominates the zooplankton in numbersand biomass (Coulter, 1991). The most commonshrimp species are: Palaemon moorei (Calman),Limnocaridina parvula (Calman) and Limnocarid-ina tanganyicae (Calman), with the first two beingmore pelagic (Mathes, 1967; Kurki et al., 1999b).The copepod portion of the community consistsof one calanoid species Tropodiaptomus simplex(Sars), and three cyclopoid species, Mesocyclopsaequatorialis (Kiefer), Tropocyclops tenellus (Sars)and Microcyclops cunningtoni (Sars) (Coulter,1991). Tropodiaptomus is the largest copepod inthe lake, for instance, Dumont (1986) establishedthe size range of an adult Calanoida to be be-tween 800-1100 μm. The lengths of adult femaleMesocyclops and Tropocyclops are about 1000 and380–445 μm, respectively, with the latter beingthe smallest cyclopoid species in the lake (Kiefer,1956). Cladocerans are reported to be absent due topredation (Harding, 1957). Copepods are an impor-tant link between pelagic primary production andplanktivorous fish, i.e. mainly two clupeid species,Stolothrissa tanganicae (Regan) and Limnothrissamiodon (Boulenger), and juveniles of centropomidLates stappersii (Boulenger) (Kurki et al., 1999a).The commercial fishery in Lake Tanganyika isessentially based on these species: clupeid speciesca. 65% by weight and L. stappersii ca. 30%by weight. Catches in the northern and southernhalves are dominated by clupeid-based and “perch”based fisheries respectively; while the central part(Kigoma) lands both species (Molsa et al., 1999;Mannini et al., 1999; Plisnier et al., 2009). Thissectional dominance is highly linked to zooplanktonand phytoplankton abundance and dynamics in thelake (Kurki et al., 1999b; Mannini et al., 1999;Descy et al., 2005, 2010).

S. tanganicae juveniles feed mostly on phyto-plankton (e.g. diatoms-Nitzschia and Navicula) inthe inshore waters and on zooplankton (especiallythe calanoid Tropodiaptomus simplex and selec-tively on the shrimp Limnocaridina spp.) as theymove into the pelagic habitat at about 50 mm(standard length, SL) (Marlier, 1957; Chene, 1975;Coulter, 1991). The endemic sardines in the LakeTanganyika depend on grazing copepod as a foodsource. However, based on characteristics of thegill–rakers and heterogeneity in the composition ofcopepods between stomach samples, Chene (1975)

concluded that feeding is unselective. Feeding in thisspecies occurs at dusk and dawn following the dielvertical movement of zooplankton (Coulter, 1991).It stays in the depth range of 8–60 m, forming largeschools at night at 8–15 m while scattering at 60 mduring the day (Coulter, 1991; Plisnier et al., 2009).S. tanganicae reaches a maximum size of 100 mmSL, while longevity is estimated at 1.5 years (Man-nini et al., 1999).

Previous zooplankton studies have emphasizedcrustacean zooplankton because of their importanceto pelagic fishes. The information on zooplanktonspatial distribution and selective feeding by plank-tivorous fishes is patchy. For example, calanoidcopepods are regarded as the most important fooditems in the diet of Stolothrissa due to their largesizes (Coulter, 1991). However, sampling methodsused in previous studies might have led to an un-derestimation of smaller groups and developmen-tal stages of zooplankton since large mesh-size nets(≈100 μm) were used (Kurki et al., 1999a). Specifi-cally this study aimed at determining the abundanceand life history of T. simplex, and examining theimpact of S. tanganicae on T. simplex.

Materials and Methods

Study site

The study was conducted at a site located04◦51.26′S and 29◦35.54′E within the KigomaBay of Lake Tanganyika. This site is identical tothe pelagic station sampled during the Food andAgriculture Organization of the United Nations(FAO)/Finnish International Development AgencyLake Tanganyika Research project (FINNIDA LTR)(Kurki et al., 1999a; Molsa et al., 1999). Themaximum depth at this site is about 500 m. Thethermocline in this area oscillates between 70and 60 m during September–early November andNovember–January (Chitamweba, 1999).

Zooplankton samples

Zooplankton samples were collected on a bi-weekly basis, in the pelagic zone at 0900 and2100 hours between October 2008 and February2009. A single sample was collected using a 7.4-litrewater sampler (Limnos Ltd., Finland) at 0, 5, 10, 15,20, 25, 30 and 40 metres. Water samples were fil-tered through 40 μm plankton net. The zooplanktonsamples were preserved with a 4% formalin solu-tion in the field and taken back to the laboratory

16 Mgana et al. / Aquatic Ecosystem Health and Management 17 (2014) 14–24

for microscopic analysis. The day and night dis-crete samplings (i.e. experiments) were not doneat the same day’s cycle in order to allow enoughtime to process live samples. Zooplankton identi-fication, examination and counting were done us-ing a dissecting (Olympus model SD-ILK) and aninverted microscope (Leica DMIL) following Du-mont and Maas (1988) and Vuorinen (1993) proto-cols. Pelagic copepods were divided into two maingroups, calanoids and cyclopoids, the former beingrepresented by Tropodiaptomus simplex only whilethe three species of cyclopoids occurring in thepelagic zone were pooled, as they were not identifiedto species level (i.e. all species and all copepodids).

Population dynamics

Live material was also collected using 40 μmplankton net hauled at a speed of not more than0.5 ms−1 from 30 m to the surface. Material waskept alive in a 5 litre bottle and in the laboratoryadult males and females with ripe ovaries of T. sim-plex were sorted out and cultured at room tempera-ture to get information on embryonic developmenttime. For the laboratory experiments, 6 males and6 females were placed in a Petri dish each contain-ing 15 ml of lake water to increase the chances ofmating and producing eggs. 10 ml of lake water (fil-tered through 40 μm) were renewed every morningand evenings in order to provide the animals withclean freshwater. Five drops of water collected froma nearby pond (Katosho), which contained high al-gal material, were added to each Petri dish/treatmentwhenever the water was renewed in the treatments.This provided a source of food to ripe females andmales. Water temperature in the Petri dishes wasrecorded every 3 h during the experiment, and atthe same time the developmental stages of the eggswere checked. Although the experiments were per-formed at room temperature, the temperature vari-ation was in an acceptable range (SD ± 1.2◦C).Females with newly laid eggs were picked out witha pipette and immersed in a separate Petri dish withthe same conditions as mentioned above. Initial timewas recorded; embryonic development was moni-tored every 3 h until nauplii were seen coming outof egg shells or moving in the water column; whichmarked the end of the experiment.

For the calculation of population dynamics thenumbers of Tropodiaptomus eggs and adults derivedfrom the day samples were corrected according tothe results from the night samples. From the rela-

tionship, day results to night results a correctionfactor was calculated and applied to the day results.

For the calculation of population dynamics thedevelopment times of the various stages are needed.The embryonic development time (0.96 days) is de-rived from our own experiments (24.0◦C, n = 9;24.4◦C, n = 11; 24.7◦C, n = 14). For the descrip-tion of the relationship between temperature anddevelopment time, the results of Hyvonen (1997)(mean experimental temperature: 26◦C) were usedsince our own result covered a narrow tempera-ture range (24.0◦C–24.7◦C). A quadratic model wasused for the description of this relationship (Herzig,1983) and the following regression equation was de-rived which is valid for temperatures ranging from22–28◦C:

ln De = 1044 − 638 (lnT◦C)

+ 97 (lnT◦C)2r2 = 0.88

Where: De = embryonic development time (days)and T = temperature (◦C). The naupliar and copopo-did development times were adopted from Hyvonen(1997) for further calculations.

A graphical method (Southwood, 1978) was ap-plied to determine the total number of individualsthat pass through each instar in a generation. Succes-sive numbers of eggs, nauplii and copepodites wereplotted against time. The area under the curve wasdetermined and divided by the mean developmenttime. The methods assume that: there is constantmortality rate throughout a given instar; mortalityduring the moulting from instar n to n+1 cannotbe distinguished from mortality during instar n+1and is attributed to the latter. Population dynamicanalyses of T. simplex were done using the AbacusConcepts in Stat View (Abacus Corporation).

Stolothrissa tanganicae gut analysis

S. tanganicae were caught using a lift net (8 mmmesh size) in the artisanal fishery by lowering it to≈70 m water depth. A total of 137 freshly caughtfish, with sizes ranging between 47 and 92 mm,were immediately injected with 4% formalin in theirbody cavity in order to arrest digestion of prey. Fishlength and weight were measured. Fish guts weredissected, and food items identified and quantified.Eggs found in the guts together with other fooditems were counted and their diameter measured.Identification was done by looking at the undigestedhard fragments such as furca of small crustaceans

Mgana et al. / Aquatic Ecosystem Health and Management 17 (2014) 14–24 17

(Chene, 1975) and other appendages like pereopod5 (P5).

Selectivity assessment

Plankton obtained during day and night (assum-ing that whenever they encounter prey item theycapture it) together with gut counts were used forcalculating an electivity index though it is believedthat Stolothrissa feed mostly at night. Prey selec-tivity by fish was calculated using Ivlev’s electivityindex modified by Jacobs (1974):

Di = ri − pi

ri + pi+2ri pi

where;Di is electivity index for prey item i, pi is the

proportion of prey item i in the lake and ri is theproportion of that same prey item in the guts. Valuesof Di range from −1 to +1; 0 indicates randomfeeding while Di > 0 or Di < 0 indicates positive ornegative selection by the fish, respectively.

Results

Zooplankton vertical distribution(day-night)

Tropodiaptomus simplex copepodid stagesshowed clear vertical migration during the wholesampling period. The night time distribution of fe-males, males and copepodids of T. simplex weresignificantly different (Kruskal-Wallis test, X2 =9.86, d.f. = 2, p < 0.05). They were dominant inthe 0–20 m water layer at night and 30–40 m duringthe day (Figure 1). Similarly, ovigerous T. simplexshowed a pronounced vertical migration. They werebelow 40 m during the day and abundant above 20 mat night especially on 26 and 27 January 2009, re-spectively (Figure 2).

Phenology

When all day and night data were pooled, twopeaks of total zooplankton were observed (Fig-ure 3), one in early November with 105,000 indi-viduals m−3 and a second one in early January, with112,500 individuals m−3. Total nauplii followed thesame trend with mean density values of 75,000 and70,000 individuals m−3, respectively. Total naupliicontributed about 69% (range: 60.4 to 77.3%) to the

total number of counted zooplankton in the studyperiod.

T. simplex population showed only one peak inearly November reaching 37,500 individuals m−3

which was followed by a sharp decrease to about15,000 individuals m−3; while in the remaining pe-riod the numbers varied between 10,000 and 15,000individuals m−3 (Figure 4). The peak was signifi-cantly contributed by nauplii (96%), while copepo-did stages and adults constituted the remaining 4%.The relative contribution of adults, copepodites, andnauplii to the total number of counted Tropodiap-tomus was 8.2, 8.6 and 83.2%, respectively. Themean density of ovigerous Tropodiaptomus was 271individuals m−3 (range: 46–574 individuals m−3)while for total adults was (mean ± SD; 1253 ± 556adults m−3). The relative contribution of copepodidsTropodiaptomus to the total copepod zooplanktonwas relatively small (3.8%) throughout the studyperiod. Three pulses of eggs (early November andDecember, finally mid-January), nauplii and cope-podid stages were observed throughout the studyperiod which if you divide the sampling period(103 days) by the generation time of Tropodiap-tomus (Hyvonen, 1997) coincide with three gener-ations (Figure 4). The pulses are observable at eachdevelopmental stage.

Population dynamics ofTropodiaptomus simplex

Three pulses, most likely generations, of Tropo-diaptomus simplex were observed over the entire103-day period (Figure 5; Table 1). In the three

Table 1. Theoretical (calculated) individual members (m−3) ofT. simplex (E = eggs, N = nauplii and C = copepodid) passingthrough various developmental stages.

Day Date E N C

01–29 (27.10.−24.11.) 65,36512–38 (07.11.−03.12.) 65,99012–59 (07.11.−24.12.) 199129–59 (24.11.−24.12.) 60,85438–71 (03.12.−05.01.) 53,07959–80 (24.12.−14-01.) 116159–103 (24.12.−06.02.) 61,72971–103 (05.01.−06.02.) 53,91080–103 (14.01.−06.02.) 694

Overall01–103 (27.10.−06.02) 187,948 172,979 3846

18 Mgana et al. / Aquatic Ecosystem Health and Management 17 (2014) 14–24

26 / 01/2009 27 / 01/2009

Percentage (%)

0 10 20 30 40 50 60

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0 5 10 15 20 25 30

Tropo. F

Tropo. F

Tropo. M

Tropo. M

Tropo. Cop

Tropo. Cop

Figure 1. Vertical distribution of post-nauplii stages of T. simplex (relative abundance) at day (open bars, total individuals = 42) andnight (black bars, total individuals = 139), Tropo. F = female T. simplex, Tropo. M = male T. simplex and Tropo. Cop = CopepodidT. simplex.

periods number passing through the naupliar stagedid not differ from the number passing throughthe embryonic stage. However, a significant differ-ence became obvious between the numbers passingthrough the copepodite stage. If one compares the

total theoretical input to the adult stage (i.e. num-ber passing through the copepodite stage) with themean (±SD) standing stock of adults (1253 ± 556individuals m−3), losses of about 60% might haveoccurred in the adult stage.

Mgana et al. / Aquatic Ecosystem Health and Management 17 (2014) 14–24 19

10/12/2008

Percentage (%)

020406080100

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0 20 40 60 80

Figure 2. Vertical distribution of ovigereous T. simplex (relative abundance) at day (open bars; total individuals on 10 December2008, 14 January 2009 and 26 January 2009 were 6, 1 and 0, respectively) and night (black bars; total individuals on 3 December2008, 5 January 2009 and 27 January 2009 were 8, 40 and 13, respectively).

Trophic relationship between Stolothrissatanganicae and Tropodiaptomus simplex

A total of 167 Stolothrissa tanganicae individu-als were analysed for gut content, 46% of which

contained food items while the remaining 54%were empty. The specimens examined had totallengths ranging from 47 mm to 92 mm. S. tangan-icae guts contained mainly female Tropodiaptomus(up to >90% of the total gut content) individuals

20 Mgana et al. / Aquatic Ecosystem Health and Management 17 (2014) 14–24

Figure 3. Abundance of total zooplankton and nauplii with time.

with some Cyclopidae (mainly adults) and malesof Tropodiaptomus constituting their diets in mid-November (Figure 6). During early December therewas reduced contribution to 68% of the total gutcontent of female Tropodiaptomus individuals. Thesame trend was also observed in the lake (Figure 5c)but shrimps and Cyclopidae increased substantiallyto almost 20% in the guts. In January Tropodiapto-mus females regained their dominance as the pre-vailing food items. Tropodiaptomus eggs were alsofound in the stomachs of S. tanganicae. The mean(±SD) egg sizes of Tropodiaptomus found in thefish guts had a relatively small diameter (119.84 ±6.19 μm) than those from the zooplankton discretesamples (126.89 ± 10.13 μm). The differences inegg sizes between the two samples were significant(t-test, t =−3.35, d.f. = 34, P < 0.05). The reductionin sizes of most eggs in the guts was probably causedby digestion. It follows therefore that a high num-ber of the consumed Tropodiaptomus females wereovigerous. The calculated electivity indices under-line the selective feeding of S. tanganicae (Figure 7).S. tanganicae showed a strong positive electivitytowards ovigerous and non-ovigerous Tropodiapto-mus females throughout the study period. Tropo-diaptomus males were also positively selected al-though not always. At the beginning of the studyperiod Tropodiaptomus copepodid were selected asfood items while in the following months no posi-tive electivity was observed. Lastly, at the beginningof this study, shrimps showed a negative electivity

Figure 4. Abundance of Tropodiaptomus simplex life stages withtime. A, C, N and Tropd. means Adult, Copepodid, Nauplii andtotal Tropodiaptomus, respectively.

Figure 5. Abundance of Tropodiaptomus simplex (a) eggs, (b)nauplius and (c) adult copepodids.

index; however, from December onwards their elec-tivity index became positive indicating that theywere selected by the fish as an important food com-ponent. No selection for Cyclopidae was observedand the electivity indices were negative throughoutthe investigation period.

Discussion

A clear strong vertical migration was displayedby ovigerous T. simplex during the whole study pe-riod. They concentrated in the lower layers (i.e.30–40 m) and occasionally staying at the 20–25 mwater layer during day time, while at night they cameup to the surface layers (5–15 m). Vuorinen et al.(1999) in their study observed that intensive visualpredation by planktivore fishes which suggested dis-tinct diel and ontogenetic vertical migration of zoo-plankton, egg-carrying females of Tropodiaptomus

Mgana et al. / Aquatic Ecosystem Health and Management 17 (2014) 14–24 21

0%

20%

40%

60%

80%

100%

Mid-Nov Early-Dec End-Dec Early-Jan

Dates

Perc

enta

ge (

% ) Tropd. Copp

Tropd. M

Shrimps

Cyclopidae

Tropd. F

2008 2009

Figure 6. Relative food composition (%) of Stolothrissa (based on number of items). Tropd. Copp = Tropodiaptomus copepodite,Tropd. M = Tropodiaptomus males and Tropd. F = Tropodiaptomus. (Color figure available online.)

have been found in deeper water than other adultstages. The observation in previous studies (Sand-strom, 1980; Narita, 1983; Mulimbwa, 1988, 1991;Flinkman et al., 1992 in Vuorinen et al., 1999) haveshown that zooplankton may migrate daily from80 m up to the surface during the season of max-imum stability because they are being selected bypredaceous clupeids which is in agreement with thisstudy. This work points out differences in the dis-tribution of females, males and copepodids in thesurface waters might be a good indication of the

importance of predation on the T. simplex popula-tion given the selectivity factors measured and thenight time distribution of fish (8–15 m). Females,males and copepodites of T. simplex revealed thesame trend of vertical migration. This behaviourcould also reflect the effect of light intensity onthe vertical and horizontal migration of zooplank-ton, such that recently photosynthesizing phyto-plankton (and higher oxygen and/or temperatures)is likely drawing T. simplex to the surface layers.This is also in accord with post naupliar distribution

Figure 7. Prey selectivity by Stolothrissa tanganicae, Tropd.F = Female Tropodiaptomus, Tropd.F+E = Ovigerous Tropodiaptomus,Tropd.M = Male Tropodiaptomus, and Tropd.Copp = Copepodite Tropodiaptomus. (Color figure available online.)

22 Mgana et al. / Aquatic Ecosystem Health and Management 17 (2014) 14–24

pattern observed during this study. It is plausiblethat T. simplex is vulnerable to predation. As a re-sult, this species has developed night feeding strat-egy by performing a clear vertical migration whichin turn reduces predation pressure from clupeids(Frost, 1988; Dawidowicz et al., 1990 in Lo et al.,2004) and photo damage (Haney, 1988 in Lo et al.,2004).

The present study recorded a 3.8% contributionof copepodids to the total number of observed zoo-plankton which was lower than in previous stud-ies (e.g. Kigoma (17%); Kurki et al., 1999a). Theaverage high abundance of ovigerous Tropodiapto-mus (271 ind. m−3) in this study compared to theliterature was due to the fact that day and nightsamples were pooled to get an idea of their “real”abundance considering the pronounced vertical mi-gration. Other studies reported lower values; forexample, Kurki et al. (1999a) recorded 3–36 ind.m−3 and 2–138 ind. m−3 during the wet season in1993–1994 and 1994–1995, respectively. The dif-ference can be attributed to sampling methods; inthe present study night samples were also used inestimating the calanoid abundance as day samplingsto a depth of 40 m would most likely underestimatethe calanoid abundance. It is worth noting that in theprevious studies quoted herein sampling was doneat depths up to 100 m.

Tropodiaptomus pulses observed from field dataduring this study are in agreement with the pro-posed Tropodiaptomus generation time by Hyvonen(1997). The results of the graphical method appliedfor the determination of total numbers of individ-uals passing through each instar are in agreementwith the field observation that there is a big gapbetween numbers of nauplii and copepodites in theT. simplex population development. Predation on thecopepodite stage could be the reason for this pat-tern. However, S. tanganicae gut contents and elec-tivity indices did not show a strong positive selec-tion towards copepodides although they were alwaysfound in the guts. The difference between calanoidsand cyclopoids as food for clupeids is significant:a calanoids nauplius is comparable with a smallcycloploid adult in biomass (Kurki et al., 1999a).However, natural mortality and food scarcity at thecopepodite stage of T. simplex, in their natural envi-ronment, could have caused the observed pattern.

It was established earlier that S. tanganicae feedspredominantly on calanoid copepods and only spo-radically on Limnocaridina spp. which is a muchlarger prey item (Chene, 1975). The results of the

present study in general correspond with Chene’s(1975) findings, but differ in that the food item‘Tropodiaptomus’ was split into various develop-mental stages. Adult females of Tropodiaptomus,especially ovigerous females, were found to be themost dominant food items during the whole period.Generally, T. simplex is the largest component of thecopepod community in Lake Tanganyika (Vuorinenet al., 1999). Increasing selectivity by a vertebratepredator with increasing prey size has been demon-strated and is predicted by optimal foraging theory,which assumes that a forager maximizes net energyintake per unit time (Werner and Hall, 1974; Bohl,1982; Pyke, 1984; Lazaro, 1987; Walton et al., 1992in Liu and Herzig, 1996). Copepodids of Cyclopi-dae were the next dominant food item after femaleT. simplex in the Stolothrissa guts. This was prob-ably due to the fact that adult female Mesocyclopsaequatorialis were larger than adult male T. sim-plex, which was the least dominant prey item in theStolothrissa guts.

Shrimps were occasionally found in the guts ofStolothrissa (Chene, 1975). This is probably due totheir larger size compared to other crustacean zoo-plankton found in the lake. Electivity indices do notshow positive selection probably due to the fact thatthe biomass of shrimp taken is a significant pro-portion compared to other prey items. The smallestshrimp size found during the study was the same asan ovigerous Tropodiaptomus; hence one may as-sume that they are also suffering heavy predationby S. tanganicae although they were not as abun-dant as other zooplankters in the environment or inS. tanganicae guts. An interesting trend was alsomade during this study: when adults of T. simplexwere much less abundant in the lake, they were alsoless abundant in the diet, even though they werepositively selected.

Electivity indices confirmed that ovigerous andnon-ovigerous females of T. simplex were highly se-lected by Stolothrissa. Ovigerous T. simplex scoredhigh in the electivity indices followed by femaleTropodiaptomus. A lot of copepod eggs (mean ±SD = 382.88 ±114.36, n = 137) were observed inthe Stolothrissa guts; however, their sizes differedsignificantly from eggs measured from Tropodiap-tomus bodies. Individual T. simplex had eggs withlarger mean diameter values than those from the gutsof Stolothrissa. This suggests that probably diges-tion process could have caused the reduction in eggsizes in the guts of Stolothrissa by almost 7.05 μmdiameter. The observation that S. tanganicae had

Mgana et al. / Aquatic Ecosystem Health and Management 17 (2014) 14–24 23

T. simplex eggs in their guts may suggest selectivepredation of ovigerous T. simplex by the species. Itfurther points into the direction that some ovigerousT. simplex were consumed as selectivity data in thisstudy indicated an existence of positive selection ofovigerous T. simplex.

Zooplankton tends to exhibit maintenance swim-ming (Dussart and Defaye, 1995) while carrying adarkish coloured sack of 2–12 eggs (this study).Carrying a sack of eggs makes them a more vis-ible target than other zooplankton to the planktonfeeder. In a similar case, Liu and Herzig (1996)established that cyprinid Pelecus cultratus was se-lectively feeding on zooplankton according to theeye size of prey rather than its length resulting ina greater preference for large sized Diaphanosoma(with large eyes) than for Leptodora which havesmaller eyes. They concluded that prey visibilitydue to pigmentation may be an important factor de-termining selective feeding behaviour in visual feed-ing fish. S. tanganicae have been reported to havegood vision as indicated by large size of the opticallobes in their brain (Coulter, 1991). Additionally, inLake Victoria, Owili et al. (2003) established thatmaximum prey capture for maximum energy gainpossibly due to relatively high water clarity (Sec-chi depth 0.5–0.9 m) recorded at their study site, asopposed to those (0.3–0.4 m) recorded at differentsites (Sondu and Awach). The predation pressureon the cladocerans was enhanced since the visuallyfeeding fishes are likely to predate on relatively big-ger prey for maximum energy gain. Good vision, incombination with the highly transparent waters ofthe lake, is instrumental in prey selection wherebyfemales (with or without eggs) of T. simplex areselectively preyed upon by S. tanganicae.

Conclusions

In conclusion, the present study found large gapsin T. simplex population development between num-bers of nauplii and copepodites. This high mortalitymight be explained by starvation of the copepoditesand/or by fish predation. However, the latter mightbe underestimated on the basis of the current gutanalyses. Optimal temperature is a key factor in em-bryonic development; in the laboratory temperaturecondition is not stable as compared to the lake. Thismight affect the mating process and embryonic de-velopment which in turn affect the number of naupliiproduced and developing to the copepodid stage andfinally adults. This study also confirms that selective

predation exists in Lake Tanganyika especially byStolothrissa tanganicae on female T. simplex (withor without eggs), and that T. simplex performs apronounced diel vertical migration as a strategy toreduce predation pressure by S. tanganicae.

Acknowledgements

We are very thankful to the field and laboratorypersonnel at TAFIRI and Neusiedler See BiologicalStation, Austria for their assistance in the develop-ment of this article. We also wish to thank Dr. A. I.Kimirei for his constructive comments on early ver-sions of the article. Finally, we would like to expressour gratitude to the Austrian Government throughADC for funding this study.

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