Journal of Fish Biology (2014) 85, 1693–1706

doi:10.1111/jfb.12529, available online at wileyonlinelibrary.com

Trophic interactions between native and introduced fishspecies in a littoral fish community

M. Monroy*†, A. Maceda-Veiga‡, N. Caiola§ and A. De Sostoa*

*Department of Animal Biology and Biodiversity Research Institute (IRBio), University ofBarcelona, E-08028, Barcelona, Spain, ‡Cardiff School of Biosciences, Cardiff University,

Cardiff CF10 3AX, U.K. and §Aquatic Ecosystems, IRTA, Sant Carles de la Ràpita, Spain

(Received 4 March 2014, Accepted 6 August 2014)

The trophic interactions between 15 native and two introduced fish species, silverside Odontesthesbonariensis and rainbow trout Oncorhynchus mykiss, collected in a major fishery area at Lake Titicacawere explored by integrating traditional ecological knowledge and stable-isotope analyses (SIA). SIAsuggested the existence of six trophic groups in this fish community based on 𝛿13C and 𝛿15N signatures.This was supported by ecological evidence illustrating marked spatial segregation between groups, buta similar trophic level for most of the native groups. Based on Bayesian ellipse analyses, niche overlapappeared to occur between small O. bonariensis (<90 mm) and benthopelagic native species (31⋅6%),and between the native pelagic killifish Orestias ispi and large O. bonariensis (39%) or O. mykiss(19⋅7%). In addition, Bayesian mixing models suggested that O. ispi and epipelagic species are likelyto be the main prey items for the two introduced fish species. This study reveals a trophic link betweennative and introduced fish species, and demonstrates the utility of combining both SIA and traditionalecological knowledge to understand trophic relationships between fish species with similar feedinghabits.

© 2014 The Fisheries Society of the British Isles

Key words: endemic fish; food web; introduced species; Lake Titicaca stable isotopes.

INTRODUCTION

Freshwater ecosystems are experiencing a worldwide biodiversity crisis, which isparticularly evident in the conservation status of many fish species (Dudgeon et al.,2006; IUCN, 2008). Common threats to freshwater ichthyofauna are habitat degrada-tion, the introduction of exotic species and overfishing (Dudgeon et al., 2006; IUCN,2008). These anthropogenic interactions can lead to species extinction, and may alsocause profound changes in community structure, thus affecting ecosystem functioning(Duffy et al., 2007; Cucherousset et al., 2012). Traditional ecological approachessuch as observational studies of species behaviour and gut content analyses can detectchanges in species performance that could be associated with ecosystem functioning(Petchey & Gaston, 2006). Such approaches may fail to detect subtle modifications inthe structure of communities, however, especially when the species examined belongto a highly diverse fish community and most species appear to have relatively similar

†Author to whom correspondence should be addressed. Tel.: +34 934021041; email: monroylopezmario@gmail.com

1693

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1694 M . M O N ROY E T A L.

feeding behaviour (Vander Zanden & Vadeboncoeur, 2002; Quevedo et al., 2009).Consequently, inferring the food web structure of fish species exhibiting generalist andsimilar feeding behaviour in large water bodies such as lakes requires the integrationof information provided by traditional ecology and food web tracers (Vander Zandenet al., 1999; Post, 2002; Clarke et al., 2005).

Stable-isotope analyses (SIA) of C and N (𝛿13C and 𝛿15N) are food-web tracers thatare widely applied in studies of fish ecology in lakes (Vander Zanden et al., 1999;Post, 2002). The predictive enrichment in 𝛿15N throughout trophic levels (i.e. 3–4‰)enables the determination of fish trophic position, and the similarity in 𝛿13C betweendiet and consumer allows the determination of fish foraging habitat (Vander Zandenet al., 1997). SIAs may then guide management strategies by showing evidence of thedifferential effect of introduced species on the native fish community by trophic com-petition or predation. Indeed, the introduction of the piscivorous silverside Odontesthesbonariensis (Valenciennes 1835) and rainbow trout Oncorhynchus mykiss (Walbaum1792) into Lake Titicaca in the 1940s to enhance fisheries resources for local humancommunities typifies one of these situations. Over a period of 32 years (1979–2011),the biomass of native fish stocks has declined by 45% (Vila et al., 2007; Aliaga &Segura, 2013), and observational studies suggest that predation by introduced fishspecies is likely to be one of the main causative factors (Vaux et al., 1988; Loubens,1989; Vila et al., 2007). Trophic competition is, however, also likely to occur becausejuveniles of both native and introduced species feed on macroinvertebrates (Vauxet al., 1988).

This study aimed to explore the trophic interactions between native and introducedfish species in Lake Titicaca by integrating the information provided by traditional ecol-ogy and SIA. Previous studies addressing the native fish species’ diet by gut contentanalyses have suggested that these species show a relatively similar trophic spectrum.Therefore, all fish species should have a similar trophic level, limiting trophic compe-tition by spatial segregation. Finally, the differential effect on native fish species of thetwo introduced fish species in terms of predation and niche overlap was examined.

MATERIALS AND METHODS

S T U DY A R E A

Lake Titicaca is the largest freshwater lake in South America, covering a surface area of c.8559 km2. It is one of the highest lakes worldwide, at an elevation of 3810 m (Vila et al., 2007)(Fig. 1). It is divided into two large basins: Lago Mayor, which reaches a maximum of 285 m indepth, and Lago Menor, with a maximum depth of 40 m. Lake Titicaca is mainly an oligotrophiclake characterized by permanent hyperhaline cool water due to geographic features and lack ofstrong seasonality (Myers et al., 2000; Vila et al., 2007). Littoral areas have the characteris-tic macrophyte carpet dominated by reeds of totora Schoenoplectus californicus ssp. tatora andjuncus Juncus articus ssp. andicola, as well other macrophyte species of the genus Chara, Pota-mogeton, Myriophyllum, Nitella and Ruppia (Lauzanne, 1991; Vila et al., 2007). Although themacrophyte carpet only covers 20% of the total lake surface, it acts as an important feedingand breeding area for the majority of native species in the lake (Lauzanne, 1991). The nativefish community in Lake Titicaca has a high degree of endemicity and is dominated by membersof the killifish genus Orestias (Teleostei, Cyprinodontidae) and the benthic catfish Trichomyc-terus rivulatus Valenciennes 1846 (I. Doadrio, pers. comm.). A variety of anthropogenic eventssuch as the introduction of alien species, overfishing and sewage discharges from urban areasand mining activities may have led many native species to be listed as vulnerable, critically

© 2014 The Fisheries Society of the British Isles, Journal of Fish Biology 2014, 85, 1693–1706

T RO P H I C I N T E R AC T I O N S I N L A K E T I T I C AC A F I S H E S 1695

16°18´

16°30´

70°00´ 68°45´ 80° 60° 40°

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0 10 20Km

PERU Copacabana

Lake TiticacaPuno

Juliaca L1L2L3L4

Lago Mayor

Lago MenorLa Paz

Huancané

Fig. 1. Map of the study area with the sampling locations (L1, Kapara; L2, Pusi, L3, Olojpata; L4, Kauta)examined in Lago Mayor in February 2010. Original map provided by National Aeronautics and SpaceAdministration (NASA).

endangered or extinct based on the International Union of Conservation of Nature (Vila et al.,2007; Van Damme et al., 2009; Monroy et al., 2014).

S A M P L I N G P RO C E D U R E

Fishes were collected in February 2010 by local fishermen using benthic nylon gillnets with amesh size of 10–30 mm in four open bays (Kapara, Pusi, Olojpata and Kauta) across 20 km ofLago Mayor (Fig. 1). The selection of this study area enabled exploration of the trophic inter-actions in an area of high fish diversity (17 of the 26 fish species reported), maximizing thechances of species interaction in an environment with little influence from other anthropogenicimpacts other than the presence of introduced fish species [Ministerio de la Producción de Perú& Agencia Española de Cooperación Internacional (AECID, 2008)]. As pollution may modifythe structure of fish communities, the good water quality of the area surveyed was corrobo-rated by determining standard water physico-chemistry variables as detailed in Table I. All fishspecies were commercial species captured in the littoral area of the four bays surveyed so thereis observational evidence of the possibility of species interactions. Fishes were identified tospecies level, measured (total length, LT, mm) and c. 3 cm2 of white muscle tissue was dissectedfrom the mid-dorsal region of each fish. As the taxonomic composition of some fish species’diet was known, only the digestive tracts of killifish Orestias gracilis Parenti 1984, Orestiastomcooni Parenti 1984, Orestias frontosus Cope 1876, Orestias incae Garman 1895, Orestiasuruni Tchernavin 1944 and Orestias farfani Parenti 1984 were fixed in 10% formaldehyde forgut content analyses (Table II).

Three samples of zooplankton and benthic invertebrates were collected at each location todetermine the isotopic composition of potential fish prey. Zooplankton samples were obtainedby pulling a 250 μm mesh net through a 1 m water column and benthic macroinvertebrates werecollected using dip nets. Prey samples were then classified in three categories: zooplankton,chironomids and amphipods. All samples were frozen at −80∘ C and transported frozen in dryice to the facilities at the University of Barcelona (Spain).

S A M P L E P R E PA R AT I O N A N D S TA B L E - I S OT O P E A NA LY S I S

Fish and prey samples were freeze-dried, ground to a powder and lipid-extracted withthree-baths of chloroform–methanol (2:1) (Logan et al., 2008). Samples were then homoge-nized and weighed to the nearest μg in ultra-clean tin capsules for 𝛿13C and 𝛿15N determinationat the Scientific-Technical Services of the University of Barcelona. Samples were analysed in a

© 2014 The Fisheries Society of the British Isles, Journal of Fish Biology 2014, 85, 1693–1706

1696 M . M O N ROY E T A L.

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© 2014 The Fisheries Society of the British Isles, Journal of Fish Biology 2014, 85, 1693–1706

T RO P H I C I N T E R AC T I O N S I N L A K E T I T I C AC A F I S H E S 1697

Table II. Characterization of the fish groups (FG) based on the information provided by tra-ditional ecological analyses through an extensive literature review. For the species whose gutcontent was examined, the percentage of individuals per species for each food category is shown

FG Species Habitat Diet Source

I Orestias gilsoni Epipelagic Zooplankton Lauzanne (1982); Parenti(1984)

I Orestias gracilis – Zooplankton (100%) Pers. obs.I Orestias

tomcooni– Zooplankton (100%) Pers. obs.

II Orestias ispi Pelagic Zooplankton, eggs Lauzanne (1982, 1991);Parenti (1984); Northcote(2000); Paca et al.(2002); Vila et al. (2007)

III Orestias mulleri Benthopelagic Zooplankton, amphipods,molluscs, insects, plantdetritus

Lauzanne (1982, 1991);Parenti (1984); Vauxet al. (1988); Loubens(1989); Vila et al. (2007)

III Orestiasfrontosus

– Zooplankton (85⋅71%),amphipods (71⋅42%)

Pers. obs.

III Orestias forgeti – Zooplankton Lauzanne (1982); Parenti(1984); Vila et al. (2007)

III Orestias agassii Benthic, pelagic,littoral

Amphipods, zooplankton,insects, plant detritus

Lauzanne (1982, 1991);Parenti (1984); Loubens(1989); Paca et al.(2002); Vila et al. (2007);Maldonado et al. (2009)

III Orestias albus Benthopelagic Molluscs, fish, zooplankton Lauzanne (1982); Parenti(1984); Vila et al. (2007)

III Orestias luteus Benthic Amphipods, molluscs,insects, algae, eggs, fish

Lauzanne (1982, 1991);Parenti (1984); Northcote(2000); Vila et al. (2007);Maldonado et al. (2009)

III Orestiascrawfordi

Benthopelagic Amphipods, molluscs,zooplankton

Lauzanne (1982, 1991);Parenti (1984); Vila et al.(2007)

III Trichomycterusrivulatus

Benthic Amphipods, molluscs,insects, eggs, algae, plantdetritus

Paca et al. (2002); Vilaet al., 2007

III Orestias incae Benthopelagic Molluscs (100%), insects(33⋅33%)

Parenti (1984); pers. obs.

IV Orestias uruni Littoral Insects (100%), molluscs(83⋅33%), amphipods(66⋅66%), zooplankton(50%)

Parenti (1984); pers. obs.

IV Orestias farfani – Insects (100%), amphipods(88⋅88%)

Pers. obs.

V Odontesthesbonariensis

Pelagic, littoral Amphipods, zooplankton,insects, fish

Vaux et al. (1988); Vilaet al. (2007)

VI Oncorhynchusmykiss

Pelagic, littoral Amphipods, zooplankton,insects, fish, frogs

Loubens (1989); Vila et al.(2007)

© 2014 The Fisheries Society of the British Isles, Journal of Fish Biology 2014, 85, 1693–1706

1698 M . M O N ROY E T A L.

Thermo-Finnigan Flash 1112 (CE Elantech; www.ceelantech.com) elemental analyser coupledto a Delta-C isotope ratio mass spectrometer via a CONFLO III interface (Thermo FinniganMAT; www.chemeurope.com). International Atomic Energy authority (IAEA) reference mate-rials were applied and inserted every 12 samples to calibrate the system. Stable-isotope ratios of13C:12C and 15N:14N were expressed as the relative difference per mil (‰), using the equation:𝛿X = [RSample × (RStandard)−1 − 1] 1000, where 𝛿X is 𝛿15N or 𝛿13C, R the corresponding ratio13C:12C or 15N:14N and RStandard is the ratio of the international reference Vienna Pee DeeBelemnite for carbon and air for nitrogen. Replicate assays of standards indicated analyticalmeasurement errors of ±0⋅1 and ±0⋅3‰ for 𝛿13C and 𝛿15N, respectively.

DATA A NA LY S E S

Defining fish groups by integrating the information provided by traditional ecologyand stable-isotopic signatures

Homogeneous fish groups based on the information provided by SIAs and traditional ecologywere created in order to facilitate the interpretation of the graphical output and to guarantee theaccuracy of the analytical procedures used in determining niche overlap and prey–consumerrelationships. First, a 𝛿13C–𝛿15N bi-plot space was performed to group fish based on the simi-larities in their isotopic signatures, as fishes showing similar isotopic signatures are more likelyto have similar ecological attributes. Secondly, the groups created by the isotopic space wererefined using the ecological information available for these species gathered via an extensiveliterature review (Lauzanne, 1982, 1991; Parenti, 1984; Vaux et al., 1988; Loubens, 1989; North-cote, 2000; Paca et al., 2002; Vila et al., 2007; Maldonado et al., 2009) (Table II). At this stage,results from SIAs were visually assessed with the previous ecological knowledge about thesespecies. This comparison also enabled assignment of poorly studied species to the most similarfish group in terms of isotopic signatures matching their ecological attributes (Table II). For thosespecies whose gut contents were analysed, the dominant prey items in the gut were determined.After integrating this information, fishes were classified into groups as follows: (I) epipelagicand strictly zooplanktophagous species, (II) pelagic and mainly zooplanktophagous species,(III) benthopelagic and mainly omnivorous species and (IV) littoral species that mainly feed oninsects and amphipods. Introduced fish species were independently sorted into two groups cor-responding to O. bonariensis (V) and O. mykiss (VI). The least abundant species (n< 5) wereremoved from trophic analyses to increase the confidence of the results, although the relevantdata are shown in the 𝛿13C–𝛿15N bi-plot space (Fig. 2).

Exploring trophic interactions using SIAUsing non-parametric statistics as the data lacked normality and homogeneity of variances

according to the Shapiro and Barlett tests, the mean 𝛿13C and 𝛿15N signatures of the six fishgroups were compared using the Kruskal–Wallis (KW) test followed by Wilcoxon’s test forpair-wise comparisons. Ontogenetic changes in the use of habitat and the trophic level wereexplored using the Spearman rank correlation between LT and 𝛿13C and 𝛿15N, respectively. Iso-topic niche overlap between fish groups was assessed using standard ellipse areas (SEA) (siberfunction in R; www.r-project.org) (Jackson et al., 2011). Briefly, ellipses reflect the covarianceof the 𝛿13C and 𝛿15N signatures, and are defined by their shape and size, with the mean of theisotopic signatures determining their location in the bi-plot isotopic space (Jackson et al., 2011).Unlike the Convex Hull method (Layman et al., 2007), SEA is recommended when there is nobalanced design (i.e. sample sizes vary between fish groups) and sample sizes are small (<50samples) (Jackson et al., 2011; Syväranta et al., 2013).

The relative contribution of the four native fish groups and invertebrate sources in the diet ofthe two introduced fish species was explored using Bayesian mixing models (siar function inR) (Parnell et al., 2008). The Bayesian approach incorporates more sources of variability andestimates various prey items ingested by a consumer, thus offering a more reliable picture of itsdiet (Jackson et al., 2011). This modelling approach, however, is very sensitive to differencesin the trophic discrimination factors exhibited by consumers (McCutchan et al., 2003; Bond& Diamonds, 2011). As there was no previous study addressing the discrimination factor of

© 2014 The Fisheries Society of the British Isles, Journal of Fish Biology 2014, 85, 1693–1706

T RO P H I C I N T E R AC T I O N S I N L A K E T I T I C AC A F I S H E S 1699

–21

2

0

6

4

10

8

14

12

–20 –19 –18

chi

zoo

amp

uru

far

IVlinc

riv

aga

III

for

fromultom

gil

gra

isp

II

I

cralutalb

–17�13C

�15 N

–16 –15 –14

Fig. 2. Mean 𝛿15N and 𝛿13C values of 15 native fish species from Lake Titicaca, the two introduced species andpotential invertebrates used as food by these fish species. Native fish species were classified into groupsto facilitate the interpretation of the analyses: (I) composed of Orestias gracilis (gra), Orestias tomcooni(tom) and Orestias gilsoni (gil); (II) containing Orestias ispi (isp); (III) Orestias frontosus (fro), Orestiasmulleri (mul), Orestias forgeti (for), Orestias luteus (lut), Orestias crawfordi (cra), Orestias incae (inc),Trichomycterus rivulatus (riv); (IV) composed of Orestias uruni (uru) and Orestias farfani (far). repre-sent food sources such as chironomids (chi), amphipods (amp) and zooplankton (zoo). Note that large andsmall (<90 mm total length) individuals of Odontesthes bonariensis (group V) are shown ( , ), and thatOncorhynchus mykiss (group VI) is shown ( ). Error bars are s.d.

O. bonariensis, the accuracy of results was increased assuming the discrimination factors of2⋅33 for 𝛥𝛿15N and 2⋅22 for 𝛥𝛿13C observed in O. mykiss for the mixing model (Pinnegar &Polunin, 1999). The different trophic behaviour of large and small specimens of O. bonarien-sis was studied with individuals >90 mm likely to be mainly piscivorous (Fig. 2). The dietarypreferences for O. mykiss were only explored in large specimens because collected individualswere all adults of >149 mm. All statistical analyses were performed in R 2.15 (R DevelopmentCore Team; www.r-project.org) and the libraries MASS (Venables & Ripley, 1999) and SIAR(Parnell et al., 2008) with default settings. The level of significance was 𝛼 = 0⋅05.

RESULTS

F I S H G RO U P S D E F I N E D B Y T R A D I T I O NA L E C O L O G I C A LK N OW L E D G E A N D S TA B L E - I S OT O P I C S I G NAT U R E S

A total of 150 individuals belonging to 17 fish species were captured in the littoralarea of Lake Titicaca with the aim of addressing the food web structure of the littoralfish community. Based on the information provided by traditional ecological stud-ies and the species location within the 𝛿13C–𝛿15N bi-plot space, the 17 fish speciescaptured were sorted into six groups: the epipelagic planktophagous group (I) com-posed of O. tomcooni, Orestias gilsoni Tchernavin 1944 and O. gracilis; the pelagicand mainly zooplanktophagous group (II) containing Orestias ispi Lauzanne 1981; the

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1700 M . M O N ROY E T A L.

benthopelagic group composed of mainly omnivorous species (III) formed by O. incae,T . rivulatus, Orestias crawfordi Tchernavin 1944, Orestias luteus Valenciennes 1846,Orestias albus Valenciennes 1846, Orestias agassii (Valenciennes 1846), Orestias for-geti Lauzanne 1981, Orestias mulleri Valenciennes 1846 and O. frontosus; the littoralshallow-water group that mainly feed on insects and amphipods (IV) composed of O.farfani and O. uruni and the introduced fish species, O. bonariensis and O. mykiss, ingroups V and VI, respectively (Fig. 2).

The designation of six fish groups was supported by the significant differencesobserved in the mean 𝛿13C (KW, 𝜒2 = 101⋅81, d.f. = 5, P< 0⋅001) and 𝛿15N signatures(KW, 𝜒2 = 62⋅28, d.f. = 5, P< 0⋅001). Specifically, the 𝛿15N values did not differsignificantly between most native groups, possibly because all such fish species havea similar trophic spectrum (e.g. amphipods and zooplankton) as demonstrated bygut content analyses. Introduced fish species showed the highest 𝛿15N values withevidence of ontogenetic dietary changes in O. bonariensis (Spearman’s 𝜌= 0⋅81,n= 26, P< 0⋅001). In particular, individuals <90 mm showed lower 𝛿15N values thanlarger individuals (W = 160, P< 0⋅001) (Fig. 2). For 𝛿13C, there were significantdifferences between all fish groups with the exception of groups containing the pelagicspecies (II) and the introduced O. mykiss (VI) (P > 0⋅05), possibly because variationin the foraging areas exists for most fish groups. Nonetheless, variation in 𝛿13C alsohad an ontogenetic component because there was a negative relationship between LTand 𝛿13C for O. ispi (Spearman’s 𝜌=−0⋅84, n= 8, P< 0⋅01), O. gracilis (Spearman’s𝜌=−0⋅76, n= 10, P< 0⋅01), O. gilsoni (Spearman’s 𝜌=−0⋅66, n= 10, P< 0⋅05)and O. bonariensis (Spearman’s 𝜌=−0⋅39, n= 26, P< 0⋅05). For O. bonariensis, theincrease in size was also related to an increase in 𝛿15N (Table III).

E X P L O R I N G N I C H E OV E R L A P A N D P R E Y–C O N S U M E RR E L AT I O N S H I P S U S I N G S I A S

Bayesian ellipse analyses revealed that large O. bonariensis overlapped with pelagicfishes (group II) by 39% and that small O. bonariensis overlapped with benthopelagicfishes (group III) by 31⋅57%. These analyses also showed an overlap of 19⋅66%between O. mykiss and pelagic fishes (group II). The location of O. bonariensis and O.mykiss along the 𝛿13C axis demonstrated the existence of spatial segregation (Fig. 2).In this regard, large O. bonariensis 𝛿13C values were similar to those observed inpelagic (group II) and benthopelagic fishes (group III), while O. mykiss 𝛿13C valueswere more similar to those observed in pelagic fishes (group II) (P> 0⋅05 for allpair-wise comparisons). In order to determine the relative contribution of native fishgroups to the diet of the two introduced fish species, Bayesian mixing models wereapplied including four native fish groups and invertebrate items as food sources.The results revealed that pelagic fish groups are likely to dominate the diet of largespecimens of both introduced fish species, while chironomids appear to be the mainprey item for small O. bonariensis (Fig. 3).

DISCUSSION

This study examined the trophic interactions between 15 native fish species andtwo introduced fish species in a littoral area of Lake Titicaca illustrating that SIAs

© 2014 The Fisheries Society of the British Isles, Journal of Fish Biology 2014, 85, 1693–1706

T RO P H I C I N T E R AC T I O N S I N L A K E T I T I C AC A F I S H E S 1701

Table III. List of species and total number of fish captured (n) for each species in Lake Tit-caca, mean fish length (LT) and size range (minimum–maximum). The relationship betweenLT and 𝛿13C or 𝛿15N (mean± s.d.) is also indicated using the Spearman rank correlation

coefficient 𝜌*

Species nLT

(mm)LT range

(mm) 𝛿13C (‰) 𝜌 P-value 𝛿15N (‰) 𝜌 P-value

Orestias gilsoni 10 95⋅05 56–112 −20⋅07± 0⋅38 −0⋅66 <0⋅05 8⋅88± 1⋅31 −0⋅45 >0⋅05Orestias gracilis 10 52⋅07 47–57 −20⋅10± 0⋅52 −0⋅76 <0⋅05 9⋅01± 0⋅66 0⋅40 >0⋅05Orestias

tomcooni9 48⋅67 39–55 −19⋅72± 0⋅45 0⋅38 >0⋅05 8⋅51± 0⋅76 0⋅19 >0⋅05

Orestias ispi 8 83⋅80 80–90 −18⋅82± 0⋅71 −0⋅84 <0⋅01 10⋅24± 0⋅67 −0⋅55 >0⋅05Orestias mulleri 6 81⋅80 70–91 −18⋅35± 0⋅57 0⋅57 >0⋅05 8⋅12± 0⋅61 −0⋅28 >0⋅05Orestias

frontosus7 144⋅57 128–168 −18⋅42± 0⋅66 0⋅50 >0⋅05 8⋅43± 0⋅64 −0⋅32 >0⋅05

Orestias forgeti 4 62⋅60 60–65 −18⋅29± 0⋅27 0⋅20 >0⋅05 7⋅77± 0⋅23 0⋅00 >0⋅05Orestias agassii 7 134⋅27 112–163 −18⋅09± 1⋅08 −0⋅60 >0⋅05 8⋅8± 1⋅53 0⋅67 >0⋅05Orestias albus 10 125⋅40 84–167 −17⋅41± 0⋅84 −0⋅29 >0⋅05 8⋅48± 1⋅93 0⋅59 >0⋅05Orestias luteus 10 134⋅92 115–148 −17⋅30± 0⋅83 −0⋅21 >0⋅05 7⋅30± 0⋅81 −0⋅23 >0⋅05Orestias

crawfordi9 76⋅30 59–85 −16⋅81± 0⋅48 0⋅19 >0⋅05 7⋅40± 0⋅49 −0⋅15 >0⋅05

Trichomycterusrivulatus

8 134⋅12 65–159 −16⋅90± 0⋅88 −0⋅49 >0⋅05 7⋅94± 1⋅14 −0⋅01 >0⋅05

Orestias incae 3 52⋅33 48–60 −16⋅37± 0⋅17 −0⋅50 >0⋅05 6⋅64± 0⋅37 1⋅00 >0⋅05Orestias uruni 6 42⋅16 39–46 −15⋅88± 0⋅42 −0⋅60 >0⋅05 5⋅87± 0⋅42 0⋅75 >0⋅05Orestias farfani 9 135⋅66 95–162 −15⋅10± 0⋅48 −0⋅68 0⋅05 6⋅41± 0⋅52 −0⋅43 >0⋅05Odontesthes

bonariensis26 158⋅00 40–422 −17⋅33± 1⋅51 −0⋅39 <0⋅05 10⋅76± 0⋅86 0⋅81 <0⋅001

Oncorhynchusmykiss

8 200⋅00 149–268 −18⋅98± 0⋅48 −0⋅16 >0⋅05 10⋅40± 0⋅61 0⋅47 >0⋅05

*Note that this list includes two species removed from ellipse analyses due to the reduced sample size (n< 5).

and traditional ecological studies can give complementary information about studiesof food web structure (Vander Zanden & Vadeboncoeur, 2002; Clarke et al., 2005;Quevedo et al., 2009). Additionally, this is the first time that ecological attributes ofsix native species are explored to investigate the differential effect of the introductionof O. bonariensis and O. mykiss on the native fish community.

N I C H E PA RT I T I O N I N G O F T H E NAT I V E F I S H C O M M U N I T Y

As predicted, most of the native fish groups showed a similar trophic level withmarked spatial segregation, possibly associated with a decrease in trophic competition.The pelagic native O. ispi was the only native fish group showing higher 𝛿15N valuesthan the remaining native fish groups. This could be attributed to the occasional con-sumption of eggs by this fish species (Paca et al., 2002). An additional reason for thehigh 𝛿15N of O. ispi might be that the pelagic condition enables O. ispi to feed on preyitems from food webs differing in baseline levels of 𝛿15N, which may lead to a higher𝛿15N (Hobson, 1999; Vander Zanden & Rasmussen, 2001; Bearhop et al., 2004).Likewise, the large deviations observed in the 𝛿13C values of T . rivulatus, O. luteusand O. agassii could be associated with the consumption of prey from different feedinggrounds. The current findings may be related to the fact that different morphotypes have

© 2014 The Fisheries Society of the British Isles, Journal of Fish Biology 2014, 85, 1693–1706

1702 M . M O N ROY E T A L.

Zooplankton

Group I

Group II

Group II

I

Group IV

Zooplankton

Chironom

ids

Amphipods

0

0·2

0·4

Prop

ortio

n

0·6

0·8

1

0

0·2

0·4

0·6

0·8

1

0

0·2

0·4

0·6

0·8

1(a)

(b)

(c)

ChironomidsSource

Amphipods

Fig. 3. Density plot showing the confidence intervals (95, 75 and 50%) of the relative proportion of Lake Titicacafish groups consumed by (a) Oncorhynchus mykiss, (b) Odontesthes bonariensis and (c) O. bonariensis<90 mm in total length. Values were corrected by a discrimination factor of 2⋅33‰ for 𝛿15N and 2⋅22‰ for𝛿13C according to Pinnegar & Polunin (1999).

been described for these three species, being particularly evident for O. agassii, butsomewhat difficult to differentiate in all cases by gross morphological examination dueto hybridization (Parenti, 1984; Lauzanne, 1991; Maldonado et al., 2009). Nonetheless,certain trophic specialization appears to be common in native ichthyofauna because anexhaustive morphological study of the native species’ feeding structures (e.g. pharyn-geal dentition and gill raker number) appears to support the notion that Orestias speciescould prey on different zooplankton groups (Northcote, 2000). In turn, although detailsregarding the trophic specialization of the native fish species were not identified in

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this study, the variations in the isotopic signatures may be related to foraging special-izations already reported in fish fauna from other great lakes [e.g. cichlids from LakeMalawi or Lake Victoria, Fry et al. (1999); Balirwa et al. (2003), respectively].

Whilst all native species considered in this study were described as littoral specieswith the exception of O. ispi, the low 𝛿13C values observed in the epipelagic group (I)could be compatible with its pelagic condition. Although the reasons behind these low𝛿13C values remain largely unknown due to poor knowledge about the ecology of thesespecies, their pelagic condition appears to be supported by the similarities observedbetween the 𝛿13C values of these species and those of the pelagic O. ispi (Parenti,1984; Vaux et al., 1988; Villwock, 1998; Vila et al., 2007). An alternative explanationfor the low 𝛿13C values of this fish group is that all these fish species are strictly zoo-planktivorous, and zooplanktonic organisms often display high variation in 𝛿13C values(Cabana & Rasmussen, 1996). Therefore, the 𝛿13C values of species belonging to theepipelagic group may be driven by the isotopic signature of zooplankton even when allthree species inhabit the littoral area. This is consistent with the pelagic condition of O.ispi because this species has been observed foraging in the littoral area, so the relativecontribution of littoral and pelagic items to its diet may explain the similar isotopiccomposition of this group and littoral species (Villwock, 1998).

The isotopic composition of the native species was also related to ontogeneticchanges in the species’ use of foraging grounds. As the 𝛿13C gradient is associatedwith enrichment in 𝛿13C in littoral primary producers relative to offshore producers(France, 1995), fish species that move from littoral areas to offshore areas when theyage show a negative relationship between LT and 𝛿13C. The negative relationshipobserved between LT and 𝛿13C for O. ispi, O. gracilis, O. gilsoni and O. bonariensissupports the field observation suggesting that the littoral macrophyte carpet is cru-cial for the juvenile stages of fish species at Lake Titicaca, even for the introducedO. bonariensis.

T RO P H I C I N T E R AC T I O N S B E T W E E N NAT I V E A N DI N T RO D U C E D F I S H S P E C I E S

As previously reported (Vaux et al., 1988; Loubens, 1989), O. bonariensis and O.mykiss are the top fish predators at Lake Titicaca. As large specimens of both intro-duced fish species appear to prey mainly on native pelagic fish groups, O. bonariensisand O. mykiss appear to depict spatial segregation according to 𝛿13C, thus minimiz-ing trophic interactions. In addition, Bayesian ellipse analyses suggested that large O.bonariensis specimens show niche overlap with the pelagic native group II. This inter-action appears to be explained by a predator–prey relationship as suggested by themixing model analyses. Interestingly, the current findings suggested that O. bonarien-sis become mainly piscivorous when smaller than the 200 mm LT suggested by Vauxet al. (1988).

Unlike in O. bonariensis, the changes in the foraging behaviour of O. mykiss were notaddressed because all individuals captured were within the size range of piscivorousindividuals (149–268 mm) reported in previous studies and consistent with the 𝛿15Nvalues observed in this study. Lauzanne (1991) reported that O. mykiss mainly feeds onO. ispi and O. agassii at Lake Titicaca. A prey–consumer interaction, however, wouldbe represented by an increase in the 𝛿15N value of O. mykiss compared to that of thesetwo species. The current findings instead only demonstrated an overlap in the isotopic

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1704 M . M O N ROY E T A L.

signatures of O. mykiss and O. ispi. As stated above, this might be explained by thenature of the O. ispi diet coupled with the fact that the isotopic signature of both fishspecies may be more influenced by the pelagic than the littoral food chain (Quevedoet al., 2009).

In conclusion, this study reveals a trophic link between native and introduced speciesand demonstrates the utility of combining both SIA and traditional ecological knowl-edge to understand trophic relationships in fish communities with a pool of speciesexhibiting similar feeding habits.

The authors would like to thank the staff of the Binational Special Project of Lake Titicaca(PELT), the Sea of Peru Institute (IMARPE) and the Lake Titicaca Agency (ALT) for theirinvaluable field and laboratory assistance, especially O. Flores and E. Yujra. Funding supportwas provided by the Spanish Agency for International Cooperation and Development (AECID)and the Department of Production of Peru and by I. González, K. Su and J. Vera provided valu-able logistic support. The authors are also grateful to F. Ramírez, L. Jover, E. García-Galea,T. Militão, R. Ramos and M. García for useful suggestions on the isotopic analysis and datainterpretation, as well as to T. Evans from the Language Service at the University of Barcelonafor revising the English. A.M.-V. was funded by a Marie Curie Fellow (EC-Funded ‘Para-Tox’project no. 327941).

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© 2014 The Fisheries Society of the British Isles, Journal of Fish Biology 2014, 85, 1693–1706