Trophic structure of a coastal fish community determined with diet and stable isotope analyses

A combination of dietary guild analysis and nitrogen (δ¹⁵N) and carbon (δ¹³C) stable‐isotope analysis was used to assess the trophic structure of the fish community in Rhode Island and Block Island Sounds, an area off southern New England identified for offshore wind energy development. In the autumn of 2009, 2010 and 2011, stomach and tissue samples were taken from 20 fish and invertebrate species for analysis of diet composition and δ¹⁵N and δ¹³C signatures. The food chain in Rhode Island and Block Island Sounds comprises approximately four trophic levels within which the fish community is divided into distinct dietary guilds, including planktivores, benthivores, crustacivores and piscivores. Within these guilds, inter‐species isotopic and dietary overlap is high, suggesting that resource partitioning or competitive interactions play a major role in structuring the fish community. Carbon isotopes indicate that most fishes are supported by pelagic phytoplankton, although there is evidence that benthic production also plays a role, particularly for obligate benthivores such as skates Leucoraja spp. This type of analysis is useful for developing an ecosystem‐based approach to management, as it identifies species that act as direct links to basal resources as well as species groups that share trophic roles.     

Seasonal and ontogenetic shifts in the diet of Arctic charr Salvelinus alpinus in a subarctic lake

Seasonal and ontogenetic shifts in the diet of Arctic charr Salvelinus alpinus were studied in a deep, ultra‐oligotrophic lake in subarctic Finland from both stomach contents and the stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotope compositions of muscle and liver tissues. Both diet and isotope results indicated that the S. alpinus population relied mainly on littoral benthic energy sources. The strong littoral reliance appeared largely independent of season or fish size, although the data lacked small (total length, L_T, <130 mm) and young (<3 years) S. alpinus. Liver isotope values of intermediate‐sized S. alpinus (200–350 mm), however, suggested exploitation of the increase in the abundance of pelagic zooplankton in the late open‐water season. The results suggest that, in general, a strong littoral reliance of fishes can be a feature in subarctic lakes throughout the year. Due to its faster isotopic turnover rate and thus higher resolution for temporal diet changes, liver could be more commonly used in stable‐isotope studies of fish trophic niche shifts instead of using only the less responsive muscle tissue.     

Feeding ecology and basal food sources that sustain the Paradoxal frog Pseudis minuta: a multiple approach combining stomach content,prey availability,and stable isotopes

In the present study, we investigated ontogenetic diet shifts, feeding strategy, prey preferences, and basal food sources that sustain the Paradoxal frog (Pseudis minuta) based on stomach content, prey availability, and stable isotope (δ¹³C, δ¹⁵N) approaches. The feeding strategy analysis showed that the population can be considered a generalist species with each individual displaying a marked opportunism for different preys. Trophic positions estimated using nitrogen isotopic ratio (δ¹⁵N) revealed that tadpoles are primary consumers, but post-metamorphic individuals shifted to secondary and tertiary trophic levels as they increase in body size. A stable isotopic mixing model revealed that most of the carbon (0.61–0.72) sustaining the post-metamorphic P. minuta is derived from the aquatic rather than the adjacent terrestrial environment. This finding suggests that the post-metamorphic P. minuta is strongly dependent on carbon sources that primarily originate in aquatic systems, regardless of the terrestrial or aquatic origins of the arthropods in its diet. Our results indicated that this species is a generalist-opportunistic predator that derives most of their carbon sources from the aquatic environment where it shows preference for aquatic preys with higher individual biomasses.     

Non‐native species modify the isotopic structure of freshwater fish communities across the globe

Multiple anthropogenic pressures including the widespread introductions of non‐native species threaten biodiversity and ecosystem functioning notably by modifying the trophic structure of communities. Here, we provided a global evaluation of the impacts of non‐native species on the isotopic structure (δ¹³C and δ¹⁵N) of freshwater fish communities. We gathered the stable isotope values (n = 4030) of fish species in 496 fish communities in lentic (lakes, backwaters, reservoirs) and lotic (running waters such as streams, rivers) ecosystems throughout the world and quantified the isotopic structure of communities. Overall, we found that communities containing non‐native species had a different isotopic structure than communities without non‐native species. However, these differences varied between ecosystem types and the trophic positions of non‐native species. In lotic ecosystems, communities containing non‐native species had a larger total isotopic niche than communities without non‐native species. This was primarily driven by the addition of non‐native predators at the top of the food chain that increased δ¹⁵N range without modifying the isotopic niche size of native species. In lentic ecosystems, non‐native primary consumers increased δ¹⁵N range and this was likely driven by an increase of resource availability for species at higher trophic levels, increasing food chain length. The introduction of non‐native secondary consumers at the centre of the isotopic niche of recipient communities decreased the core isotopic niche size, the δ¹³C range of recipient communities and the total isotopic niche of coexisting native species. These results suggested a modified contribution of the basal resources consumed (e.g. multi‐chain omnivory) and an increase level of competition with native species. Our results notably imply that, by affecting the isotopic structure of freshwater fish communities at a global scale, non‐native species represent an important source of perturbations that should be accounted for when investigating macro‐ecological patterns of community structure and biotic interactions.     

Dietary ontogeny and niche shift to piscivory in lacustrine brown trout Salmo trutta revealed by stomach content and stable isotope analyses

The feeding ecology and ontogeny of a large size range of brown trout Salmo trutta in Lake Fyresvatnet, southern Norway, were examined by stomach content and stable isotope analyses. According to the stomach contents, the S. trutta changed their diet at c. 30 cm total length (L_T). The smaller size classes fed on benthic invertebrates and surface insects, whereas larger S. trutta (>30 cm) fed mainly on whitefish Coregonus lavaretus. A similar, but more gradual shift to piscivory in the size range 25–30 cm was found when using the stable isotope mixing model SIAR to reveal dietary ontogeny. The δ¹³C isotopic signature confirmed that S. trutta independent of size predominantly relied upon benthic energy sources, suggesting that the littoral zone was the primary foraging habitat for both invertebrate and piscivorous feeders. The δ¹⁵N values and trophic position increased with predator length, ranging from an average of 3·60 for small‐sized S. trutta (<15 cm) to 4·15 for large‐sized fish (>35 cm). The S. trutta exhibited a relatively slow growth rate during the predominant invertebrate feeding stages up to 7 years of age and 28 cm L_T, whereas fish above this size and age displayed a rapid growth rate of 9–11 cm year^?1, demonstrating the profitability of piscivorous feeding.     

Trophic niche segregation allows range‐extending coral reef fishes to co‐exist with temperate species under climate change

Changing climate is forcing many terrestrial and marine species to extend their ranges poleward to stay within the bounds of their thermal tolerances. However, when such species enter higher latitude ecosystems, they engage in novel interactions with local species, such as altered predator–prey dynamics and competition for food. Here, we evaluate the trophic overlap between range‐extending and local fish species along the east coast of temperate Australia, a hotspot for ocean warming and species range extensions. Stable isotope ratios (δ¹⁵N and δ¹³C) of muscle tissue and stomach content analysis were used to quantify overlap of trophic niche space between vagrant tropical and local temperate fish communities along a 730 km (6°) latitudinal gradient. Our study shows that in recipient temperate ecosystems, sympatric tropical and temperate species do not overlap significantly in their diet—even though they forage on broadly similar prey groups—and are therefore unlikely to compete for trophic niche space. The tropical and temperate species we studied, which are commonly found in shallow‐water coastal environments, exhibited moderately broad niche breadths and local‐scale dietary plasticity, indicating trophic generalism. We posit that because these species are generalists, they can co‐exist under current climate change, facilitating the existence of novel community structures.     

Questioning assumptions of trophic behavior in a broadly ranging marine predator guild

We evaluated whether existing assumptions regarding the trophic ecology of a poorly‐studied predator guild, northwest (NW) Atlantic skates (family: Rajidae), were supported across broad geographic scales. Four hypotheses were tested using carbon (δ¹³C) and nitrogen (δ¹⁵N) stable isotope values as a proxy for foraging behavior: 1) species exhibit ontogenetic shifts in habitat and thus display a shift in ¹³C with differential use of the continental shelf; 2) species exhibit ontogenetic prey shifts (i.e. from smaller to larger prey items) and become enriched in ¹⁵N; 3) individuals acquire energy from spatially confined local resource pools and exhibit limited displacement; and 4) species exhibit similarly sized and highly overlapping trophic niches. We found some evidence for ontogenetic shifts in habitat‐use (δ¹³C) for thorny and little skate and diet (δ¹⁵N) of thorny and winter skate and hypothesize that individuals exhibit gradual trophic niche transition, especially in δ¹⁵N space, rather than a clear and distinct shift in diet throughout ontogeny. Spatial isoscapes generated for little, thorny, and winter skate highlighted distinct spatial patterns in isotopic composition across the coastal shelf. For little and thorny skate, patterns mimicked expected spatial variability in the isotopic composition of phytoplankton/POM, suggesting limited displacement and utilization of spatially confined resource pools. Winter skate, however, exhibited a much narrower range of δ¹³C and δ¹⁵N values, suggesting individuals may use resources from a more confined latitudinal range. Although high total trophic niche overlap was observed between some species (e.g. little and thorny skate), sympatric species (e.g. little and winter skate) exhibited a degree of trophic niche separation. These findings offer new insight into the trophic dynamics of a poorly‐studied, vulnerable group of predators, and highlight a need to re‐examine assumptions pertaining to aspects of their ecology.     

Trophic behaviour of juvenile reef fishes inhabiting interlinked mangrove–seagrass habitats in offshore mangrove islets

Stable isotope (δ¹³C and δ¹⁵N) and gut content analyses were used to investigate size‐related feeding habits of four reef fishes (the beaugregory Stegastes leucostictus, the french grunt Haemulon flavolineatum, the schoolmaster snapper Lutjanus apodus and the yellowtail snapper Ocyurus chrysurus) inhabiting an offshore (non‐estuarine) mangrove islet off Belize, Central America. Comparisons of isotopic niche space and Schoener diet similarity index suggested a low to moderate degree of niche overlap between fish size groups. The δ¹³C gradient between mangrove and seagrass prey as well as results of Bayesian mixing models revealed that sampled fishes relied mostly on seagrass prey items. Only small and large juveniles of the carnivorous species L. apodus derived a part of their diet from mangroves by targeting mangrove‐associated Grapsidae crabs and fish prey, respectively. Isotopic niche shifts were particularly obvious for carnivorous fishes that ingested larger prey items (Xanthidae crabs and fishes) during their ontogeny. The utilization of mangrove food resources is less than expected and depends on the ecology and life history of the fish species considered. This research highlights that mangrove‐derived carbon contributed relatively little to the diets of four fish taxa from an offshore mangrove islet.     

Stable isotopes indicate that zebra mussels (Dreissena polymorpha) increase dependence of lake food webs on littoral energy sources

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Global test of Eltonian niche conservatism of nonnative freshwater fish species between their native and introduced ranges

Despite growing evidence that biotic interactions limit the distribution of species and their potential redistribution under climate change, the recent surge of interest in niche conservatism has predominantly focused on the Grinellian (abiotic) niche, whereas few studies have attempted to quantify potential lability in the Eltonian (biotic or trophic) niche. Here, we test for conservatism in the Eltonian niche of 32 freshwater fish species between their introduced and native ranges from 435 populations across the globe. We used stable isotope data to quantify niche shifts along the horizontal (δ¹³C: indicating the origin of the resources consumed) and vertical (δ¹⁵N: describing the trophic position) dimensions of the isotopic niche, as well as shifts in overall isotopic niche breadth. Using an assemblage centroid standardized isotope vector analysis and controlling for phylogenetic relatedness among species, we demonstrated that introduced freshwater fishes exhibited flexibility in both resource use and trophic position that was beyond levels of natural variability observed in their native ranges. By contrast, niche breadth showed variability only within the limits recorded in native populations and varied independently from shifts in mean isotopic niche positions. Across all species and introduction histories, we found a consistent shift towards more balanced acquisition of resources with mixed origins and at intermediate trophic positions, suggesting a general mechanism by which fish species successfully establish into recipient communities. The mechanisms that promote or inhibit species from shifting their Eltonian niche remains unknown, but trophic flexibility is likely to contribute to both the success and the ecological impacts of invasive species and range shifts of native species under future global change.     

Stable isotope analysis as a tool to monitor dietary trends in little penguins Eudyptula minor

Long‐term dietary monitoring of seabirds can be used to relate population fluctuations to at‐sea events. Stomach flushing is a conventional dietary monitoring technique, but has a number of disadvantages. Stable isotope analysis (SIA) is a less invasive method that provides unbiased dietary information over a longer period. We evaluated stable isotope analysis as a potential tool for monitoring long‐term little penguin Eudyptula minor diet. We determined diet composition during the chick feeding stage using stomach flushing and SIA at three separate colonies, using spatial variation in diet as a surrogate for potential temporal variation. Bayesian isotopic mixing models were generated for blood and feathers to evaluate their ability to discriminate broad‐scale (fish, squid, crustaceans) and fine‐scale (individual prey species) diet composition. Differences in stable carbon and nitrogen isotope ratios were found between colonies: broad‐scale isotopic mixing models predicted different proportional contributions of broad taxa (fish, cephalopod, crustacean) to diet than was indicated by stomach samples, reflecting the bias incurred by one‐off stomach contents analysis. Fine‐scale isotopic mixing models predicted proportional contributions of prey items with less certainty. Blood isotopic mixing models had narrower confidence intervals than models for feathers, but trends in δ¹⁵N for feathers mirrored those for blood. Our results suggest that relying on stomach contents analysis to detect shifts in prey consumption in little penguins could be very misleading, resulting in a less‐than‐complete idea of total prey consumption. SIA of little penguin tissues could be used to monitor dietary shifts across dissimilar taxa that may affect population numbers, but would fail to detect shifts between fish species.     

Feeding habits and trophic levels of bluefin tuna Thunnus thynnus of different size classes in the Mediterranean Sea

Possible changes in diet and trophic levels in relation to size of Mediterranean bluefin tuna, Thunnus thynnus, were investigated using labelled carbon (δ¹³C) and nitrogen (δ¹⁵N) stable isotopes. Samples were obtained from two locations in the southern Tyrrhenian Sea (Western Mediterranean Sea) in May and October 2004. The δ¹³C and δ¹⁵N analyses revealed at least three significant isotopic groups [small juveniles (0.7–2.2 kg), sub‐adults (15–50 kg) and adults (70 to 225 kg)]. δ¹³C was negatively dependent on weight, while δ¹⁵N was positively dependent on weight [TW = 8.2 (±0.16) + 0.03 (± 0.0) *δ¹⁵N (n = 49; r = 0.91; P < 0.001)]. Different prey contribution to the diet was highlighted for each class. The diet of juveniles comprised zooplankton, small pelagic fish and some coastal fish; sub‐adults relied on medium pelagic fish, shrimps and cephalopods, and adults relied mainly on cephalopods and larger fish. The trophic level (TL) of tunas belonging to each size class was closely correlated to weight, starting from ca 3.0 TL for Group I and reaching 4.4–4.8 TL for the giants. Bluefin tuna, from small juveniles to giants, showed a shift in feeding preferences due to different use of habitats and food items as a function of the life stage.     

Impacts of zebra mussels (Dreissena polymorpha) on isotopic niche size and niche overlap among fish species in a mesotrophic lake

Zebra mussels (Dreissena polymorpha) filter feed phytoplankton and reduce available pelagic energy, potentially driving fish to use littoral energy sources in lakes. However, changes in food webs and energy flow in complex fish communities after zebra mussel establishment are poorly known. We assessed impacts of zebra mussels on fish littoral carbon use, trophic position, isotopic niche size, and isotopic niche overlap among individual fish species using δ¹³C and δ¹⁵N data collected before (2014) and after (2019) zebra mussel establishment in Lake Ida, MN. Isotope data were collected from 11 fish species, and from zooplankton and littoral invertebrates to estimate baseline isotope values. Mixing models were used to convert fish δ¹³C and δ¹⁵N into estimates of littoral carbon and trophic position, respectively. We tested whether trophic position, littoral carbon use, isotopic niche size, and isotopic niche overlap changed from 2014 to 2019 for each fish species. We found few effects on fish trophic position, but 10 out of 11 fish species increased littoral carbon use after zebra mussel establishment, with mean littoral carbon increasing from 43% before to 67% after establishment. Average isotopic niche size of individual species increased significantly (2.1-fold) post zebra mussels, and pairwise-niche overlap between species increased significantly (1.2-fold). These results indicate zebra mussels increase littoral energy dependence in the fish community, resulting in larger individual isotopic niches and increased isotopic niche overlap. These effects may increase interspecific competition among fish species and could ultimately result in reduced abundance of species less able to utilize littoral energy sources.

     

Prey composition and ontogenetic shift in coastal populations of longnose gar Lepisosteus osseus

Longnose gar Lepisosteus osseus were collected from May 2012 to July 2013 in the Charleston Harbor and Winyah Bay estuaries (SC, U.S.A.). This study examined trends in stomach fullness, described major prey components and their importance in the diet of L. osseus, compared stomach content‐based trophic level estimates with the stable‐isotope‐based proxy: δ¹⁵N and tested for the occurrence of an ontogenetic diet shift using stomach content analysis and stable C and N isotopes (δ¹³C and δ¹⁵N). Dominant prey families were Clupeidae, Sciaenidae, Penaeidae, Fundulidae and Mugilidae, with the highest consumption rates in autumn. Trophic levels calculated using stomach contents did not correspond to δ¹⁵N (P > 0·05). Stomach contents and stable‐isotope signatures indicate ontogenetic prey composition shifts from low trophic level benthic prey (fundulids) to higher trophic level pelagic prey (clupeids) as the fish grow between 400 and 600 mm in standard length. Due to their biomass, abundance and top predator status, L. osseus play a significant ecological role in the estuarine community composition, although this effect has often been overlooked by past researchers and should be considered in future estuarine community studies.     

Intra‐lake stable isotope ratio variation in selected fish species and their possible carbon sources in Lake Kyoga (Uganda): implications for aquatic food web studies

The stable isotopes of nitrogen (δ¹⁵N) and carbon (δ¹³C) provide powerful tools for quantifying trophic relationships and carbon flow to consumers in food webs; however, the isotopic signatures of organisms vary within a lake. Assessment of carbon and nitrogen isotopic signatures in a suite of plants, invertebrates, and fishes in Lake Kyoga, indicated significant variation between two sites for δ¹³C (paired t = 6.305; df = 14, P < 0.001 and δ¹⁵N paired t = 1.292; df = 14; P < 0.05). The fish fauna in Bukungu was generally more ¹³C enriched (mean δ¹³C = –16.37 ± 1.64‰) than in Iyingo (mean δ¹³C = –20.80 ± 2.41‰) but more δ¹⁵N depleted (mean δ¹⁵N = 5.57 ± 0.71‰) than in Iyingo (mean δ¹⁵N = 6.92 ± 0.83‰). The simultaneous shifts in phytoplankton and consumer signatures confirmed phytoplankton as the major source of carbon for the food chain leading to fish. Limited sampling coverage within lakes may affect lake wide stable isotope signatures, and the same error is transferred into trophic position estimation. Consideration of potential intra‐lake spatial variability in isotope ratios and size is essential in evaluating the spatial and trophic structure of fish assemblages.     

Growth- and feeding-related isotopic dilution and enrichment patterns in young-of-the-year yellow perch (Perca flavescens)

1. Isotopic signatures (δ¹⁵N and δ¹³C) from young‐of‐the‐year (YOY) yellow perch (Perca flavescens) were collected over the initial 4 month summer growing period from three separate and distinctive sites in northern Alberta, Canada. Data were analysed to test the hypotheses that there are within‐ and among‐population differences in the patterns of isotopic δ¹⁵N and δ¹³C change over the growing season, and that observed isotopic dilution and/or enrichment patterns were influenced by site‐specific physical and chemical factors. 2. Increases in δ¹⁵N relative to spawned egg masses were observed in immediate posthatch (emergent) YOY and attributed to enrichment associated with the assimilation of yolk during embryonic development. 3. Posthatch dilution of YOY δ¹⁵N signatures associated with ontogenetic dietary shifts from yolk to exogenous feeding and zooplanktivory to benthivory occurred at all sites and was associated at most sites with a concomitant increase in δ¹³C. 4. The rate and pattern of δ¹⁵N dilution and δ¹³C enrichment observed for the study populations varied between and within sites and depended on maternal trophic status and timing of ontogenetic dietary shifts, as determined by prey availability and site‐specific biogeochemical factors. 5. Comparisons of isotopic dilution patterns among species, using results from this study and literature‐derived values, indicated that dilution rates and patterns are species dependent and may vary in relation to key life‐history events. 6. Seasonal and spatial isotopic variability among populations and between species complicates field sampling. In particular, the connectivity to site‐specific conditions found here suggests that for locally resident juvenile fishes, spatial, as well as temporal variability must be included in isotopic sampling programmes designed to characterise littoral zone foodweb relationships.     

Ecological variation in invasive brown trout (Salmo trutta) within a remote coastal river catchment in northern Patagonia complicates estimates of invasion impact

Salmonids were first introduced into the Chilean fresh waters in the 1880s, and c. 140 years later, they are ubiquitous across Chilean rivers, especially in the southern pristine fresh waters. This study examined the brown trout (Salmo trutta) and native taxa ecology in two adjacent but contrasting rivers of Chilean Patagonia. During spring 2016 and spring–fall 2017 we examined the variation in benthic macroinvertebrate and fish community composition and characterized fish size structure, stomach contents, and stable isotopes (δ¹³C and δ¹⁵N) to understand population structure, fish diet, and trophic interactions between S. trutta and native taxa. The native Galaxias maculatus (puye) dominated the fish community (74% of abundance). S. trutta was less abundant (16% of survey catch) but dominated the fish community (over 53%) in terms of biomass. S. trutta showed distinct diets (stomach content analysis) in the two rivers, and individuals from the larger river were notably more piscivorous, consuming native fish with a relatively small body size (<100-mm total length). Native fishes were isotopically distinct from S. trutta, which showed a wider isotopic niche in the smaller river, indicating that their trophic role was more variable than in the larger river (piscivorous). This study provides data from the unstudied pristine coastal rivers in Patagonia and reveals that interactions between native and introduced species can vary at very local spatial scales.     

Dietary niche partitioning in sympatric gadid species in coastal Newfoundland: evidence from stomachs and C-N isotopes

The feeding habits of co-occurring gadid species Atlantic cod (Gadus morhua) and Greenland cod (Gadus ogac) in coastal Newfoundland waters, examined using stable isotope (δ ¹³C and δ ¹⁵N) and stomach content analysis, indicated little dietary niche overlap and interspecific competition for food resources despite similar trophic levels. Both species consumed a variety of invertebrates and fish but showed a preference for different prey items. Polychaetes, fish and small crustaceans dominated G. ogac stomach contents while small crustaceans, in particular hyperiid amphipods and fish, dominated those of G. morhua. In general, G. morhua consumed more pelagic prey and had a significantly more pelagic (more negative) δ ¹³C signature while G. ogac consumed primarily benthic prey and had a more benthic (more positive) δ ¹³C signature. δ ¹⁵N levels were similar in these species suggesting similar trophic positions, with levels increasing with fish length in both species. Dietary overlap was not significant in both stomach and stable isotope analyses. We conclude that interspecific competition for food is low between G. ogac and G. morhua and is unlikely to be a factor in the slow rebuilding of Atlantic cod in this region.     

Food web pathways for fish communities in small tropical streams

1. It is often assumed that invertebrate consumers in small tropical streams are dependent on allochthonous sources, although recent studies indicate that algae can form the base of food webs in tropical streams. Fish in tropical streams can feed across several trophic levels and the origin and path of energy and nutrient flow is uncertain for many species.
2. We collected fish, insects, periphyton, and leaf litter from 20 streams across four Atlantic Forest catchments. We analysed stomach contents of fish to define trophic guild and fish dietary trophic position. We also analysed stable isotopes of carbon and nitrogen of fish and their resources to identify the main basal resources of the food web and to estimate trophic positions and identify the path of energy flow.
3. We found that autochthonous sources were the primary resource base for fish communities. Trophic positions estimated from diet and isotopes were similar and correlated for insectivore and algivore–insectivore fish, but not for algivore–detritivore or omnivore fish. Using path analysis, fish classified as algivore–detritivores appear to have derived their biomass through a diet of primary consumer insects and periphytic algae and thus, are more likely to play a trophic role as algivore–insectivores in these streams. However, omnivores probably derived much of their biomass from aquatic insects.
4. Our findings support other studies of tropical systems in which the main basal resource is autochthonous, even in small streams. We also show that the assignment to a specific trophic guild for some fish species, based on gut contents, does not reflect what they assimilate into their bodies. In some species, food sources that are uncommon can make a disproportionately important contribution to their biomass.
5. This study affirms the important role of inconspicuous algal resources in aquatic food webs, even in small forested streams, and demonstrates the effectiveness of taking a combined approach of diet analysis, isotopic tracing, and modelling to resolve food web pathways where the level of omnivory is high.

     

Difference in diet and size‐related trophic level in two sympatric fish species,the red mullets Mullus barbatus and Mullus surmuletus,in the Gulf of Lions (north‐west Mediterranean Sea)

Stable‐isotope ratios (δ¹⁵N and δ¹³C) and diet of the red mullets Mullus barbatus and Mullus surmuletus were analysed in two zones differently subjected to the Rhône River runoff in the Gulf of Lions (north‐west Mediterranean sea) in May and October 2004. δ¹⁵N and δ¹³C increased significantly with fish size in M. barbatus in both zones and seasons, whereas no significant trend was evidenced in M. surmuletus. A clear size‐related shift in diet was observed in M. barbatus, with an increase in polychaete and shrimp consumption with size and a decrease in small crustacean consumption. In M. surmuletus, a diet shift was observed only between medium and large individuals. Large M. surmuletus preyed on shrimps, polychaetes, bivalves, ophiurids and amphipods, and ingested prey of lower mean mass than M. barbatus of similar size. Difference in size‐related increase in δ¹⁵N between the two mullid fish species were related to difference of trophic level of their main prey. Sub‐surface deposit‐feeding polychaetes, carnivorous polychaetes, shrimps and brachyurans presented higher δ¹⁵N values than bivalves, small crustaceans and ophiurids. The lower δ¹³C values observed in M. barbatus compared to M. surmuletus were related to a higher consumption of sub‐surface polychaetes in the former species. Significantly, lower δ¹³C were recorded in fishes collected off the Rhône River, particularly in spring, suggesting an influence of river inputs as a source of particular organic matter for mullids in this zone after the flooding season. Thus, these closely related sympatric fish species displayed diet divergences that were reflected in their stable isotopic signatures.