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.     

Foraging habits in a generalist predator: Sex and age influence habitat selection and resource use among bottlenose dolphins (Tursiops truncatus)

This study examines resource use (diet, habitat use, and trophic level) within and among demographic groups (males, females, and juveniles) of bottlenose dolphins (Tursiops truncatus). We analyzed the δ¹³C and δ¹⁵N values of 15 prey species constituting 84% of the species found in stomach contents. We used these data to establish a trophic enrichment factor (TEF) to inform dietary analysis using a Bayesian isotope mixing model. We document a TEF of 0‰ and 2.0‰ for δ¹³C and δ¹⁵N, respectively. The dietary results showed that all demographic groups relied heavily on low trophic level seagrass‐associated prey. Bayesian standard ellipse areas (SEA_b) were calculated to assess diversity in resource use. The SEA_b of females was nearly four times larger than that of males indicating varied resource use, likely a consequence of small home ranges and habitat specialization. Juveniles possessed an intermediate SEA_b, generally feeding at a lower trophic level compared to females, potentially an effect of natal philopatry and immature foraging skills. The small SEA_b of males reflects a high degree of specialization on seagrass associated prey. Patterns in resource use by the demographic groups are likely linked to differences in the relative importance of social and ecological factors.     

Quantitative diet analysis of four mesopredators from a coral reef

The diets of four common mesopredator fishes were examined in the back‐reef habitat of a subtropical fringing reef system during the summer months. Quantitative gut content analyses revealed that crustaceans, represented >60% of ingested prey (% mass) by the latticed sand‐perch Parapercis clathrata, brown dottyback Pseudochromis fuscus and half‐moon grouper Epinephelus rivulatus. Dietary analyses also provided insights into ontogenetic shifts. Juvenile P. fuscus ingested large numbers of crustaceans (amphipods and isopods); these small prey were rarely found in larger individuals (<1% of ingested mass). Fishes also made an important contribution to the diets of all three species representing 10–30% of ingested mass. Conversely, the sand lizardfish Synodus dermatogenys fed exclusively on fishes including clupeids, gobies and labrids. Differences in the gut contents of the four species recorded were not apparent using stable isotope analysis of muscle tissues. The similarity of δ¹³C values in muscle tissues suggested that carbon within prey was derived from primary producers, with comparable carbon isotope signatures to corals and macroalgae, whilst similarities in δ¹⁵N values indicated that all four species belonged to the same trophic level. Thus, interspecific differences between mesopredator diets were undetectable when using stable isotope analysis which suggests that detailed elucidation of trophic pathways requires gut content analyses.     

Diet segregation between two colonies of little penguins Eudyptula minor in southeast Australia

We studied foraging segregation between two different sized colonies of little penguins Eudyptula minor with overlapping foraging areas in pre‐laying and incubation. We used stomach contents and stable isotope measurements of nitrogen (δ¹⁵N) and carbon (δ¹³C) in blood to examine differences in trophic position, prey‐size and nutritional values between the two colonies. Diet of little penguins at St Kilda (small colony) relied heavily on anchovy while at Phillip Island (large colony), the diet was more diverse and anchovies were larger than those consumed by St Kilda penguins. Higher δ¹⁵N values at St Kilda, differences in δ¹³C values and the prey composition provided further evidence of diet segregation between colonies. Penguins from each colony took anchovies from different cohorts and probably different stocks, although these sites are only 70 km apart. Differences in diet were not reflected in protein levels in the blood of penguins, suggesting that variation in prey between colonies was not related to differences in nutritional value of the diet. Anchovy is currently the only available prey to penguins throughout the year and its absence could have a negative impact on penguin food supply, particularly at St Kilda where the diet is dominated by this species. While it is difficult to establish whether diet segregation is caused by inter‐ or intra‐colony competition or spatial differences in foraging areas, we have shown that colonies with broadly overlapping foraging ranges could have significant differences in trophic position, diet composition and prey size while maintaining a diet of similar nutritional value.     

Trophic ecology of three stingrays (Myliobatoidei: Dasyatidae) off the Brazilian north-eastern coast: Habitat use and resource partitioning

Understanding the ecological role of species with overlapping distributions is central to inform ecosystem management. Here we describe the diet, trophic level and habitat use of three sympatric stingrays, Hypanus guttatus, H. marianae and H. berthalutzae, through combined stomach content and stable isotope (δ¹³C and δ¹⁵N) analyses. Our integrated approach revealed that H. guttatus is a mesopredator that feeds on a diverse diet of benthic and epibenthic marine and estuarine organisms, principally bivalve molluscs, Alpheus shrimp and teleost fishes. Isotopic data supported movement of this species between marine and estuarine environments. H. berthalutzae is also a marine generalist feeder, but feeds primarily on teleost fishes and cephalopods, and consequently occupies a higher trophic level. In contrast, H. marianae is a mesopredator specialized on shrimps and polychaetas occurring only in the marine environment and occupying a low niche breadth. While niche overlap occurred, the three stingrays utilized the same prey resources at different rates and occupied distinct trophic niches, potentially limiting competition for resources and promoting coexistence. These combined data demonstrate that these three mesopredators perform different ecological roles in the ecosystems they occupy, limiting functional redundancy.     

Spatial,temporal, age,and sex related variation in the diet of South American sea lions in southern Chile

We analyzed spatial, temporal, and age/sex variation in the diet of the South American sea lions (SASL, Otaria byronia) in southern Chile via δ¹³C and δ¹⁵N isotope analysis, with emphasis on exotic farmed and feral salmonids as a source of prey for this generalist predator. Isotope mixing models show that the primary prey consumed by SASL were demersal and benthopelagic fishes. Individuals sampled in summer and in the outer coast had higher δ¹⁵N values than those from winter or from the inner coast due to consumption of high trophic level demersal fish. We also found evidence of an ontogenetic shift in isotopic niche width as measured by δ¹³C and δ¹⁵N standard ellipse areas, with juveniles showing a restricted isotopic niche width (1.0‰²) in comparison to subadult (2.2‰²) and adult males (1.9‰²). The proportion of salmonids in the diet of the SASL was negligible and lower than reported in previous studies, which may be related to a recent drop in the production of salmon aquaculture in the region. Our study suggests that the SASL are able to adapt to shifts in spatiotemporal variations in the abundance of potential prey, including feral and farmed salmonids.     

Utility of fish scales from stock assessment surveys in stable isotope analysis for initial assessments of trophic relationships in riverine fish communities

The utility of using fish scales collected during stock assessment exercises to assess the trophic relationships of riverine fishes using their stable isotopes of d¹³C and d¹⁵N was tested using three riverine fish communities in England (Rivers Great Ouse, Ivel and Goyt). In each river, European barbel Barbus barbus was an important species, with other cyprinid species, including chub Squalius cephalus, present. Stable isotope analyses was completed using relatively small sample sizes per species (<11) from fish samples collected in 2001, 2005 and 2006 when up to 5 scales were collected from each fish. The calculation of standard ellipse areas (as a measure of trophic niche size) revealed that relative to other fishes, B. barbus occupied high trophic positions with minimal overlap in their trophic niche with other species, especially S. cephalus. As the analysed fish samples comprised species of various length ranges and as length has strong ontogenetic consequences for fish diet composition, generalized linear models were developed in which length was the covariate; model outputs included length‐adjusted mean δ¹³C and δ¹⁵N for each species. In each fish community, significant differences in δ¹³C and δ¹⁵N were apparent between B. barbus and S. cephalus, but were less apparent between B. barbus and other fishes. Thus, whilst the utility of using fish scales from stock assessments in stable isotope analyses are limited due to the differing length ranges of the sampled fishes, they can be useful in identifying trophic differences between species when methods such as stomach content analyses are unavailable.     

Diet of harbor porpoises along the Dutch coast: A combined stable isotope and stomach contents approach

High stranding frequency of porpoises, Phocoena phocoena, along the Dutch coast since 2006 has led to increased interest in the ecology of porpoises in the North Sea. Stranded porpoises were collected along the Dutch coast (2006–2008) and their diet was assessed through stomach content and stable isotope analysis (δ¹³C and δ¹⁵N) of porpoise muscle and prey. Stable isotope analysis (SIAR) was used to estimate the contribution of prey species to the porpoises' diet. This was compared to prey composition from stomach contents, to analyze differences between long‐ and short‐term diet. According to stomach contents, 90.5% of the diet consisted of gobies, whiting, lesser sandeel, herring, cod, and sprat. Stable isotope analysis revealed that 70‐83% of the diet consisted of poor cod, mackerel, greater sandeel, lesser sandeel, sprat, and gobies, highlighting a higher importance of pelagic, schooling species in the porpoises' diet compared to stomach contents. This could be due to prey distribution as well as differences in behavior of porpoises and prey between the coastal zone and offshore waters. This study supports the need for multi‐method approaches. Future ecological and fishery impact assessment studies and management decisions for porpoise conservation should acknowledge this difference between the long‐ and short‐term diet.     

Stable isotope analysis indicates trophic differences among forest floor carabids in Japan

Differences in trophic niches among carabid beetles (Coleoptera: Carabidae) co‐occurring on the forest floors of warm temperate forests in central Japan were studied using carbon (δ¹³C) and nitrogen (δ¹⁵N) stable isotope analyses. Different carabid species showed similar δ¹⁵N values, which were higher than those of their possible invertebrate prey (herbivores and detritivores) collected from the litter layer, indicating that these species were consumers in the same trophic level. In contrast, δ¹³C values differed among carabid species, indicating interspecific differences in prey animals. The variation in the δ¹³C value was larger in summer than in autumn. In summer, δ¹³C values indicated that some carabids depended highly on either grazing (low δ¹³C values) or detrital sources (high δ¹³C values) within the food chain [Chlaenius posticalis Motschulsky and Haplochlaenius costiger (Chaudoir), respectively], although other species with intermediate δ¹³C values likely depended on both. The latter group of species comprised mostly two dominant genera (Carabus and Synuchus). Although congeners might have similar feeding habits, the stable isotope ratios indicated trophic niche differences between adults of different species and between adults and larvae of the same genus.     

Consistency of elemental and isotope‐ratio patterns across multiple scales from individual fish

Assays of elemental and stable‐isotope ratios across growth increments of scales have the potential to provide a non‐lethal alternative to otolith chemistry for identifying migration and ontogenetic trophic shifts. A central assumption when employing scales as otolith analogues is that any scale from an individual will provide equivalent information about the chemical history of that fish. This assumption was investigated with multiple scales from wild and captive euryhaline Atlantic tarpon Megalops atlanticus from the north‐west Gulf of Mexico. Elemental (Sr:Ca) and isotope‐ratio (δ¹³C and δ¹⁵N) life‐history profiles were compared among multiple scales from each fish. All three chemical proxies showed highly consistent patterns among non‐regenerated scales, while patterns in regenerated scales diverged, indicating rapid regrowth of interior scale material at the onset of regeneration. Patterns of Sr:Ca and δ¹³C covaried, supporting their use as salinity proxies, while δ¹⁵N patterns were consistent with ontogenetic diet shifts. Water samples taken from aquaria holding captive fish were used to calculate partition coefficients for a suite of elements in M. atlanticus scales for future quantification of migratory movements in the region. Together, these results support the assumption that non‐regenerated scales from individual M. atlanticus provide equivalent chemical histories, further validating their use as a viable non‐lethal alternative to otoliths.     

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.     

The influence of climate and habitat on stable isotope signatures and the isotopic niche of nestling White‐headed Woodpeckers (Dryobates albolarvatus)

The majority of landbird species feed their nestlings arthropods and variation in arthropod populations can impact reproductive outcomes in these species. Arthropod populations in turn are influenced by climate because temperature affects survival and reproduction, and larval development. Thus, climate factors have the potential to influence many bird species during their reproductive phases. In this study, we assessed climate factors that impact the diet of nestling White‐headed Woodpecker (Dryobates albolarvatus), an at‐risk keystone species in much of its range in western North America. To do this, we measured stable isotope signatures (δ¹³C and δ¹⁵N) in 152 nestlings across six years and linked variation in isotopic values to winter (December–February) and spring (June) precipitation and temperature using mixed effects models. We also explored habitat factors that may impact δ¹³C and δ¹⁵N and the relationship between δ¹⁵N and nest productivity. Last, we estimated isotopic niche width for nestlings in different watersheds and years using Bayesian standard ellipses, which allowed us to compare dietary niche width and overlap. We found that colder winter temperatures were associated with an increase in δ¹⁵N and δ¹⁵N levels had a weak positive relationship with nest productivity. We also found that sites with a more diverse tree community were associated with a broader isotopic niche width in nestlings. Our findings suggest that nestling diet is affected by climate, and under future warming climate scenarios, White‐headed Woodpecker nestling diet may shift in favor of lower trophic level prey (prey with lower δ¹⁵N levels). The impact of such changes on woodpecker populations merits further study.     

Ontogenetic diet changes in bottlenose dolphins (Tursiops truncatus) reflected through stable isotopes

The ability of stable isotope analysis to provide insight into ontogenetic dietary changes was examined using bottlenose dolphin tooth and skin samples. Teeth were subsampled to compare tissue produced early in life (outer tooth) to that produced later in life (inner tooth). Outer tooth had significantly higher δ¹⁵N values than the corresponding inner sample from the same tooth (n= 60, P= 0.0041), indicating that there was a temporal shift to a lower δ¹⁵N diet. There were no significant δ¹³C differences. Higher δ¹⁵N values in young have previously been attributed to the period of suckling. Analysis of skin tissue from stranded animals of different developmental stages similarly indicated that the δ¹⁵N values were significantly higher in young animals. Further comparisons indicated that the primary influence for this difference was animals with lengths less than or equal to the largest neonatal dolphin. This difference likely reflects an ontogenetic dietary shift from a sole reliance on milk to a combination of milk and prey species during the first year of life.     

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.     

Evolutionary convergence in Neotropical cichlids and Nearctic centrarchids: evidence from morphology,diet, and stable isotope analysis

Despite divergent evolutionary histories, Neotropical cichlids (Cichlidae) and Nearctic sunfishes (Centrarchidae) appear to have similar functional morphotypes and occupy similar ecological niches. We applied an integrative approach analyzing morphological traits, stomach contents, and stable isotope ratios (δ¹³C, δ¹⁵N) to investigate whether local assemblages of cichlids (Venezuela, Peru) and centrarchids (Texas) reveal one‐to‐one patterns of morphological and ecological convergence. Multivariate ordinations performed on diet and morphology datasets identified a broad overlap between cichlid and centrarchid assemblages. The functional morphology of the two groups has diversified in a convergent manner within the confines of ram‐suction modes of prey ingestion. Both groups had the same set of ecomorph types that corresponded to the same trophic niches, including substrate‐sifting invertivores, epibenthic invertebrate gleaners, and piscivores; the one exception was a molluscivorous sunfish, comprising a niche that was not represented in the two cichlid assemblages. Estimates of trophic positions based on stable isotope analysis revealed convergent vertical trophic structure; with few exceptions, fishes with similar morphologies had similar trophic positions. Large‐bodied piscivores had highest trophic positions, whereas small and medium‐bodied generalists and invertivores had low to intermediate trophic positions. Consistent patterns of ecomorphological convergence in these two perciform groups provide strong evidence for adaptation involving constrains in functional morphology associated with feeding. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 109 , 146–164.     

Comparing pellet and stable isotope analyses of nestling Bonelli's Eagle Aquila fasciata diet

Diet analyses are central to the study of avian trophic ecology, and stable isotope analyses have made an increasing contribution in the last two decades. Few isotopic studies have assessed the diet of raptor species, which are more frequently analysed by conventional diet methods such as pellet analysis. In this study, we compare prey consumption estimates of nestling Bonelli's Eagles Aquila fasciata from conventional pellet analysis (in terms of items and biomass) and stable isotopic mixing models (SIAR) using δ¹³C, δ¹⁵N and δ³⁴S of feathers. The pellet analysis showed that European Rabbits Oryctolagus cuniculus, pigeons (mainly Common Wood Pigeons Columba palumbus and Domestic Pigeons Columba livia dom.), Red‐legged Partridges Alectoris rufa, passerines, Yellow‐legged Gulls Larus michahellis and Eurasian Red Squirrels Sciurus vulgaris were the main prey, so they were selected for diet reconstructions in SIAR. At the population level, mean prey consumption estimates were similar for pellets (both items and biomass) and SIAR. At the territory level, the weighted kappa statistic showed good ordinal scale agreement in main prey consumption between items or biomass and SIAR. Although the intraclass correlation coefficient showed poor method agreement when considering all prey in the same analysis, the intraclass correlation coefficients for each prey category showed significant agreement between pellets and SIAR when estimating the consumption of Rabbits, pigeons and Gulls, with lower agreement for passerines and Squirrels. Lastly, there was poor method agreement for estimates of Partridges. Our results suggest an overall agreement between the pellet analysis and SIAR when estimating nestling Bonelli's Eagle diet at both the population and, to a lesser extent, the territory level, supporting the usefulness of isotopic mixing models when identifying the terrestrial and marine components of raptor diets.     

A trophic pathway from biogenic methane supports fish biomass in a temperate lake ecosystem

Although some primary consumers such as chironomid larvae are known to exploit methane‐derived carbon via microbial consortia within aquatic food webs, few studies have traced the onward transfer of such carbon to their predators. The ruffe Gymnocephalus cernuus is a widespread benthivorous fish which feeds predominantly on chironomid larvae and is well adapted for foraging at lower depths than other percids. Therefore, any transfer of methanogenic carbon to higher trophic levels might be particularly evident in ruffe. We sampled ruffe and chironomid larvae from the littoral, sub‐littoral and profundal areas of Jyväsjärvi, Finland, a lake which has previously been shown to contain chironomid larvae exhibiting the very low stable carbon isotope ratios indicative of methane exploitation. A combination of fish gut content examination and stable isotope analysis was used to determine trophic linkages between fish and their putative prey. Irrespective of the depth from which the ruffe were caught, their diet was dominated by chironomids and pupae although the proportions of taxa changed. Zooplankton made a negligible contribution to ruffe diet. A progressive decrease in δ¹³C and δ¹⁵N values with increasing water column depth was observed for both chironomid larvae and ruffe, but not for other species of benthivorous fish. Furthermore, ruffe feeding at greater depths were significantly larger than those feeding in the littoral, suggesting an ontogenetic shift in habitat use, rather than diet, as chironomids remained the predominant prey item. The outputs from isotope mixing models suggested that the incorporation of methane‐derived carbon to larval chironomid biomass through feeding on methanotrophic bacteria increased at greater depth, varying from 0% in the littoral to 28% in the profundal. Using these outputs and the proportions of littoral, sub‐littoral or profundal chironomids contributing to ruffe biomass, we estimated that 17% of ruffe biomass in this lake was ultimately derived from chemoautotrophic sources. Methanogenic carbon thus supports considerable production of higher trophic levels in lakes.     

Spatial,ontogenetic and interspecific variability in stable isotope ratios of nitrogen and carbon of Merluccius capensis and Merluccius paradoxus off South Africa

General linear models (GLMs) were used to determine the relative importance of interspecific, ontogenetic and spatial effects in explaining variability in stable isotope ratios of nitrogen (δ¹⁵N) and carbon (δ¹³C) of the co‐occurring Cape hakes Merluccius capensis and Merluccius paradoxus off South Africa. Significant GLMs were derived for both isotopes, explaining 74 and 56% of observed variance in Merluccius spp. δ¹⁵N and δ¹³C, respectively. Spatial effects (west or south coast) contributed most towards explaining variability in the δ¹⁵N model, with Merluccius spp. off the west coast having higher (by c. 1·4‰) δ¹⁵N levels than Merluccius spp. off the south coast. Fish size and species were also significant in explaining variability in δ¹⁵N, with both species showing significant linear increases in δ¹⁵N with size and M. capensis having higher (by c. 0·7‰) δ¹⁵N values than M. paradoxus. Species and coast explained most and similar amounts of variability in the δ¹³C model, with M. capensis having higher (by c. 0·8‰) δ¹³C values than M. paradoxus, and values being lower (by c. 0·7‰) for fishes off the west coast compared with the south coast. These results not only corroborate the knowledge of Merluccius spp. feeding ecology gained from dietary studies, in particular the ontogenetic change in trophic level corresponding to a changing diet, but also that M. capensis feeds at a slightly higher trophic level than M. paradoxus. The spatial difference in Merluccius spp. δ¹⁵N appears due to a difference in isotopic baseline, and not as a result of Merluccius spp. feeding higher in the food web off the west than the south coast, and provides new evidence that corroborates previous observations of biogeographic differences in isotopic baselines around the South African coast. This study also provides quantitative data on the relative trophic level and trophic width of Cape hakes over a large size range that can be used in ecosystem models of the southern Benguela.     

Combined gut‐content and stable isotope trophic analysis of the pelagic stingray Pteroplaytrygon violacea (Bonaparte, 1832) diet from the western North Atlantic Ocean

The understanding of trophic relationships is vital for correctly modeling ecosystems and ecosystem effects of fisheries removals. The pelagic stingray is found in epipelagic sub‐tropical and tropical waters worldwide and is a common bycatch in pelagic longline fisheries. Between August 2008 and November 2011, 156 specimens (81 males; 75 females) were collected during pelagic longline fishing operations in the US South Atlantic Bight and Gulf of Mexico. Stomach content analyses found that the major prey items were cephalopod molluscs (59.18%), followed by actinopterygiian fishes (37.75%), and decapod crustaceans (35.71%). These concentrations of prey items found in the stomachs coincide with previous studies done in the Pacific Ocean. In contrast to previous studies that found high percentages of empty stomachs (63%), the current percentage of empty stomachs was much lower (25.6%), likely due to shorter times between collection and inspection. Stable isotope analysis (δ¹³C and δ¹⁵N) was performed on white muscle in order to correlate the trophic position with gut‐content analysis. The δ¹³C values ranged from ‐18.81‰ to ‐16.70‰, while the δ¹⁵N ranged from 6.11‰ to 11.88‰. Modeling of stable isotope data suggest that while squid are occasionally an important part of the pelagic stingray diet, prey usually consist of shrimp and other pelagic crustaceans. Pelagic stingrays fed within two trophic levels, but their prey appeared to feed on different carbon sources than those found in other pelagic elasmobranchs. A deeper understanding of the pelagic stingray diet sources can help fisheries management as it begins to transition into ecosystem‐based management.     

Ontogenetic dietary information of the California sea lion (Zalophus californianus) assessed using stable isotope analysis

We used stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotopes to examine ontogenetic dietary changes in 289 California sea lions (Zalophus californianus) at San Miguel Island, California during 2004–2007. Tissues analyzed included fur, red blood cells, plasma, and serum. For all tissues, pups had higher δ¹⁵N values and lower δ¹³C values compared to adults, which indicated that pups were feeding higher trophically than older conspecifics and on a lipid‐rich milk diet prior to weaning. Yearling δ¹⁵N values were slightly lower than pup or nearly indistinguishable from adult values depending on the tissue analyzed, indicating a dietary shift from maternal dependency to independent foraging. Juveniles (2–4 yr) and adults (>4 yr) had similar δ¹⁵N values indicating they fed at a similar trophic level. There did not appear to be a pronounced dietary shift in δ¹³C values. However, δ¹³C values integrated with telemetry data indicated that postweaned individuals fed in similar foraging areas. Dietary changes during early life stages may be due to differences in physiology, morphology, experience, or energetic requirements; however, young animals are able to attain the skills needed to consume adult prey types near the end of their second year of life.