What do the empty stomachs of northern pike (<Emphasis Type="Italic">Esox lucius</Emphasis>) reveal? Insights from carbon (δ<Superscript>13</Superscript>C) and nitrogen (δ<Superscript>15</Superscript>N) stable isotopes

The frequency of individuals with empty stomachs (FES) can vary greatly among northern pike populations. However, the FES has only seldom been analyzed in this species and its meaning is still not fully understood. It has been suggested that a high FES may reflect a strongly piscivorous behaviour while low FES could reflect a higher utilization of invertebrates. We compared the stomach contents and the trophic position of northern pike in 16 populations of individuals feeding mainly on fish or benthic invertebrates. We tested the hypothesis that northern pike with empty stomachs or with fish in their stomachs have a higher trophic position than individuals feeding on invertebrates. Carbon (δ¹³C) and nitrogen (δ¹⁵N) stable isotope signatures were used to estimate the trophic position of individuals. We found no significant difference in the trophic position among piscivores, invertebrate feeders, and northern pike with empty stomachs. The average trophic position of northern pike was high (mean ± SD = 4.3 ± 0.4, n = 66) and was correlated with total length. These results indicate that, although invertebrates could be an important part of the diet of northern pike in Canadian Shield lakes, fish are still the dominant prey. Hence, feeding on invertebrates in our study lakes would reflect an opportunistic rather than a specialized feeding strategy.     

Trophic polymorphism in bluegill sunfish (<Emphasis Type="Italic">Lepomis macrochirus</Emphasis>) introduced into Lake Biwa: evidence from stable isotope analysis

Trophic polymorphism was recently reported in introduced bluegill (Lepomis macrochirus) in Lake Biwa, Japan, where three morphs are specialized in benthic invertebrates (benthivorous type), submerged aquatic plants (herbivorous type), and zooplankton (planktivorous type). We evaluated the long-term effects of food resource utilization by these trophic morphs using stable isotope ratios, δ¹⁵N and δ¹³C. A significant difference in δ¹⁵N was found between the benthivorous and planktivorous types. The planktivorous type had the higher δ¹⁵N value, which corresponded with the value expected from its prey, zooplankton. The lower δ¹⁵N value of the benthivorous type would be derived from the lower δ¹⁵N values of benthic prey organisms compared to zooplankton. These results support previous findings that the benthivorous and planktivorous types have different food resource utilization. In contrast, the δ¹⁵N and δ¹³C values of the herbivorous type were distinctly different from the expected values, indicating that this type was unlikely to utilize aquatic plants substantially, contradicting the results of the dietary analysis.     

Variability and directionality of temporal changes in δ<Superscript>13</Superscript>C and δ<Superscript>15</Superscript>N of aquatic invertebrate primary consumers

Seasonal oscillations in the carbon (δ¹³C) and nitrogen (δ¹⁵N) isotope signatures of aquatic algae can cause seasonal enrichment–depletion cycles in the isotopic composition of planktonic invertebrates (e.g., copepods). Yet, there is growing evidence that seasonal enrichment–depletion cycles also occur in the isotope signatures of larger invertebrate consumers, taxa used to define reference points in isotope-based trophic models (e.g., trophic baselines). To evaluate the general assumption of temporal stability in non-zooplankton aquatic invertebrates, δ¹³C and δ¹⁵N time series data from the literature were analyzed for seasonality and the influence of biotic (feeding group) and abiotic (trophic state, climate regime) factors on isotope temporal patterns. The amplitude of δ¹³C and δ¹⁵N enrichment–depletion cycles was negatively related to body size, although all size-classes of invertebrates displayed a winter-to-summer enrichment in δ¹³C and depletion in δ¹⁵N. Among feeding groups, periphytic grazers were more variable and displayed larger temporal changes in δ¹³C than detritivores. For nitrogen, temporal variability and magnitude of directional change of δ¹⁵N was most strongly related to ecosystem trophic state (eutrophic > mesotrophic, oligotrophic). This study provides evidence of seasonality in the isotopic composition of aquatic invertebrates across very broad geographical and ecological gradients as well as identifying factors that are likely to modulate the strength and variability of seasonality. These results emphasize the need for researchers to recognize the likelihood of temporal changes in non-zooplankton aquatic invertebrate consumers at time scales relevant to seasonal studies and, if present, to account for temporal dynamics in isotope trophic models.     

Effects of food quality on tissue-specific isotope ratios in the mussel <Emphasis Type="Italic">Perna perna</Emphasis>

Investigations into trophic ecology and aquatic food web resolution are increasingly accomplished through stable isotope analysis. The incorporation of dietary and metabolic changes over time results in variations in isotope signatures and turnover rates of producers and consumers at tissue, individual, population and species levels. Consequently, the elucidation of trophic relationships in aquatic systems depends on establishing standard isotope values and tissue turnover rates for the level in question. This study investigated the effect of diet and food quality on isotopic signatures of four mussel tissues: adductor muscle, gonad, gill and mantle tissue from the brown mussel Perna perna. In the laboratory, mussels were fed one of the two isotopically distinct diets for 3 months. Although not all results were significant, overall δ¹³C ratios in adductor, mantle and gill tissues gradually approached food source signatures in both diets. PERMANOVA analyses revealed significant changes over time in tissue δ¹³C (mantle and gill) with both diets and in δ¹⁵N (all tissues) and C:N ratios (mantle and gill) for one diet only. The percentage of replaced carbon isotopes were calculated for the 3 month period and differed among tissues and between diets. The tissue with the highest and lowest amount of replaced isotopes over 81 days were mantle tissue on the kelp diet (33.89%) and adductor tissue on the fish food diet (4.14%), respectively. Percentages could not be calculated for any tissue in either diet for δ¹⁵N due to the lack of significant change in tissue nitrogen. Fractionation patterns in tissues for both diets can be linked to nutritional stress, suggesting that consumer isotopic signatures are strongly dependent on food quality, which can significantly affect the degree of isotopic enrichment within a trophic level.     

Vertical heterogeneity of a forest floor invertebrate food web as indicated by stable-isotope analysis

Diverse populations of invertebrates constitute the food web in detritus layers of a forest floor. Heterogeneity in trophic interactions within such a species-rich community food web may affect the dynamic properties of biological communities such as stability. To examine the vertical heterogeneity in trophic interactions among invertebrates in litter and humus layers, we studied differences in species composition and variations in carbon and nitrogen stable-isotope ratios (δ¹³C and δ¹⁵N) using community-wide metrics of the forest floors of temperate broadleaf forests in Japan. The species composition differed between the two layers, and the invertebrates in the litter layer were generally larger than those in the humus layer, suggesting that these layers harbored separate food webs based on different basal resources. However, the δ¹³C of invertebrates, an indicator of differences in the basal resources of community food webs, did not provide evidence for separate food webs between layers even though plant-derived organic matter showed differences in stable-isotope ratios according to decomposition state. The minimum δ¹⁵N of invertebrates also did not differ between layers, suggesting sharing of food by detritivores from the two layers at lower trophic levels. The maximum and range of δ¹⁵N were greater in the humus layer, suggesting more trophic transfers (probably involving microorganisms) than in the litter layer and providing circumstantial evidence for weak trophic interactions between layers at higher trophic levels. Thus, the invertebrate community food web was not clearly compartmentalized between the detrital layers but still showed a conspicuous spatial (vertical) heterogeneity in trophic interactions.     

Preliminary assessment of Greenland halibut diet in Cumberland Sound using stable isotopes

We provide preliminary carbon (δ¹³C) and nitrogen (δ¹⁵N) stable isotope assessment of the Greenland halibut (Reinhardtius hippoglossoides) diet in Cumberland Sound, with focus on two possible prey sources: pelagic represented by capelin (Mallotus villosus) and epibenthic represented by shrimp (Lebbeus polaris). The δ¹³C for the Greenland halibut stock indicated a pelagic carbon source in Cumberland Sound while stable isotope mixing models, IsoSource and MixSIR, indicated a 99% dietary composition of capelin relative to the shrimp. The δ¹⁵N did not vary across Greenland halibut size ranges and placed them at a fourth trophic position relative to a primary herbivore. This study provides the starting point for more elaborate Cumberland Sound research on the local Greenland halibut feeding ecology by confirming pelagic feeding and establishing relative trophic position as well as identifying stable isotopes as a useful tool for the study of diet in cold water fish species.     

Consumer‐resource stoichiometry as a predictor of trophic discrimination (Δ13C, Δ15N) in aquatic invertebrates

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Changes in trophic linkages to shortfin eels (<Emphasis Type="Italic">Anguilla australis</Emphasis>) since the collapse of submerged macrophytes in Lake Ellesmere,New Zealand

Lake Ellesmere (Te Waihora) is a nationally important coastal brackish lake in New Zealand, however degradation in water quality and loss of submerged macrophytes over past decades have raised concerns in regards to the declining status of the lake’s commercial and customary fisheries, predominantly targeted at shortfin eels (Anguilla australis). We investigated foodweb dynamics and trophic linkages to shortfin eels in Lake Ellesmere using a combination of abundance assessments, dietary studies, and stable isotope analyses. Data from our study are compared with historical data sets on benthic invertebrate community composition and shortfin eel diets to trace changes in the trophic linkages to top predators that have occurred since the late 1960s. Stable isotope analyses indicate that the foodweb is predominantly driven by epipelic and phytoplankton derived carbon sources, although it was difficult to discriminate between these two carbon pools because of wind-driven resuspension of lake sediments. Comparison of our survey results with historical data sets indicates a clear shift in benthic biota from being dominated by phytofaunal species such as Potamopyrgus antipodarum (comprising 90% of total invertebrate biomass) during the 1960s, to now being almost entirely comprised of subterranean species such as Chironomus zealandicus and oligochaetes (together comprising 82% of total invertebrate biomass). This shift in benthic communities has resulted in significant changes in the size-specific diet of juvenile shortfin eels (<400 mm) from those reported for Lake Ellesmere during the mid 1970s, with Chironomus larvae now comprising 65% of the diets of juvenile eels, whereas historically P. antipodarum was the dominant food item (>30% of total biomass). This shift towards foraging on smaller sediment-dwelling species could have implications for juvenile eel bioenergetics, and may help explain why juvenile shortfin growth rates have significantly decreased in past decades. Juvenile shortfins now appear to switch to foraging on preyfish (mainly common bullies, Gobiomorphus cotidianus) at a smaller size (≈400 mm) than historically recorded (>500 mm). Dietary and stable isotope signatures indicated that small shortfins (100–299 mm) have considerable overlap in trophic position (δ¹³C = −20.4‰, δ¹⁵N = 13.6‰) with common bullies (δ¹³C = −20.5‰, δ¹⁵N = 13.7‰), the dominant fish in Lake Ellesmere (92% of total abundance CPUE), potentially indicating that these two species may directly compete for food resources. These findings again highlighted the importance of C. zealandicus in sustaining the fish populations of the lake. Handling editor: S. Declerck     

Individual specialization and trophic adaptability of northern pike (Esox lucius): an isotope and dietary analysis

Northern pike (Esox lucius) are often considered to be specialist piscivores, but under some circumstances will continue to eat invertebrates as adults. To examine effects of fish assemblage composition on the trophic ecology of pike, we combined stable isotope analysis (SIA) of carbon and nitrogen and stomach content analysis (SCA) on pike from five lakes in northern Alberta, three of which contain only pike (“pike-only”) and two that also contain yellow perch (Perca flavescens) or white sucker (Catostomus commersoni) (“pike-other”). Fish were more important as prey and empty stomachs, which often characterize piscivores, were significantly more frequent in pike-other than in pike-only lakes. However, even though invertebrates were more important for pike in pike-only lakes, SIA and SCA indicated that invertebrates were also an important component of pike diets in pike-other lakes. SIA and SCA also revealed considerable intrapopulation variation in trophic ecology, with individuals in some populations differing by as much as two trophic levels. Comparisons of stomach contents and isotope signatures of the same fish suggested that within these variable populations, specialization on invertebrates or fish was a long-term trait of some individuals. SIA indicated that trophic position increased and diets shifted to a greater importance of littoral prey as pike grew in pike-only lakes, but not in lakes with other fish present. Trophic adaptability in northern pike is expressed at both the population level, where the trophic ecology is sensitive to differences in prey regimes, and at the organismal level, in the form of intrapopulation variation and individual specialization. Received: 1 July 1998 / Accepted: 3 February 1999     

Contributions of stable-isotope data to elucidating food webs of Mediterranean rocky littoral fishes

The food webs of rocky infra-littoral ecosystems in the Mediterranean have been little studied. In this investigation stable isotopes and dietary data were compared in an attempt to describe features of the food webs concerned. δ¹³C and δ¹⁵N were determined for plants, invertebrates and fishes from the Bay of Calvi, Corsica. Dietary data were derived from the literature. δ¹³C of plants ranged from –8.59‰ to –33.74‰, of benthic invertebrates from –17.0‰ to –20.52‰, of planktonic invertebrates from –20.08‰ to –22.34‰ and of fishes from –16.27‰ to –19.59‰. δ¹⁵N was generally greater at higher trophic levels. δ¹⁵N of plants was 0.95–2.92‰, of benthic invertebrates 1.69–6.54‰, of planktonic invertebrates 3.51–6.82‰ and of fishes 4.63–9.77‰. ¹³C enrichment tended to be associated with benthic food chains and ¹³C depletion with planktonic chains. Stable-isotope data suggested more varied diets for many species than implied by gut-contents data. Omnivory and trophic plasticity were widespread, and many consumers fed lower down the food chain than previous studies had suggested. Both stable-isotope and gut-contents analysis resolved differences between fishes feeding on planktonic and benthic prey and indicated that the herbivorous fish Sarpa salpa fed on a diet substantially different from that of other fishes. Zooplankton were important in the diets of several consumers (both primary and secondary), as was plankton derived detritus. One species of fish previously identified as planktivorous was shown to feed largely on benthic organisms, whilst several species of benthic invertebrates may feed on plankton-derived detritus. Although herbivores seemed to obtain most of their C from macroalgae, δ¹⁵N data suggested that many of these animals supplemented their intake of N, although gut-contents analysis did not provide evidence for such uptake. The isotopic data have elucidated several features of the food web which we would not otherwise have detected. Received: 26 April 1999 / Accepted: 24 September 1999     

Trophic niches of thirteen damselfishes (Pomacentridae) at the Grand Récif of Toliara,Madagascar

The damselfishes, with more than 340 species, constitute one of the most important families that live in the coral reef environment. Most of our knowledge of reef-fish ecology is based on this family, but their trophic ecology is poorly understood. The aim of the present study was to determine the trophic niches of 13 sympatric species of damselfishes by combining stable isotope (δ¹⁵N and δ¹³C) and stomach content analyses. Isotopic signatures reveal three main groups according to their foraging strategies: pelagic feeders (Abudefduf sexfasciatus, A. sparoides, A. vaigiensis, Chromis ternatensis, C. dimidiata, Dascyllus trimaculatus and Pomacentrus caeruleus), benthic feeders (Chrysiptera unimaculata, Plectroglyphidodon lacrymatus and Amphiprion akallopisos) and an intermediate group (D. aruanus, P. baenschi and P. trilineatus). Stomach contents reveal that planktonic copepods and filamentous algae mainly represent the diets of pelagic feeders and benthic feeders, respectively. The intermediate position of the third group resulted from a partitioning of small planktonic prey, small vagile invertebrates and filamentous algae. In this last feeding group, the presence of a wide range of δ¹³C values in P. trilineatus suggests a larger trophic niche width, related to diet-switching over time. Some general considerations about the feeding habits of damselfishes reveal that their choice of habitat on the reef and their behavior appear to be good predictors of diet in this group. Benthic (algae and/or small invertebrates) feeders appear to be solitary and defend a small territory on the bottom; zooplankton feeders remain in groups just above the reef, in the water column.     

Fast carnivores and slow herbivores: differential foraging strategies among grizzly bears in the Canadian Arctic

Categorizing animal populations by diet can mask important intrapopulation variation, which is crucial to understanding a species’ trophic niche width. To test hypotheses related to intrapopulation variation in foraging or the presence of diet specialization, we conducted stable isotope analysis (δ¹³C, δ¹⁵N) on hair and claw samples from 51 grizzly bears (Ursus arctos) collected from 2003 to 2006 in the Mackenzie Delta region of the Canadian Arctic. We examined within-population differences in the foraging patterns of males and females and the relationship between trophic position (derived from δ¹⁵N measurements) and individual movement. The range of δ¹⁵N values in hair and claw (2.0–11.0‰) suggested a wide niche width and cluster analyses indicated the presence of three foraging groups within the population, ranging from near-complete herbivory to near-complete carnivory. We found no linear relationship between home range size and trophic position when the data were continuous or when grouped by foraging behavior. However, the movement rate of females increased linearly with trophic position. We used multisource dual-isotope mixing models to determine the relative contributions of seven prey sources within each foraging group for both males and females. The mean bear dietary endpoint across all foraging groups for each sex fell toward the center of the mixing polygon, which suggested relatively well-mixed diets. The primary dietary difference across foraging groups was the proportional contribution of herbaceous foods, which decreased for both males and females from 42–76 to 0–27% and 62–81 to 0–44%, respectively. Grizzlies of the Mackenzie Delta live in extremely harsh conditions and identifying within-population diet specialization has improved our understanding of varying habitat requirements within the population.     

Distribution and ecology of <Emphasis Type="Italic">Hemimysis anomala</Emphasis>, the latest invader of the Great Lakes basin

Since 2006, the known distribution of Hemimysis anomala has greatly expanded in the Great Lakes ecosystem, with, to date, 45 sites of occurrence among 91 monitored sites, located in four of the Great Lakes and the upper St. Lawrence River. By means of carbon and nitrogen stable isotopes, a first assessment of the feeding ecology of Hemimysis was completed. The δ¹³C values of 18 individuals collected in Lake Erie (Port Mainland) on a single date (Sept. 23, 2008) ranged from −30.2 to −24.5‰, indicating that Hemimysis could feed on multiple carbon sources including pelagic and littoral autochthonous and terrestrial carbon. In Lake Erie, variation in δ¹³C was related to δ¹⁵N, indicating the importance of food source for determining the trophic position of Hemimysis. The δ¹⁵N signatures of individuals were strongly related to their C/N ratios, suggesting that variations in the nutritional value of Hemimysis may depend on trophic position. Isotopic variation among individuals in Lake Erie was complemented by temporal variation in Lake Ontario. Monthly changes (from June to December 2008) in carbon isotope signatures were observed and related to changes in water temperature, highlighting the variations in the baseline prey signatures that fuel Hemimysis diets. The observed variation in stable isotope signatures occurring among individuals within a localized Hemimysis assemblage and temporally should be considered as a key design feature in further studies attempting to identify the possible effects of Hemimysis on nearshore food webs in the Great Lakes.     

Trophic linkages between periphyton and grazing macroinvertebrates in rivers with different levels of catchment development

Stable isotope signatures (δ¹³C and δ¹⁵N) were used to compare trophic linkages between epilithic periphyton and three families of macroinvertebrates (Baetidae, Leptophlebiidae and Gripopterygidae) in riffles of two rivers with developed catchments (including agriculture, urbanization, impoundment and flow regulation) and two with undeveloped catchments (native forest with no major impoundments) in the Murrumbidgee River system, New South Wales, Australia. Periphyton had much higher average δ¹⁵N values and lower average C:N ratios in the developed rivers than in the undeveloped rivers, probably because of the combined effects of nutrient enrichment, upstream impoundment and alteration of riparian vegetation. The invertebrates were generally slightly depleted in ¹³C and ¹⁵N relative to expected values if they were assimilating whole periphyton alone, which suggests that they were assimilating periphyton components selectively or also consuming other foods. The match in isotope signatures between periphyton and invertebrates was only slightly weaker in the developed than in the undeveloped rivers, suggesting that development did not greatly disrupt trophic linkages between periphyton and these invertebrates. Handling editor: M. Power     

Stable isotopes and trophic positions of littoral fishes from a Mediterranean marine protected area

Stable isotope analyses were employed to explore feeding and foraging habitats and trophic levels of littoral fishes in a western Mediterranean Marine Protected Area (Egadi Islands, Sicily, Italy). Carbon and nitrogen stable isotope ratios were measured in primary producers, invertebrates and fishes collected in December 2001 and January 2002. Fishes of the littoral region of the Egadi Islands had isotopic signatures that fell into a wider range for δ ¹³C (about 6‰) than for δ ¹⁵N (about 3‰). Carbon isotope ratios were consistent with a food web based on mixed sources and two trophic pathways leading to different fish species. Differences in the isotopic composition between islands were higher for benthivorous than for planktivorous fishes. The overall picture gained from this study is of a isotopic distinction between planktivorous and benthivorous fishes, resource partitioning facilitating the coexistence of similar species within the same ecosystem, and spatial variability in the isotopic signatures and trophic level of fishes. Asymmetrical analysis of variance showed that estimated trophic levels were lower in the area with the highest level of protection (Zone A) for only two out of the nine fishes analysed. As a consequence, overall spatial differences do not seem to be a consequence of protection, since in most cases trophic levels did not change significantly between zone A and zones C where professional fishing (trawling apart) is permitted, but of natural sources of variation (e.g. variability in food availability and site-specific food preferences of fishes). However, the results of this study suggest a different response at the species compared to the community level.     

δ13C and δ15N changes after dietary shift in veliger larvae of the slipper limpet Crepidula fornicata: an experimental evidence

δ¹³C and δ¹⁵N measurements are still poorly conducted in benthic invertebrate larvae. To assess the δ¹³C and δ¹⁵N changes occurring after a dietary shift, experiments were conducted on veliger larvae of Crepidula fornicata fed with two cultured microalgae (Isochrysis galbana and Pavlova lutheri) of known isotopic composition, ¹³C-enriched and ¹⁵N-depleted compared to the initial values of the larvae. Rapid changes in larval δ¹³C and δ¹⁵N were observed after the dietary shift, with an increase in δ¹³C and a decrease in δ¹⁵N. After 19 days of feeding, isotopic equilibrium was still not reached, a period which is close to the duration of the pelagic life of the larvae. This implies that the isotopic composition measured in field-collected larvae might only partly reflect actual larval feeding but also the parental isotopic signature, especially during the early developmental stages. Isotopic measurements in marine invertebrate larvae should thus be interpreted cautiously. In planktonic food web investigations, the study of field-collected larvae of different size/developmental stage may reduce potential misinterpretations.     

Stable isotope characterization of mammalian predator–prey relationships in a South African savanna

This paper characterizes predator–prey interactions amongst African mammals from C₄ savanna environments using stable carbon and nitrogen isotope proxies for diet. Stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotope data from hair and faeces of large African mammal carnivores, and herbivores as potential prey, are presented for a diverse range of taxa. Carbon-isotope data imply that most carnivores from the “lowveld” savanna of South Africa form part of C₄ grass-based food webs. Nitrogen isotope data show clear differences between trophic levels, although it appears that the magnitude of these differences varies between predators feeding on invertebrates and vertebrates, respectively. Whilst the number of carnivore samples for which data are available is relatively few, and data for prey are restricted mainly to large ungulate herbivores, results clearly demonstrate the potential for future applications of this technique to predator–prey food webs in African savannas. In tandem with traditional approaches, stable isotopes can help elucidate patterns of predator impacts on prey populations, domestic livestock, and resolving similar food webs in palaeoenvironmental contexts.     

Stable carbon and nitrogen isotope ratio profiling of sperm whale teeth reveals ontogenetic movements and trophic ecology

Teeth from male sperm whales (Physeter macrocephalus) stranded in the North-eastern Atlantic were used to determine whether chronological profiles of stable isotope ratios of C (δ¹³C) and N (δ¹⁵N) across dentine growth layers could be used to detect known ontogenetic benchmarks in movements and trophic ecology. Profiles showed a general decrease in δ¹³C (median = 1.91‰) and an increase in δ¹⁵N (median = 2.42‰) with age. A marked decline in δ¹³C occurred for all 11 teeth around 9–10 years and again for six individuals around 20 years. After the early twenties the δ¹³C continued to decline with age for all teeth. These results are consistent with males segregating from natal groups in low latitudes with the onset of puberty between 4 and 15 years and gradually dispersing pole-ward into ¹³C-depleted temperate waters. Penetration into further depleted, productive high latitudes after the age of 20 might facilitate the spurt of accelerated growth rate observed around this age. Breeding migrations back to lower latitudes were not reflected in the δ¹³C profiles possibly due to being short compared to the time spent feeding in high latitudes. The timings of marked isotopic change in the δ¹⁵N profiles reflect those of the δ¹³C profiles, suggesting a link between dietary changes and movements. The observed increase in δ¹⁵N with age is likely to be caused by a trophic level increase as males grow in size, probably feeding on larger prey. An additional explanation could be that, in the higher latitudes of the North Atlantic, the main prey source is the high trophic level squid Gonatus fabricii. Also, the lower latitudes from where males disperse are depleted in basal ¹⁵N. Profiles of δ¹³C and δ¹⁵N in sperm whale teeth gathered from different regions, sexes, and periods in time, could provide a unique way to understand the ecology of this species across different oceans.     

Stable isotope profiles in whale shark (<Emphasis Type="Italic">Rhincodon typus</Emphasis>) suggest segregation and dissimilarities in the diet depending on sex and size

We investigated the sex- and size-related differences in the diet of whale sharks from the Arabian Sea (north-western Indian Ocean) using carbon and nitrogen stable-isotope analyses in white muscle. The samples were collected during the commercial fishing season between April and May of 2001 in Veraval (Gujarat, India). The overall isotope signature was similar to that of the pelagic-neritic zooplanktivore Ilisha melastoma, which suggests that both species are feeding on similar prey. In whale sharks, a positive relationship was found between δ¹⁵N and δ¹³C. This, together with a significant enrichment of both heavy stable isotopes with total length indicates that the contribution to the diet of small fish and/or larger zooplankton of higher trophic level increases with the movement from offshore areas to coastal areas as they grow. Gender differences in the isotopic ratios were not statistically significant, but small sample size cannot rule out completely the existence of some degree of spatial or dietary segregation between sexes.     

Fine‐scale spatial differences in humpback whale diet composition near Kodiak,Alaska

On the North Pacific feeding grounds, humpback whales (Megaptera novaeangliae) are recovering from commercial whaling at a rapid rate (6.8%). The potential effect that this recovery will have on trophic dynamics involving these predators is currently unknown. To better elucidate complex trophic dynamics, variability in diet composition of apex predators on their respective feeding grounds needs to be understood. Thus, we explored the diet composition of two defined subaggregations of humpback whales of the Kodiak Archipelago population (“North,” “South”) using stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotope ratios of humpback whale skin and regional prey samples in Bayesian dietary mixing models. Humpback whales in the “North” region consumed proportionally more fish, dominated by capelin (Mallotus villosus), whereas, whales in the “South” region consumed predominantly krill. The difference in diet composition appears to reflect regional differences in prey availability. Thus, regional variability in diet composition by humpback whales may have disproportionate impacts on prey resources of sympatric predators depending on available prey biomass. As a result, we suggest fine‐scale studies of apex predator diets are needed to better model trophic dynamics with accuracy.