Fueling phocids: Divergent exploitation of primary energy sources and parallel ontogenetic diet switches among three species of subarctic seals

Determining how marine predators partition resources is hindered by the difficulty in obtaining information on diet and distribution. Stable isotopes (SI) of carbon (¹³C/¹²C, δ¹³C) and nitrogen (¹⁵N/¹⁴N, δ¹⁵N) provide a two‐dimensional estimate of the dietary space of consumers; an animal's isotopic composition is directly influenced by what they consume and where they feed. Harp (Pagophilus groenlandicus) and hooded (Cystophora cristata) seals are abundant phocid species found in the North Atlantic. We measured and contrasted SI values between seals sampled at nearshore and offshore sites to test for effects of sampling location, sex, age‐class, and body size to gain insight into how these species partition space and prey resources. In addition we contrasted previously published results for gray seals (Halichoerus grypus). Isotope values differed significantly by age class and location in harp and hooded seals. We found significant differences in SI values (mean δ¹³C and δ¹⁵N ± SE) between all species. Hooded seals, a continental shelf‐edge, deep‐diving species, exhibited low SI values (juveniles: ?20.9‰ ± 0.03‰, 13.36‰ ± 0.05‰; adults: ?20.41‰ ± 0.03‰, 14.81‰ ± 0.04‰) characteristic of feeding on meso‐ to bathypelagic prey. Harp seals, which dive to moderate depths primarily on the shelf had intermediate SI values (juveniles: ?20.53‰ ± 0.01‰, 13.91‰ ± 0.01‰; adults: ?20.13‰ ± 0.01‰, 14.96‰ ± 0.01‰) characteristic of feeding on epipelagic prey, whereas gray seals, which feed on or near the sea floor in shallow shelf waters, had high SI values (juveniles: ?19.74‰ ± 0.04‰, 17.51‰ ± 0.05‰; adults: ?18.86‰ ± 0.01‰, 17.23‰ ± 0.02‰) characteristic of feeding on demersal prey. In all species, δ¹³C values increased with body size and age in the same manner, indicating that seals exploit or forage in deeper habitats as they get larger and older. We hypothesize that the consistent ontogenetic shift in foraging niche, despite large differences between species in their diving behavior, geographic range and habitat use, not only reflects increased access to different prey due to increased diving capacity, but a progressive adjustment to balance energy budgets by reducing foraging costs.     

Isotopic niche differs between seal and fish‐eating killer whales (Orcinus orca) in northern Norway

Ecological diversity has been reported for killer whales (Orcinus orca) throughout the North Atlantic but patterns of prey specialization have remained poorly understood. We quantify interindividual dietary variations in killer whales (n = 38) sampled throughout the year in 2017–2018 in northern Norway using stable isotopic nitrogen (δ¹⁵N: ¹⁵N/¹⁴N) and carbon (δ¹³C: ¹³C/¹²C) ratios. A Gaussian mixture model assigned sampled individuals to three differentiated clusters, characterized by disparate nonoverlapping isotopic niches, that were consistent with predatory field observations: seal‐eaters, herring‐eaters, and lumpfish‐eaters. Seal‐eaters showed higher δ¹⁵N values (mean ± SD: 12.6 ± 0.3‰, range = 12.3–13.2‰, n = 10) compared to herring‐eaters (mean ± SD: 11.7 ± 0.2‰, range = 11.4–11.9‰, n = 19) and lumpfish‐eaters (mean ± SD: 11.6 ± 0.2‰, range = 11.3–11.9, n = 9). Elevated δ¹⁵N values for seal‐eaters, regardless of sampling season, confirmed feeding at high trophic levels throughout the year. However, a wide isotopic niche and low measured δ¹⁵N values in the seal‐eaters, compared to that of whales that would eat solely seals (δ_N‐measured = 12.6 vs. δ_N‐expected = 15.5), indicated a diverse diet that includes both fish and mammal prey. A narrow niche for killer whales sampled at herring and lumpfish seasonal grounds supported seasonal prey specialization reflective of local peaks in prey abundance for the two fish‐eating groups. Our results, thus, show differences in prey specialization within this killer whale population in Norway and that the episodic observations of killer whales feeding on prey other than fish are a consistent behavior, as reflected in different isotopic niches between seal and fish‐eating individuals.     

Niche overlap and diet composition of three sympatric coastal dolphin species in the southwest Atlantic Ocean

Sympatric species are expected to differ in ecological requirements to minimize niche overlap and avoid competition. Here we assess the trophic interactions among three coexisting dolphin species from southern Brazil: the franciscana dolphin (Pontoporia blainvillei), the Guiana dolphin (Sotalia guianensis), and the Lahille's bottlenose dolphin (Tursiops truncatus gephyreus). We evaluated temporal variation in carbon (δ¹³C) and nitrogen (δ¹⁵N) isotope values of bone collagen to examine potential dietary shifts resulting from increased fishing activity over the past three decades. We estimated the degree of niche overlap among these species and the contribution of potential prey sources to their diet. δ¹⁵N values were consistent among species and across years, while δ¹³C values increased for Guiana dolphins and decreased for bottlenose dolphins, suggesting changes in diet and/or foraging habitats through time. The similar δ¹³C and δ¹⁵N values and the high niche overlap between Guiana and bottlenose dolphins indicate that these species are primarily feeding on demersal prey. The franciscana diet is primarily composed of pelagic prey, resulting in a lower niche overlap in comparison with the other dolphin species. Our study provides further information about the foraging ecology of this unique dolphin community in southern Brazil with implications for its management and conservation.     

Using Satellite Tracking and Isotopic Information to Characterize the Impact of South American Sea Lions on Salmonid Aquaculture in Southern Chile

Apex marine predators alter their foraging behavior in response to spatial and/or seasonal changes in natural prey distribution and abundance. However, few studies have identified the impacts of aquaculture that represents a spatially and temporally predictable and abundant resource on their foraging behavior. Using satellite telemetry and stable isotope analysis we examined the degree of spatial overlap between the South American sea lion (SASL) and salmon farms, and quantify the amount of native prey versus farmed salmonids in SASL diets. We instrumented eight SASL individuals with SRDL-GPS tags. Vibrissae, hair and skin samples were collected for δ¹³C and δ¹⁵N analyses from five of the tagged individuals and from four males captured in a haul-out located adjacent to salmon farms. Tracking results showed that almost all the foraging areas of SASL are within close proximity to salmon farms. The most important prey for the individuals analyzed was farmed salmonids, with an estimated median (±SD) contribution of 19.7 ± 13.5‰ and 15.3 ± 9.6‰ for hair and skin, respectively. Using vibrissae as a temporal record of diet for each individual, we observed a remarkable switch in diet composition in two SASL, from farmed salmonids to pelagic fishes, which coincided with the decrease of salmon production due to the infectious salmon anemia virus that affected salmon farms in Chile at the end of 2008. Our study demonstrates the usefulness of integrating stable isotope derived dietary data with movement patterns to characterize the impacts of a non-native prey on the foraging ecology of an apex marine predator, providing important applied implications in situations where interactions between aquaculture and wildlife are common.     

Trophic interaction between striped marlin and swordfish using different timescales in waters around Baja California Sur,Mexico

Isotopic niche and resource partitioning were examined between striped marlin (Kajikia audax) and swordfish (Xiphias gladius) using stable isotope ratios from spine bone and muscle samples. The δ¹³C (¹³C/¹²C) and δ¹⁵N (¹⁵N/¹⁴N) values were measured in annual growth bands from fin spines and muscle collected from striped marlin and swordfish off Baja California Sur to evaluate the trophic interaction between these two species, the relative contribution of the main prey and the isotopic niche within two different timescales. Mean trophic level (TL) values were different when tissues were compared with the highest value found in muscle samples from K. audax. When TL was analysed in all growth bands for both species, no changes were detected throughout time, suggesting only minor differences in feeding habits between these species. We determined that Dosidicus gigas, Sthenoteuthis oualaniensis, and Ancistrocheirus lesueurii were the most important prey for both species, but the relative contribution of each of them to the diet differed. There is likely no trophic overlap between K. audax and X. gladius off Baja California Sur, as K. audax occupied a larger isotopic niche with more diverse prey than X. gladius.     

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.     

Reductions in the dietary niche of southern sea otters (Enhydra lutris nereis) from the Holocene to the Anthropocene

The sea otter (Enhydra lutris) is a marine mammal hunted to near extinction during the 1800s. Despite their well‐known modern importance as a keystone species, we know little about historical sea otter ecology. Here, we characterize the ecological niche of ancient southern sea otters (E. lutris nereis) using δ¹³C analysis and δ¹⁵N analysis of bones recovered from archaeological sites spanning ~7,000 to 350 years before present (N = 112 individuals) at five regions along the coast of California. These data are compared with previously published data on modern animals (N = 165) and potential modern prey items. In addition, we analyze the δ¹⁵N of individual amino acids for 23 individuals to test for differences in sea otter trophic ecology through time. After correcting for tissue‐specific and temporal isotopic effects, we employ nonparametric statistics and Bayesian niche models to quantify differences among ancient and modern animals. We find ancient otters occupied a larger isotopic niche than nearly all modern localities; likely reflecting broader habitat and prey use in prefur trade populations. In addition, ancient sea otters at the most southerly sites occupied an isotopic niche that was more than twice as large as ancient otters from northerly regions. This likely reflects greater invertebrate prey diversity in southern California relative to northern California. Thus, we suggest the potential dietary niche of sea otters in southern California could be larger than in central and northern California. At two sites, Año Nuevo and Monterey Bay, ancient otters had significantly higher δ¹⁵N values than modern populations. Amino acid δ¹⁵N data indicated this resulted from shifting baseline isotope values, rather than a change in sea otter trophic ecology. Our results help in better understanding the contemporary ecological role of sea otters and exemplify the strength of combing zooarchaeological and biological information to provide baseline data for conservation efforts.     

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.     

Elemental turnover rates and trophic discrimination in juvenile Lebranche mullet Mugil liza under experimental conditions

The aim of this study was to determine the isotopic‐turnover rate (R_IT) and trophic‐discrimination factor (F_TD) in muscle tissues of Lebranche mullet Mugil liza fed an experimental diet (δ¹³C = ?27·1‰; δ¹⁵N = 1·0‰). Juvenile M. liza exhibited a relatively fast R_IT, with a half‐life (t₅₀) of only 16 and 14 days for δ¹³C and δ¹⁵N respectively and a nearly complete isotopic turnover (t₉₅) of 68 and 60 days for δ¹³C and δ¹⁵N.     

Shared dietary niche between sexes in Magellanic Penguins

Prey use by male and female Magellanic Penguins (Spheniscus magellanicus) was quantified during chick rearing in northern Golfo San Jorge, Argentina and the dietary niches and diet composition of each sex were assessed and compared. A total of 67 males and 45 females were flushed during the chick stages of 2011 and 2012. Argentine Anchovy (Engraulis anchoita) was the main prey in terms of importance by mass in males and females in all stages and years (52.4–86.6% and 83.8–88.7%, respectively), except during the old chick stage in 2011 when the main prey in males and females was the Shortfin Squid (Illex argentinus) (63.1% and 44.4%, respectively). Analyses of similarity showed that diet composition in terms of contribution by mass of the main prey species was similar between sexes. Based on the isotopic values of δ¹³C and δ¹⁵N corresponding to 38 males and 21 females, the Bayesian mixing model outputs showed that Argentine Anchovy was also the main prey. In both years and chick stages, the isotopic niche of males and females was similar. The similarity in the dietary niche between male and female Magellanic Penguins raising chicks may be due to the high availability of Argentine Anchovy during the study period and the consequent dominance of this forage species in the diet of both sexes.     

Niche dynamics of sympatric non-breeding shearwaters under varying prey availability

Variation in prey availability can cause changes in species interactions among marine predators. Foraging theory predicts that niche breadth will expand when resources become limited, possibly leading to higher niche overlap among sympatric species; however, a species’ niche can become constrained by interactions with other similar species, resulting in an inability to shift niche breadth or position. In coastal Newfoundland, Capelin Mallotus villosus is the main forage fish species and its availability (i.e. biomass) during the summer has varied considerably following a population collapse in the 1990s. During the summer, non-breeding Great and Sooty Shearwaters Ardenna gravis and A. grisea migrate and aggregate at annually persistent Capelin spawning sites. We used stable isotope ratios (δ¹³C, δ¹⁵N) of blood components (plasma, red blood cells) to investigate variation in isotopic niche breadth (95% prediction ellipse areas) and overlap of the two shearwater species during 2014, 2015 and 2016. Capelin availability varied among years, illustrated by lower peak biomass in 2015 (0.126 g/m²) and 2016 (0.027 g/m²) relative to 2014 (0.254 g/m²). The isotopic niche breadth (plasma) of both shearwater species expanded similarly from 2014 (0.65–0.66‰²) to 2015 (2.22–2.57‰²) and 2016 (1.15–1.42‰²), suggesting the incorporation of alternative prey types into their diet during years of lower prey availability. Isotopic niche overlap between Great and Sooty Shearwaters remained high across years (44–63%), however, providing little evidence for dietary niche partitioning during years of lower prey availability. Findings suggest that both shearwater species are flexible foragers and can modify their diet during the non-breeding season to accommodate fluctuations in prey availability.     

INVESTIGATING TROPHIC RELATIONSHIPS OF PINNIPEDS IN ALASKA AND WASHINGTON USING STABLE ISOTOPE RATIOS OF NITROGEN AND CARBON

We measured stable-nitrogen (δ¹⁵N) and stable-carbon (δ¹³C) isotope ratios in muscle and hair from 7 northern fur seals (Callorhinus ursinus) from the Pribilof Islands, Alaska, and 27 Steller sea lions (Eumetopias jubatus), and 14 harbor seals (Phoca vitulina) from the Gulf of Alaska and coast of Washington State, in order to contrast dietary information derived from isotopic vs. available conventional dietary studies. Stable-nitrogen-isotope analysis of muscle revealed that harbor seals were enriched over sea lions (mean δ¹⁵N = 18.6‰vs. 17.5‰) which were in turn enriched over northern fur seals (mean δ¹⁵N = 16.6‰). Trophic segregation among these species likely results primarily from differential reliance on herring (Clupea harengus), Atka mackerel (Pleurogrammus monopterygius), and large vs. small walleye pollock (Theregra chalcogramma). According to their δ¹⁵N values, adult male Steller sea lions showed a higher trophic position than adult females (mean δ¹⁵N: 18.0‰vs. 17.2‰), whereas adult female northern fur seals were trophically higher than juvenile male fur seals (mean δ¹⁵N: 16.5‰vs. 15.0‰). Each of these observed differences likely resulted from differential reliance on squid or differences in the size range of pollock consumed. Three northern fur seal pups showed higher δ¹⁵N enrichment over adults (mean 17.7‰vs. 15.8‰) due to their reliance on their mother's milk. Stable-carbon isotope measurements of hair revealed a cline toward more negative values with latitude. Segregation in hair δ¹³C between Steller sea lions and harbor seals off the coast of Washington (mean δ¹³C: ?13.6‰vs.?15.0‰) reflected the greater association of harbor seals with freshwater input from the Columbia River. Our study demonstrates the utility of the stable isotope approach to augment conventional dietary analyses of pinnipeds and other marine mammals.     

Trophic level delineation and resource partitioning in a South African afromontane forest bird community using carbon and nitrogen stable isotopes

Southern African forests are naturally fragmented yet hold a disproportionately high number of bird species. Carbon and nitrogen stable isotopes were measured in feathers from birds captured at Woodbush (n = 27 species), a large afromontane forest in the eastern escarpment of Limpopo province, South Africa. The δ¹³C signatures of a range of forest plants were measured to categorise the food base. Most plants sampled, including two of five grass species, had δ¹³C signatures typical of a C₃ photosynthetic pathway (?29.5 ± 1.9‰). Three grass species had a C₄ signature (?12.0 ± 0.6‰). Most bird species had δ¹³C values representing a predominantly C₃‐based diet (?24.8‰ to ?20.7‰). δ¹⁵N values were as expected, with higher levels of enrichment associated with a greater proportion of dietary animal matter. The cohesive isotopic niche defining most species (n = 22), where the ranges for δ¹³C and δ¹⁵N were 2.4‰ and 3.4‰, respectively, highlight the difficulties in understanding diets of birds in a predominantly C₃‐based ecosystem using carbon and nitrogen stable isotopes. However, variation in isotopic values between and within species provides insight into possible niche width and the use of resources by different birds within a forest environment.     

Movement,connectivity and population structure of the intertidal fish Lipophrys pholis as revealed by otolith oxygen and carbon stable isotopes

The shanny Lipophrys pholis is an intertidal fish commonly found in Portuguese coastal waters. Spawning takes place from early autumn to mid spring, after which demersal eggs hatch and larvae disperse along the coast. Two to three months later, young juveniles return to the tide pools to settle. However, information on fish movement, habitat connectivity and population structure is scarce for this species. One hundred and twenty early juveniles (16–35?mm) were collected in April 2014 from six rocky beaches along the western and south Portuguese coasts (Agudela, Cabo do Mundo, Boa Nova, Peniche, Sines and Olhos de Água). δ¹⁸O and δ¹³C were determined by isotope-ratio mass spectrometry. Data were analysed to determine whether isotopic signatures could be used to assess the degree of separation between individuals collected from different locations. Mean δ¹³C and δ¹⁸O values ranged from ?0.02‰ to 1.14‰ and ?7.77‰ to ?6.62‰, respectively. Both seawater temperature and salinity caused differences in otolith δ¹⁸O among the four main sampling areas. The variation among areas in δ¹³C was most likely related to slight differences in the diet, growth and metabolism of fish. The distinct isotopic signatures, at least for the northern and central areas, suggested low levels of connectivity across large spatial scales during the juvenile stage. Furthermore, similar isotopic signatures within the same area indicated some degree of larval oceanic retention at short spatial scales. This study suggests that stable isotope records in otoliths could provide information about the home residency, movements and habitat connectivity of intertidal fishes.     

Trophic shift in young‐of‐the‐year Mugilidae during salt‐marsh colonization

This study investigated the trophic shift of young‐of‐the‐year (YOY) thinlip grey mullet Liza ramada and golden grey mullet Liza aurata during their recruitment in a salt marsh located on the European Atlantic Ocean coast. Stable‐isotope signatures (δ¹³C and δ¹⁵N) of the fishes followed a pattern, having enrichments in ¹³C and ¹⁵N with increasing fork length (L_F): δ¹³C in fishes < 30 mm ranged from ?19.5 to ?15.0‰, whereas in fishes > 30 mm δ¹³C ranged from ?15.8 to ?12.7‰, closer to the level in salt‐marsh food resources. Large differences between the δ¹⁵N values of mugilids and those of food sources (6·0‰ on average) showed that YOY are secondary consumers, similar to older individuals, when feeding in the salt marsh. YOY mugilids shift from browsing on pelagic prey to grazing on benthic resources from the salt marsh before reaching 30 mm L_F. The results highlight the role of European salt marshes as nurseries for juvenile mugilids.     

Stable isotope enrichment in laboratory ant colonies: effects of colony age,metamorphosis, diet,and fat storage

Ecologists use stable isotopes to infer diets and trophic levels of animals in food webs, yet some assumptions underlying these inferences have not been thoroughly tested. We used laboratory‐reared colonies of Solenopsis invicta Buren (Hymenoptera: Formicidae: Solenopsidini) to test the effects of metamorphosis, diet, and lipid storage on carbon and nitrogen stable isotope ratios. Effects of metamorphosis were examined in ant colonies maintained on a control diet of domestic crickets and sucrose solution. Effects of a diet shift were evaluated by adding a tuna supplement to select colonies. Effects of lipid content on stable isotopes were tested by treating worker ants with polar and non‐polar solvents. δ¹³C and δ¹⁵N values of larvae, pupae, and workers were measured by mass spectrometry on whole‐animal preparations. We found a significant effect of colony age on δ¹³C, but not δ¹⁵N; larvae, pupae, and workers collected at 75 days were slightly depleted in ¹³C relative to collections at 15 days (Δδ¹³C = ?0.27‰). Metamorphosis had a significant effect on δ¹⁵N, but not δ¹³C; tissues of each successive developmental stage were increasingly enriched in ¹⁵N (pupae, +0.5‰; workers, +1.4‰). Availability of tuna resulted in further shifts of about +0.6‰ in isotope ratios for all developmental stages. Removing fat with organic solvents had no effect on δ¹³C, but treatment with a non‐polar solvent resulted in enriched δ¹⁵N values of +0.37‰. Identifying regular patterns of isotopic enrichment as described here should improve the utility of stable isotopes in diet studies of insects. Our study suggests that researchers using ¹⁵N enrichment to assess trophic levels of an organism at different sites need to take care not to standardize with immature insect herbivores or predators at one site and mature ones at another. Similar problems may also exist when standardizing with holometabolous insects at one site and spiders or hemimetabolous insects at another site.     

Foraging ecology and niche overlap in pygmy (Kogia breviceps) and dwarf (Kogia sima) sperm whales from waters of the U.S. mid‐Atlantic coast

A complementary approach of stomach content and stable isotope analyses was used to characterize the foraging ecology and evaluate niche overlap between pygmy (Kogia breviceps) and dwarf (K. sima) sperm whales stranded on the U.S. mid‐Atlantic coast between 1998 and 2011. Food habits analysis demonstrated both species were primarily teuthophagous, with 35 species of cephalopods, and 2 species of mesopelagic fishes represented in their overall diets. Pianka's Index of niche overlap suggested high overlap between whale diets (O_n = 0.92), with squids from the families Histioteuthidae, Cranchidae, and Ommastrephidae serving as primary prey. Pygmy sperm whales consumed slightly larger prey sizes (mean mantle length [ML] = 10.8 cm) than dwarf sperm whales (mean ML = 7.8 cm). Mean prey sizes consumed by pygmy sperm whales increased with growth, but showed no trend in dwarf sperm whales. Significant differences were not detected in δ¹⁵N and δ¹³C values of muscle tissues from pygmy (10.8‰ ± 0.5‰, ?17.1‰ ± 0.6‰), and dwarf sperm whales (10.7‰ ± 0.5‰, ?17.0‰ ± 0.4‰), respectively. Isotopic niche widths also did not differ significantly and dietary overlap was high between the two species. Results suggest the feeding ecologies of the pygmy and dwarf sperm whales are similar and both species occupy equivalent trophic niches in the region.     

Altitudinal gradients of grassland carbon and nitrogen isotope composition are recorded in the hair of grazers

Aim The hair of grazers provides an isotopic record of environmental and nutritional signals. Here, we assess the effect of altitude on the carbon and nitrogen isotope composition of the hair of ruminant grazers and its relation to grassland vegetation, to evaluate the use of hair isotope data for ecosystem reconstruction, animal nutritional ecology and biogeochemical studies in montane environments. Location European Alps. Methods We sampled grassland vegetation (pure C₃) and the hair of ruminants along an altitudinal gradient (400–2500 m), and analysed their isotope composition (δ¹³C and δ¹⁵N). Results were compared with published effects of altitude on ¹³C in C₃ plants at the species level and on ¹⁵N at the community level. The study was complemented with a comparison of diet and hair isotope composition in ruminants held in confinement. Results δ¹³C of hair increased (c. 1.1‰ km⁻¹) and δ¹⁵N decreased (c. 1.1‰ km⁻¹) with altitude. The same changes occurred in local grassland vegetation, and in regional to global grassland data sets. Offsets between hair and vegetation ¹³C or ¹⁵N (‘diet–hair shift’) were independent of altitude. Sheep (Ovis aries) and cattle (Bos taurus) exhibited a ¹³C shift near +3‰, but that of goats (Capra hircus) was larger (+4.2‰) in alpine environments and in confinement. The diet–hair shift for ¹⁵N was more variable (+2.1 to +3.6‰). Main conclusions Grazer hair provides a faithful spatially and temporally integrated record of grassland isotope composition, useful for ecosystem and environment reconstruction. The effect of altitude on hair ¹⁵N is important for studies of trophic relationships: an altitude shift of 2000 m produced the same effect in hair ¹⁵N as would a shift from an animal tissue‐based to a plant‐based diet. The similarity of altitude effects on δ¹³C of individual plant species, vegetation and hair indicates that the effect of altitude on species‐level ‘intrinsic water use efficiency’ scales up linearly to the community and landscape level.     

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.     

Seabird species‐ and assemblage‐level isotopic niche shifts associated with changing prey availability during breeding in coastal Newfoundland

Shifting prey availability can lead to altered species interactions, indicated by variation in the dietary niche breadth and position of species within an assemblage. On the Newfoundland coast, annual inshore spawning migration of the dominant forage fish, Capelin Mallotus villosus, provides an excellent opportunity to investigate the influence of varying prey availability on dietary niche breadth and position among species. During June–August 2017, we investigated species‐ and assemblage‐level dietary responses to shifting Capelin availability of three Capelin‐eating, sympatrically breeding auk species, the Atlantic Puffin Fratercula arctica, Razorbill Alca torda and Common Murre Uria aalge. The diet of Leach's Storm Petrels Oceanodroma leucorhoa, which breed alongside the three auk species but are not known to rely on Capelin, was also examined to determine dietary shifts throughout breeding that were unrelated to Capelin availability. We quantified stable isotope ratios (δ¹⁵N, δ¹³C) in seabird blood components (plasma, cellular component) collected both before and after spawning Capelin arrived in the study area and compared isotopic niche breadth within a Bayesian framework. At the species level, auk trophic position increased and isotopic niche breadth narrowed after Capelin arrived, suggesting a more Capelin‐based diet. Simultaneously, trophic diversity of the auk assemblage, reflecting the extent of spacing among niches of species, decreased after spawning Capelin arrived inshore. Contrastingly, increased trophic position but broader isotopic niche breadth during higher relative to lower Capelin availability for Leach's Storm Petrel confirm that this species is probably not affected by the inshore arrival of Capelin, but instead that isotopic changes may be more related to a shift in breeding stage to chick‐rearing. Overall, our findings reiterate the importance of Capelin as a prey resource for breeding auks in coastal Newfoundland, but that the degree of reliance on Capelin varies among species, possibly allowing coexistence of these ecologically similar species. The findings highlight potential changing species interactions, such as increased competition, under declines in Capelin biomass.