Seals and sea lions are what they eat, plus what? Determination of trophic discrimination factors for seven pinniped species

The influence of biopsy site and pregnancy on stable isotope ratios in humpback whale skin

RATIONALE

Stable isotope analysis (SIA) of free-swimming mysticetes using biopsies is often limited in sample size and uses only one sample per individual, failing to capture both intra-individual variability and the influence of demographic and physiological factors on isotope ratios.

METHODS

We applied SIA of δ13C and δ15N to humpback whale (Megaptera novaeangliae) biopsies taken during the foraging season along the western Antarctic Peninsula to quantify intra-individual variation from repeatedly sampled individuals, as well as to determine the effect of biopsy collection site, sex, and pregnancy on isotope ratios.

RESULTS

There was substantial variability in δ13C from multiple biopsies taken from the same individuals, though δ15N was much more consistent. Side of the body (left versus right) and biopsy location (dorsal, anterior, ventral, and posterior) did marginally affect the isotopic composition of δ15N but not δ13C. Pregnancy had a significant effect on both δ13C and δ15N, where pregnant females were depleted in both when compared to non-pregnant females and males.

CONCLUSIONS

These results indicate that isotopic signatures are influenced by multiple endogenous and exogenous factors and emphasize value in accounting for intra-individual variability and pregnancy status within a sampled population. Placed within an ecological context, the endogenous variability in δ13C observed here may be informative for future isotopic analyses.

Tracing the Food Web of Changing Arctic Ocean: Trophic Status of Highly Abundant Fish, Gasterosteus aculeatus (L.), in the White Sea Recovered Using Stomach Content and Stable Isotope Analyses

Studies of dietary preferences of migratory species are of great importance as these species connect food webs of habitats across the migration route and thus represent trophic relationships between the spatially disjointed communities. Here we described the dietary preferences of threespine stickleback G. aculeatus in the White Sea during the spawning season using stable isotope and stomach content analyses. The two analyses suggested that during the spawning season, when sticklebacks spend the majority of their time inshore, their diet consists mostly of benthic species, while at the beginning of the spawning season when fish migrating from the offshore were feeding on plankton. Additionally, we demonstrated that stickleback eggs contributed greatly to the diet of both male and female fish. Using Bayesian mixing modeling, we showed that dietary preferences in females were broader than in males, and more variable during the spawning season. While guarding their nests, males fed almost exclusively on eggs. Both stomach contents and isotope signatures demonstrate that by the end of the spawning season sticklebacks again increase the consumption of plankton. Isotope analysis proved to be a more reliable tool to trace this change than stomach content analysis. Our results show that stable isotope and stomach content analyses are complementary in understanding seasonal changes in the dietary composition of stickleback.

Growth and Shedding of Harbor Seal (Phoca vitulina) Whiskers

Accurate and precise estimates of the timing of whisker growth and shedding are necessary to interpret biochemical information stored in whiskers. This fine-scale examination of harbor seal (Phoca vitulina Linnaeus, 1758) whisker growth and shedding identified an asymptotic growth pattern with initial rapid growth that decays over time that can be defined with the von Bertalanffy curve. Initial growth rates were similar among whiskers, but estimated growth coefficients (K) and asymptotic lengths (L_∞ ) differed by follicle positions suggesting that differences in total whisker lengths result from dissimilarly shaped growth curves. In other words, longer whisker length is attained by delaying the growth rate decay. There was substantial intra- and inter-seal variation in shedding dates; whisker shedding began at the caudal margin of the whisker bed and progressed toward the nose. Shedding of marked whiskers from the three study seals took from 78 to 133 d, however, selecting whiskers only from the most caudal follicle of the bottom three rows constrained the period of whisker shedding to 7 to 43 d. These differences in growth and shedding of harbor seal whiskers emphasize the importance of considering follicle position to select whiskers that are the most similar for analyses of information stored in whiskers.

Leopard seal diets in a rapidly warming polar region vary by year, season, sex, and body size

Background

Resolving the preferred prey items and dietary proportions of leopard seals is central to understanding food-web dynamics in the rapidly-warming Antarctic Peninsula region. Previous studies have identified a wide range of prey items; however, due to anecdotal or otherwise limited information, leopard seal diets remain unresolved by seal sex, individual, body size, region, and season. Over the 2013, 2014, and 2017 field seasons we collected scat, tissue samples (red blood cells and plasma; n = 23) for stable isotope analyses, and previously-reported animal-borne video from 19 adult leopard seals foraging near mesopredator breeding colonies at Cape Shirreff, Livingston Island. We summarized a priori diet information from scat and video analysis and applied a three-isotope (δ¹³C, δ¹⁵N, δ³⁴S), four-source (fish, fur seal, krill, penguin) Bayesian mixing model to examine temporal variability in both prey sources and leopard seal tissues.

Results

The austral spring diets of males and females focused on Antarctic krill (31.7–38.0%), notothen fish (31.6–36.5%), and penguin (24.4–26.9%) and were consistent across all 3 years. Several lines of evidence suggest the transition to summer foraging was distinct for males and females. Female diets transitioned rapidly to higher δ¹⁵N values (+2.1‰), indicating increased consumption of penguin (29.5–46.2%) and energy-dense Antarctic fur seal pup (21.3–37.6%).

Conclusions

The seasonal increase in leopard seal δ¹⁵N values, and thus fur seal in their diet, was predictably related to larger body size; it may also be forcing reductions to the largest Antarctic fur seal colony in the Antarctic Peninsula. Our ensemble sampling approach reduces historical biases in monitoring marine apex predator diets. Further, our results are necessary to best inform regional fisheries management planning.

Seasonal variability and individual consistency in gray seal (Halichoerus grypus) isotopic niches

Although it is often assumed that individuals in generalist populations are equivalent, recent research indicates that individual dietary specialization can be common in marine predators. Gray seals (Halichoerus grypus (Fabricius, 1791)) were considered locally extinct in United States waters by 1958 but have since recolonized the region. Although considered generalists, less is known about gray seal foraging ecology in the United States. To address this, we used carbon and nitrogen stable isotope analyses to investigate the foraging niches of adult gray seals in Massachusetts, USA. We examined skin, fur, and blood components to investigate seasonal variability and individual consistency in foraging niches, and serially sampled vibrissae to quantify the degree of individual foraging specialization in this population. Our results suggest that seals shift from coastal foraging habitats before molt to offshore habitats after molt, with a coincident shift from higher to lower trophic-level prey. Adult gray seals also exhibited individual consistency in foraging niches independent of population-level shifts and reflect a generalist population composed of individual foraging specialists. These findings serve as a baseline for subsequent research on gray seals in United States waters that could help to determine the mechanisms which promote individual specialization in this population.

Evaluating the use of stable isotope analysis to infer the feeding ecology of a growing US gray seal (Halichoerus grypus) population

Gray seals (Halichoerus grypus) have been rapidly recolonizing the Northeast US coast, eliciting concern from the fishing industry. However, the ecological effect of this recovery is still unknown and as such, research is needed to better understand how the diet composition of gray seals in US waters will contribute to the ecological impact. While previous research on seal diets has focused on the analysis of hard prey remains, stable isotope analysis presents an alternative method that can be used to describe marine mammal diets when direct observation is impossible. To address this issue, we used stable isotope analysis of gray seal pup vibrissae and lanugo from Monomoy Island, Cape Cod, MA during the 2015/2016 winter breeding season to estimate adult female diet composition during pregnancy. Stable isotope mixing models (SIMM) suggested adult female gray seals were consuming greater amounts of cephalopod prey and less sand lance than previously indicated from analysis of hard prey remains. However, using SIMMs to estimate the diet composition of gray seals remains difficult due to the large number of isotopically similar prey species and uncertainty in tissue-specific, stable isotope trophic enrichment factors. Even so, by combining prey sources into ecologically informative groups and integrating prior information into SIMMs it is possible to obtain additional insights into the diet of this generalist predator.

Growth rate and stable carbon and nitrogen isotope trophic discrimination factors of lion and leopard whiskers

RATIONALE

Stable isotope analysis (SIA) of whiskers has been used to identify temporal feeding habits, intra-population diet variation, as well as individual dietary specialisation of marine and terrestrial carnivores. However, the potential of the method to disclose such dietary information for large wild felids is hampered by lack of information on species-specific whisker growth rates, whisker growth patterns and whisker-diet trophic discrimination factors (TDFs).

METHODS

Whisker growth rates and growth patterns were measured for four lions (Panthera leo) and one leopard (Panthera pardus) held at the National Zoological Gardens, Pretoria, South Africa. Actively growing whiskers of the felids were 'marked' four times over 185 days using ¹³ C-depleted, C₃ -based giraffe (Giraffa camelopardalis) meat. The periods with low δ¹³ C values, identified following serial sectioning of the regrown whiskers at 1 mm intervals and isotopic analysis, were then correlated to specific giraffe meat feeding bouts and hence growth periods. δ¹³ C and δ¹⁵ N whisker-diet TDFs were estimated for five lions whose diet remained consistent over multiple years.

RESULTS

The whisker growth rates of three lionesses and the leopard were similar (mean = 0.65 mm day^-1 ), despite species, sex and age differences. There was a decrease in whisker growth rate over time, suggesting a non-linear whisker growth pattern. However, linear and non-linear growth simulations showed slight differences between the two growth patterns for the proximal ~50 mm of whiskers. δ¹³ C and δ¹⁵ N lion whisker-diet TDFs were also similar amongst individuals (mean = 2.7 ± 0.12 ‰ for δ¹³ C values and 2.5 ± 0.08 ‰ for δ¹⁵ N values), irrespective of age and sex.

CONCLUSIONS

The whisker growth rate and δ¹³ C and δ¹⁵ N lion whisker-diet TDFs obtained in this study can be applied in future studies to assign dietary information contained in analysed felid whiskers to the correct time period and improve deductions of prey species consumed by wild felids.

Carbon, nitrogen and sulphur isotopic fractionation in captive juvenile hooded seal (Cystophora cristata): Application for diet analysis

RATIONALE

Intrinsic biogeochemical markers, such as stable isotope ratios of carbon, nitrogen and sulphur, are increasingly used to trace the trophic ecology of marine top predators. However, insufficient knowledge of fractionation processes in tissues continues to hamper the use of these markers.

METHODS

We performed a controlled feeding experiment with eight juvenile hooded seals (Cystophora cristata) that were held on a herring-based diet (Clupea harengus) for two years. Stable isotope ratios were measured via isotope ratio mass spectrometry in three of their tissues and related to values of these markers in their diet.

RESULTS

Diet-tissue isotope enrichment (trophic enrichment factor, TEF) values between dietary herring and seal tissues for carbon (Δ¹³ C) were +0.7 ‰ for red blood cells, +1.9 ‰ for hair and +1.1 ‰ for muscle. The TEFs for nitrogen trophic (Δ¹⁵ N) were +3.3 ‰ for red blood cells, +3.6 ‰ for hair and +4.3 ‰ for muscle. For sulphur, the Δ³⁴ S values were +1.1 ‰ for red blood cells, +1.0 ‰ for hair and +0.9 ‰ for muscle.

CONCLUSIONS

These enrichment values were greater than those previously measured in adult seals. This increase may be related to the higher rate of protein synthesis and catabolism in growing animals. This study is the first report on sulphur isotope enrichment values for a marine mammal species.