Sex differences in grey seal diet reflect seasonal variation in foraging behaviour and reproductive expenditure: evidence from quantitative fatty acid signature analysis

Sources of intraspecific variation in the isotopic niche of a semi-aquatic predator in a human-modified landscape

Intraspecific variation modulates patterns of resource use by species, potentially affecting the structure and stability of food webs. In human-modified landscapes, habitat disturbance modifies trophic interactions and intraspecific niche variation, impacting population persistence. Here, we investigated the relationship of sex, ontogeny, and habitat factors with the trophic niche of Caiman crocodilus in an agricultural landscape. We evaluated temporal variation in the trophic niche parameters using carbon and nitrogen stable isotope analysis from different body tissues. We found that caimans exploit the same carbon and nitrogen pools through time, with low isotopic variability between seasons, partly due to the slow isotope turnover rates of tissues in crocodilians. Conversely, the trophic niche of caimans varied across habitats, but with no evidence of a difference between natural and anthropogenic habitats. It apparently results from the influence of habitat suitability, connectivity, and caiman movements during the foraging. Our findings highlight the broader niches of juvenile caimans relative to adults, possibly in response of territorialism and opportunistic foraging strategy. Although using similar resources, females had a larger niche than males, probably associated with foraging strategies during nesting. Considering the sex and body size categories, caimans occupied distinct isotopic regions in some habitats, indicating apparent niche segregation. Ontogenetic trophic shifts in the isotopes (δ13C and δ15N) depended on sex, leading to resource partitioning that can potentially reduce intraspecific competition. Decision-makers and stakeholders should consider the trophic dynamics of sex and body size groups for the sustainable management and conservation of caiman populations, which implies in the maintenance of wetland habitats and landscape heterogeneity in the Formoso River floodplain.

Measuring repeatability of compositional diet estimates: An example using quantitative fatty acid signature analysis

By measuring the temporal consistency, or repeatability, in the diets of predators, we can gain a better understanding of the degree of individual specialization in resource utilization and implications for predator–prey interactions, population dynamics, and food web structure. To measure repeatability, we require repeated diet estimates of individuals over time, such as those derived from quantitative fatty acid signature analysis (QFASA), a popular diet estimation technique. However, diet estimates are often lengthy compositional vectors with many zeros, as some prey will not be consumed by all individuals, precluding the use of previously proposed measures of repeatability. In this paper, we propose a novel approach for inferring repeatability for multivariate data and, in particular, compositional diet estimates. We extend the commonly used measure of repeatability for univariate data to the multivariate compositional setting by utilizing the mean squares obtained from a nonparametric multivariate analysis of variance, and an appropriate choice of statistical distance. Our measure and its extension are compatible with both balanced and unbalanced data sets. Associated confidence intervals via nonparametric bootstrapping are also developed for the case of QFASA diet estimates that incorporate both sampling error and measurement error, where the latter error arises because the diets of predators are estimated. Simulation study results suggest that for practical levels of repeatability, our methods yield confidence intervals with the desired coverage probability even when the sample size relative to the dimension of the data (i.e., number of prey species eaten) is small. We tested our methods using QFASA diet estimates for free‐ranging Northwest Atlantic grey seals. Given the importance of understanding how predator diets vary over time and space, our method may find broad application to other compositional diet estimates, including those derived from the stomach or fecal contents, and stable isotope analyses.

Regional, seasonal and age class blubber fatty acid signature analysis of harbour seals in Alaska from 1997 to 2010

Alaskan harbour seal populations are currently listed as a species of special concern. Although there is evidence of recent stabilization or even partial recovery of harbour seal numbers in areas of historic decline, most populations have not made substantial recoveries. To date, few data exist regarding spatial and seasonal changes in blubber fatty acids (FAs) for Alaskan harbour seal populations. The purpose of this study was to qualitatively investigate harbour seal blubber FA profiles for regional, seasonal and age class differences. Blubber FA concentrations were analysed using MANOVA and linear discriminant analysis (LDA) from 760 individual harbour seals across Bristol Bay, Kodiak, Prince William Sound and Southeast Alaska from 1997 to 2010. Our results suggest spatial and seasonal differences are largely driven by monounsaturated FAs, most notably 14:1n-5, 16:1n-7 and 18:1n-7. In addition, our data revealed a progression in blubber FAs from pups to adults, with a shift from saturated FAs and short-chained monounsaturated FAs in the pup blubber to more long-chain monounsaturated FAs and polyunsaturated FAs in adults. Lastly, harbour seals pups had elevated saturated FA 16:0 concentrations when compared to other age classes, regardless of location or period. With this vast spatial and seasonal FA information, we believe future sampling of blubber FAs from Alaskan harbour seal populations could be a useful tool in assessing the response of this species and its ecosystem to changes associated with natural and anthropogenic pressures.

Habitat and resource segregation of two sympatric seals in the North Sea

The study of ecological niche segregation in sympatric species is essential to understand ecosystem functioning and its response to potential changes. In the North Sea, sympatric grey and harbour seals may present competition for food resources sustained by intense fishing activities and recent increase of seal populations. In order to coexist and reduce inter-specific competition, sympatric species must segregate at least one aspect of their ecological niches: temporal, spatial or resource segregation. We aim to study the foraging resources and foraging distributions of grey seals and harbour seals and the potential competition between these species in the North Sea. Therefore, we analysed stable isotopic composition of C, N and S (δ¹³C, δ¹⁵N and δ³⁴S values), and the concentrations of Hg and Se in blood of harbour and grey seals from the North Sea. Blood samples were collected on 45 grey seals and 37 harbour seals sampled along German and Scottish coasts. Stable isotope ratios were performed with an isotope ratio mass spectrometer coupled to an N-C-S elemental analyser for automated analyses. Total mercury concentrations (T-Hg) were determined by atomic absorption spectroscopy and Se concentrations by ICP-MS. The multi-tracer approach shown spatial and resource partitioning within grey and harbour seal living along German and Scottish coasts. Data indicate 1) the offshore foraging distribution of grey seals as reflected by the lower δ¹⁵N values and T-Hg concentrations and higher Se concentrations and 2) the inshore foraging distribution of harbour seals because of higher δ¹⁵N values and T-Hg concentrations and lower Se concentrations. The SIAR mixing model revealed 3) a more selective diet of grey seals compared to harbour seals and 4) the importance of sandeels in grey seal diet reflected by their high δ³⁴S values. Lastly, diet ellipse overlaps between grey seals and harbour seals sampled along the German coasts suggested 5) a potential sharing of food resources, possibly due to the increase number of grey seals number in this area during the foraging season - all year except breeding and moulting periods. The multi-tracer approach of this study provides a more robust discrimination among diet resources and spatial foraging distributions of grey seals and harbour seals in the North Sea.

Flatfish may be underestimated in the diet of gray seals (Halichoerus grypus)

The United States east coast population of the gray seal (Halichoerus grypus (Fabricius, 1791)) was once hunted to near extirpation, but the population has since rebounded due to protection by the Marine Mammal Protection Act of 1972. Although this population growth is seen as a success by conservationists, others are concerned about the economic and ecological impact of gray seals on New England fisheries. The study objective was to quantify flatfish presence in the diet of gray seals using an analysis of prey DNA in seal scat (molecular scatology). This may reduce a potential bias in other diet analysis methods since flatfish may not be swallowed whole, and therefore, their identifying otoliths may not be present. Scats were collected from Muskeget and Monomoy islands off Cape Cod, Massachusetts, USA, in 2016, 2017, and 2018 and analyzed for flatfish presence using species-specific polymerase chain reaction (PCR) assays and a hard parts analysis using otoliths. Frequency of occurrence for flatfish species from DNA was higher than estimated in previous literature on gray seal diet and a concurrent otolith analysis of the same samples, suggesting that previous analyses may have potentially underestimated the importance of flatfish in the diet.

Exposure of the Gulf of St. Lawrence grey seal Halichoerus grypus population to potentially zoonotic infectious agents

The population of grey seals Halichoerus grypus in Canadian waters is currently used as a commercial source of meat for human consumption. As with domestic livestock, it is important to understand the occurrence in these seals of infectious agents that may be of public health significance and thus ensure appropriate measures are in place to avoid zoonotic transmission. This study examined the prevalence of antibodies against Brucella spp., Erysipelothrix rhusiopathiae, 6 serovars of Leptospira interrogans, and Toxoplasma gondii in 59 grey seals and determined by polymerase chain reaction (PCR) the presence of these potentially zoonotic agents in specific organs and tissues of seropositive animals. The presence of encysted Trichinella spp. larvae was also investigated by digestion of tongue, diaphragm and other muscle samples, but none were detected. Seroprevalence against Brucella spp. and E. rhusiopathiae was low (5 and 3%, respectively). All 59 seals tested had antibodies against L. interrogans, but no carrier of this bacterium was detected by PCR. Seroprevalence against T. gondii was 53%, and DNA of this protozoan was detected by PCR in 11/30 (37%) seropositive animals. Standard sanitary measures mandatory for commercialization of meat products for human consumption should greatly reduce the potential for exposure to these infectious agents. However, special consideration should be given to freezing seal meat for at least 3 d to ensure destruction of tissue cysts of T. gondii.

Foraging behaviour and habitat-use drives niche segregation in sibling seabird species

To mediate competition, similar sympatric species are assumed to use different resources, or the same but geographically separated resources. The two giant petrels (Macronectes spp.) are intriguing in that they are morphologically similar seabirds with overlapping diets and distributions. To better understand the mechanisms allowing their coexistence, we investigated intra- and interspecific niche segregation at Marion Island (Southern Indian Ocean), one of the few localities where they breed in sympatry. We used GPS tracks from 94 individuals and remote-sensed environmental data to quantify habitat use, combined with blood carbon and nitrogen stable isotope ratios from 90 individuals to characterize their foraging habitat and trophic ecology. Females of both species made distant at-sea foraging trips and fed at a similar trophic level. However, they used distinct pelagic habitats. By contrast, males of both species mainly foraged on or near land, resulting in significant sexual segregation, but high interspecific habitat and diet overlap. However, some males showed flexible behavioural strategies, also making distant, pelagic foraging trips. Using contemporaneous tracking, environmental and stable isotope data we provide a clear example of how sympatric sibling species can be segregated along different foraging behaviour dimensions.

Dietary complexity and hidden costs of prey switching in a generalist top predator

Variation in predator diet is a critical aspect of food web stability, health, and population dynamics of predator/ prey communities. Quantifying diet, particularly among cryptic species, is extremely challenging, however, and differentiation between demographic subsets of populations is often overlooked.We used prey remains and data taken postmortem from otter Lutra lutra to determine the extent to which dietary variation in a top predator was associated with biotic, spatial, and temporal factors.Biotic data (e.g., sex, weight, and length) and stomach contents were taken from 610 otters found dead across England and Wales between 1994 and 2010. Prey remains were identified to species where possible, using published keys and reference materials. Multi-model inference followed by model prediction was applied to test for and visualize the nature of associations.Evidence for widespread decline in the consumption of eels (Anguilla anguilla) reflected known eel population declines. An association between eel consumption and otter body condition suggested negative consequences for otter nutrition. Consumption of Cottus gobio and stickleback spp. increased, but was unlikely to compensate (there was no association with body condition). More otters with empty stomachs were found over time. Otter sex, body length, and age-class were important biotic predictors of the prey species found, and season, region, and distance from the coast were important abiotic predictors.Our study is unique in its multivariate nature, broad spatial scale, and long-term dataset. Inclusion of biotic data allowed us to reveal important differences in costs and benefits of different prey types, and differences between demographic subsets of the population, overlaid on spatial and temporal variation. Such complexities in otter diet are likely to be paralleled in other predators, and detailed characterization of diet should not be overlooked in efforts to conserve wild populations.

Contemporary diets of walruses in Bristol Bay, Alaska suggest temporal variability in benthic community structure

Background

Pacific walruses (Odobenus rosmarus divergens) are a conspicuous and important component of the Bristol Bay ecosystem and human social systems, but very little is known about walrus ecology in this region, principally their feeding ecology. The present work provides contemporary data on the diets of walruses at four haulout locations throughout Bristol Bay between 2014 and 2018.

Methods

We analyzed scat and gastrointestinal tract samples from these animals using quantitative polymerase chain reaction to amplify prey DNA, which allowed for diet estimates based on frequencies of prey item occurrence and on the relative importance of dietary items as determined from DNA threshold cycle scores.

Results

Diets were highly diverse at all locations, but with some variation in composition that may be related to the time of year that samples were collected (summer vs. autumn), or to spatial variability in the distribution of prey. Overall, polychaetes and tunicates had the highest frequencies of occurrence and relative abundances in 2014-15, but a major change in diet appears to have occurred by 2017-18. While some sample sizes were small, diets in these later years contrasted sharply, with a greater prevalence of sea cucumbers and mollusks, and reduced importance of decapods and fishes compared to the earlier years. Prey identified in scat samples from one collection site also contrasted sharply with those reported from the same location in 1981. The apparent temporal shifts in walrus prey may represent a changing benthic ecosystem due to warming waters in recent decades.

Pathological findings in North Sea and Baltic grey seal and harbour seal intestines associated with acanthocephalan infections

Grey seals Halichoerus grypus and harbour seals Phoca vitulina are common seal species in the North and Baltic seas and final hosts of Corynosoma acanthocephalans. C. strumosum and C. magdaleni infect the small intestines of both seal species. In contrast to harbour seals, Baltic grey seals in the past have regularly displayed severe C. semerme infections in the caecum and colon, with associated tunica muscularis hypertrophy, inflammation and ulcerations as part of the Baltic seal disease complex (BSDC). Pathogenesis and correlation of acanthocephalan infections with these lesions are still unknown. This study describes the intestinal pathology and parasitic distribution in each seal species. Grey seal (n = 83) and harbour seal (n = 1156) intestines of all age groups and sexes, collected in Poland, Germany and Estonia from 1998 to 2017, were investigated. Most harbour seals came from the North Sea, whereas grey seals were predominantly derived from the Baltic Sea. Both species featured mild to moderate small intestinal infections. Grey seals showed colonic infections not found in harbour seals and featured a chronic erosive to ulcerative, eosinophilic or lympho-plasmacytic colitis with tunica muscularis hypertrophy, indicating still prevailing clinical signs of the BSDC. Harbour seals displayed granulomatous, eosinophilic, lympho-plasmacytic or catarrhal enteritis. The prevalence of acanthocephalan infections in harbour seals increased from 2012 onwards. Furthermore, significant associations between acanthocephalan infection and the presence of intestinal inflammation were found for both seal species. This study suggests that the level of acanthocephalan infection and associated lesions are suitable seal population health indicators, with the colon being a specific target organ for Baltic grey seal health monitoring.

Sex-differences in fine-scale home-range use in an upper-trophic level marine predator

BACKGROUND

The distribution of prey in the ocean is spatially and temporally patchy. How predators respond to this prey patchiness may have consequences on their foraging success, and thus physical condition. The recent ability to record fine-scale movements of marine animals combined with novel home-range analyses that incorporate the dimension of time should permit a better understanding of how individuals utilise different regions of space and the consequences on their foraging success.

METHODS

Over a six-year study, we used T-LoCoH (Time-Local Convex Hull) home-range software to model archival GPS (Global Positioning System) data from 81 grey seals to investigate the fine-scale spatio-temporal use of space and the distribution of apparent foraging effort. Regions of home-ranges were classified according to the frequency of return visits (site fidelity) and duration of visits (intensity of use). Generalized linear mixed -effects models were used to test hypotheses on seasonal changes in foraging distribution and behaviour and the role of space-use and state on determining foraging success.

RESULTS

Male grey seals had larger home-ranges and core areas than females, and both sexes showed a contraction in home-range and core area in fall leading up to the breeding season compared with summer. Heavier individuals had smaller core areas than lighter ones, suggesting access to higher quality habitat might be limited to those individuals with greater foraging experience and competitive ability. The size of the home-range or core area was not an important predictor of the rate of mass gain. A fine-scale spatio-temporal analysis of habitat use within the home-range provided evidence of intra-annual site fidelity at presumed foraging locations, suggesting predictably in prey distribution. Neither sex nor season were useful predictors for classifying behaviour. Rather, individual identity explained much of the variation in fine-scale behaviour.

CONCLUSIONS

Understanding how upper-trophic level marine predators use space provides opportunities to explore the consequences of variation in foraging tactics and their success on fitness. Having knowledge of the drivers that shape this intraspecific variation can contribute toward predicting how these predators may respond to both natural and man-made environmental forcing.

Foraging behaviour of a continental shelf marine predator, the grey seal (Halichoerus grypus), is associated with in situ, subsurface oceanographic conditions

BACKGROUND

The heterogeneous oceanographic conditions of continental shelf ecosystems result in a three-dimensionally patchy distribution of prey available to upper-trophic level predators. The association of bio-physical conditions with movement patterns of large marine predators has been demonstrated in diverse taxa. However, obtaining subsurface data that are spatio-temporally relevant to the decisions made by benthically-foraging species can be challenging.

METHODS

Between 2009 and 2015, grey seals were captured on Sable Island, Nova Scotia, Canada during summer and fall and instrumented with high-resolution archival GPS tags. These tags recorded location data as well as depth (m), temperature (°C), and light level measurements during dives, until animals returned to the haulout site to breed. Hidden Markov models were used to predict apparent foraging along movement tracks for 79 individuals (59 females, 20 males) every 3 h. In situ measurements were used to estimate chlorophyll-a concentration (mg m^- 3) and temperature within the upper-water column (50 m) and temperature and depth at the bottom of dives. As chlorophyll-a could only be estimated from 10:00 to 14:00 AST for dive depths ≥50 m, we formulated two generalized linear mixed-effects models to test the association of predicted grey seal behavioural states with oceanographic conditions and phytoplankton biomass: the first representing conditions of the upper-water column likely to influence primary productivity, and a second model including environmental conditions encountered by grey seals at the bottom of dives, when seals were more likely to be foraging.

RESULTS

Predicted grey seal behavioural states were associated with fine-scale chlorophyll-a concentrations and other environmental conditions they encountered across the continental shelf. In the Water Column Model, season had no influence on the probability of observing apparent foraging, but chlorophyll-a, upper-water column temperature, and sex did, with females having a greater probability of foraging than males. In the Bottom Conditions Model, again season had no influence on the probability of apparent foraging, but females were over twice as likely as males to be foraging.

CONCLUSIONS

The results of this study highlight the value of in situ measurements of oceanographic properties that can be collected at high temporal resolution by animal-borne data loggers. These data provide insight into how inferred behavioural decisions made by large marine predators, such as the grey seal, may be influenced by fine-scale oceanographic conditions.

Seasonal differences in foraging and isotopic niche width related to body size in Gulf of Alaska harbor seals

Harbor seals (Phoca vitulina Linnaeus, 1758) use different foraging strategies based on body size and sex, but this difference can be difficult to evaluate across seasons. We used stable isotope analysis of harbor seal whiskers from 32 individuals to assess seasonal foraging of seals inhabiting tidewater glacial habitat in Southeast Alaska. We analyzed stable isotope ratios from serial sections of whiskers, estimated deposition date for each section, and used mixed models to determine if sex and body size influence stable isotope ratios. Seals were grouped by size (>1.4 m or ≤1.4 m curvilinear length) as a proxy for sexual maturity to describe isotopic differences between groups using standard ellipse corrected area. Mean carbon and nitrogen isotope ratios differed significantly between size classes (p < 0.005), with no effect of sex. Larger seals exhibited a broader isotopic niche in the fall, winter, and spring relative to smaller seals, but not in the summer. Our results suggest that seals using tidewater glacial habitat share common foraging behavior in the summer, while larger seals exhibit more diverse foraging throughout the rest of the year. These results highlight the importance of tidewater glacial habitat for this population of harbor seals during the summer.

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.

Ecophysiological determinants of sexual size dimorphism: integrating growth trajectories, environmental conditions, and metabolic rates

Sexual size dimorphism (SSD) often results in dramatic differences in body size between females and males. Despite its ecological importance, little is known about the relationship between developmental, physiological, and energetic mechanisms underlying SSD. We take an integrative approach to understand the relationship between developmental trajectories, metabolism, and environmental conditions resulting in extreme female-biased SSD in the crab spider Mecaphesa celer (Thomisidae). We tested for sexual differences in growth trajectories, as well as in the energetics of growth, hypothesizing that female M. celer have lower metabolic rates than males or higher energy assimilation. We also hypothesized that the environment in which spiderlings develop influences the degree of SSD of a population. We tracked growth and resting metabolic rates of female and male spiderlings throughout their ontogeny and quantified the adult size of individuals raised in a combination of two diet and two temperature treatments. We show that M. celer's SSD results from differences in the shape of female and male growth trajectories. While female and male resting metabolic rates did not differ, diet, temperature, and their interaction influenced body size through an interactive effect with sex, with females being more sensitive to the environment than males. We demonstrate that the shape of the growth curve is an important but often overlooked determinant of SSD and that females may achieve larger sizes through a combination of high food ingestion and low activity levels. Our results highlight the need for new models of SSD based on ontogeny, ecology, and behavior.

Male and female bees show large differences in floral preference

Background

Intraspecific variation in foraging niche can drive food web dynamics and ecosystem processes. In particular, male and female animals can exhibit different, often cascading, impacts on their interaction partners. Despite this, studies of plant-pollinator interaction networks have focused on the partitioning of the floral community between pollinator species, with little attention paid to intraspecific variation in plant preference between male and female bees. We designed a field study to evaluate the strength and prevalence of sexually dimorphic foraging, and particularly resource preferences, in bees.

Study design

We observed bees visiting flowers in semi-natural meadows in New Jersey, USA. To detect differences in flower use against a shared background of resource (flower) availability, we maximized the number of interactions observed within narrow spatio-temporal windows. To distinguish observed differences in bee use of flower species, which can reflect abundance patterns and sampling effects, from underlying differences in bee preferences, we analyzed our data with both a permutation-based null model and random effects models.

Findings

We found that the diets of male and female bees of the same species were often dissimilar as the diets of different species of bees. Furthermore, we demonstrate differences in preference between male and female bees. We show that intraspecific differences in preference can be robustly identified among hundreds of unique species-species interactions, without precisely quantifying resource availability, and despite high phenological turnover of both bees and plant bloom. Given the large differences in both flower use and preferences between male and female bees, ecological sex differences should be integrated into studies of bee demography, plant pollination, and coevolutionary relationships between flowers and insects.

Kinematic signatures of prey capture from archival tags reveal sex differences in killer whale foraging activity

Studies of odontocete foraging ecology have been limited by the challenges of observing prey capture events and outcomes underwater. We sought to determine whether subsurface movement behavior recorded from archival tags could accurately identify foraging events by fish-eating killer whales. We used multisensor bio-logging tags attached by suction cups to Southern Resident killer whales (Orcinus orca) to: (1) identify a stereotyped movement signature that co-occurred with visually confirmed prey capture dives; (2) construct a prey capture dive detector and validate it against acoustically confirmed prey capture dives; and (3) demonstrate the utility of the detector by testing hypotheses about foraging ecology. Predation events were significantly predicted by peaks in the rate of change of acceleration (‘jerk peak’), roll angle and heading variance. Detection of prey capture dives by movement signatures enabled substantially more dives to be included in subsequent analyses compared with previous surface or acoustic detection methods. Males made significantly more prey capture dives than females and more dives to the depth of their preferred prey, Chinook salmon. Additionally, only half of the tag deployments on females (5 out of 10) included a prey capture dive, whereas all tag deployments on males exhibited at least one prey capture dive (12 out of 12). This dual approach of kinematic detection of prey capture coupled with hypothesis testing can be applied across odontocetes and other marine predators to investigate the impacts of social, environmental and anthropogenic factors on foraging ecology.

Spatial variation in a top marine predator's diet at two regionally distinct sites

In ecological studies it is often assumed that predator foraging strategies and resource use are geographically and seasonally homogeneous, resulting in relatively static trophic relationships. However, certain centrally placed foragers (e.g. seals) often have terrestrial sites for breeding, resting, and moulting that are geographically distinct, and associated with different habitat types. Therefore, accurate estimations of predator diet at relevant spatial and temporal scales are key to understanding energetic requirements, predator-prey interactions and ecosystem structure. We investigate geographic variation in the diet of grey seals (Halichoerus grypus), a relatively abundant and widely distributed central place forager, to provide insights into geographic variation in resource use. Prey composition was identified using scat samples collected over concurrent timescales and a multivariate approach was used to analyse diet from two contrasting habitats. Regional differences in prey assemblages occurred within all years (2011-2013) and all seasons (ANOSIM, all p<0.05), apart from in winter. Telemetry data were used to identify core foraging areas and habitats most likely associated with scat samples collected at the two haul-out sites. Regional differences in the diet appear to reflect regional differences in the physical habitat features, with seals foraging in deeper waters over sandy substrates showing a higher prevalence of pelagic and bentho-pelagic prey species such as blue whiting and sandeels. Conversely, seals foraging in comparatively shallow waters had a greater contribution of demersal and groundfish species such as cephalopods and flatfish in their diet. We suggest that shallower waters enable seals to spend more time foraging along the benthos while remaining within aerobic dive limits, resulting in more benthic species in the diet. In contrast, the diet of seals hauled-out in areas adjacent to deeper waters indicates that either seals engage in a more pelagic foraging strategy, or that seals can spend less time at the benthos, resulting in comparatively more pelagic prey recovered in the diet. The substantial differences in prey assemblages over a small spatial scale (<300 km) demonstrates the importance of using regionally-specific diet information in ecosystem-based models to better account for different trophic interactions.

Complementary methods assessing short and long-term prey of a marine top predator ‒ Application to the grey seal-fishery conflict in the Baltic Sea

The growing grey seal (Halichoerus grypus) population in the Baltic Sea has created conflicts with local fisheries, comparable to similar emerging problems worldwide. Adequate information on the foraging habits is a requirement for responsible management of the seal population. We investigated the applicability of available dietary assessment methods by comparing morphological analysis and DNA metabarcoding of gut contents (short-term diet; n = 129/125 seals, respectively), and tissue chemical markers i.e. fatty acid (FA) profiles of blubber and stable isotopes (SIs) of liver and muscle (mid- or long-term diet; n = 108 seals for the FA and SI markers). The methods provided complementary information. Short-term methods indicated prey species and revealed dietary differences between age groups and areas but for limited time period. In the central Baltic, herring was the main prey, while in the Gulf of Finland percid and cyprinid species together comprised the largest part of the diet. Perch was also an important prey in the western Baltic Proper. The DNA analysis provided firm identification of many prey species, which were neglected or identified only at species group level by morphological analysis. Liver SIs distinguished spatial foraging patterns and identified potentially migrated individuals, whereas blubber FAs distinguished individuals frequently utilizing certain types of prey. Tissue chemical markers of adult males suggested specialized feeding to certain areas and prey, which suggest that these individuals are especially prone to cause economic losses for fisheries. We recommend combined analyses of gut contents and tissue chemical markers as dietary monitoring methodology of aquatic top predators to support an optimal ecosystem-based management.

Prevalence of alopecia in gray seals Halichoerus grypus atlantica in Massachusetts, USA, 2004-2013

There has been an increase in the presence of alopecia among gray seals Halichoerus grypus atlantica in Massachusetts, USA. To understand the prevalence and describe the presentation of this condition, data records and photographs of 10070 gray seals from 2004-2013 were reviewed; there were sufficient data to confidently assess the presence or absence of alopecia in 2134 seals. Mild hair loss presented in multifocal patches with minimal to no skin lesions or erythema. In severe cases, alopecia was concentrated over the dorsal head, neck, and shoulders and extended down the ventrum, affecting >50% of the body. Associated skin lesions and erythema were often present. Alopecia was documented in 7.1% of the surveyed seals, and was centered in Nantucket. Alopecia was more prevalent in stranded/sighted animals in spring and summer, with 81% of cases documented from April to July. There was no sex bias, and weanlings were the most affected age class (38%). The etiology for alopecia in Massachusetts gray seals is unknown. Possible causes of alopecia in wildlife are infectious disease, nutritional deficiencies, endocrinopathies, or chronic physiologic stress. High population density around Nantucket may escalate intraspecific competition for resources, which may indirectly lead to stress-induced immunosuppression or nutritional deficiencies. Crowded haul-out sites increase the opportunity for disease transmission. The weanling age class may be prone to alopecia due to naïve immune systems and inexperienced foraging capabilities. Diagnostic sample collection from gray seals will be required to characterize the etiology, pathogenesis, and significance of alopecia in this population.

Large-scale molecular diet analysis in a generalist marine mammal reveals male preference for prey of conservation concern

Sex-specific diet information is important in the determination of predator impacts on prey populations. Unfortunately, the diet of males and females can be difficult to describe, particularly when they are marine predators. We combined two molecular techniques to describe haul-out use and prey preferences of male and female harbor seals (Phoca vitulina) from Comox and Cowichan Bay (Canada) during 2012-2013. DNA metabarcoding quantified the diet proportions comprised of prey species in harbor seal scat, and qPCR determined the sex of the individual that deposited each scat. Using 287 female and 260 male samples, we compared the monthly sex ratio with GLMs and analyzed prey consumption relative to sex, season, site, and year with PERMANOVA. The sex ratio between monthly samples differed widely in both years (range = 12%-79% males) and showed different patterns at each haul-out site. Male and female diet differed across both years and sites: Females consumed a high proportion of demersal fish species while males consumed more salmonid species. Diet composition was related to both sex and season (PERMANOVA: R 2 = 27%, p < 0.001; R 2 = 24%, p < 0.001, respectively) and their interaction (PERMANOVA: R 2 = 11%, p < 0.001). Diet differences between males and females were consistent across site and year, suggesting fundamental foraging differences, including that males may have a larger impact on salmonids than females. Our novel combination of techniques allowed for both prey taxonomic and spatiotemporal resolution unprecedented in marine predators.

Density-dependent resource selection by a terrestrial herbivore in response to sea-to-land nutrient transfer by seals

Sea-to-land nutrient transfers can connect marine food webs to those on land, creating a dependence on marine webs by opportunistic species. We show how nitrogen, imported by gray seals, Halichoerus grypus, and traced through stable isotope (δ¹⁵ N) measurements in marram grass, Ammophila breviligulata, significantly alters foraging behavior of a free-roaming megaherbivore (feral horses, Equus ferus caballus) on Sable Island, Canada. Values of δ¹⁵ N correlated with protein content of marram and strongly related to pupping-seal densities, and positively influenced selective foraging by horses. The latter was density dependent, consistent with optimal foraging theory. We present the first demonstration of how sea-to-land nutrient transfers can affect the behavioral process of resource selection (resource use relative to availability) of terrestrial consumers. We hypothesize that persistence of horses on Sable Island is being facilitated by N subsidies. Our results have relevance to advancing theory on trophic dynamics in island biogeography and metaecosystem ecology.

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.

Hooded seal Cystophora cristata foraging areas in the Northeast Atlantic Ocean-Investigated using three complementary methods

Identifying environmental characteristics that define the ecological niche of a species is essential to understanding how changes in physical conditions might affect its distribution and other aspects of its ecology. The present study used satellite relay data loggers (SRDLs) to study habitat use by Northeast Atlantic hooded seals (N = 20; 9 adult females, 3 adult males, and 8 juveniles). Three different methods were used in combination to achieve maximum insight regarding key foraging areas for hooded seals in this region, which have decline by 85% in recent decades: 1) first passage time (FPT); 2) vertical transit rate and; 3) change in dive drift rate. Generalized additive mixed models (GAMM) were applied to each method to determine whether specific habitat characteristics were associated with foraging. Separate models were run for the post-molting and the post-breeding seasons; sex and age classes were included in the GAMMs. All three methods highlighted a few common geographic areas as being important foraging zones; however, there were also some different areas identified by the different methods, which highlights the importance of using multiple indexes when analyzing tracking and diving data to study foraging behavior. Foraging occurred most commonly in relatively shallow areas with high Sea Surface Temperatures (SST), corresponding to continental shelf areas with Atlantic Water masses. All age and sex classes overlapped spatially to some extent, but the different age and sex groups showed differences in the bathymetry of their foraging areas as well as in their vertical use of the water column. When foraging, pups dove in the upper part of the water column in relatively deep areas. Adult females foraged relatively shallowly in deep water areas too, though in shallower areas than pups. Adult males foraged close to the bottom in shallower areas.

Intrinsic and extrinsic factors drive ontogeny of early-life at-sea behaviour in a marine top predator

Young animals must learn to forage effectively to survive the transition from parental provisioning to independent feeding. Rapid development of successful foraging strategies is particularly important for capital breeders that do not receive parental guidance after weaning. The intrinsic and extrinsic drivers of variation in ontogeny of foraging are poorly understood for many species. Grey seals (Halichoerus grypus) are typical capital breeders; pups are abandoned on the natal site after a brief suckling phase, and must develop foraging skills without external input. We collected location and dive data from recently-weaned grey seal pups from two regions of the United Kingdom (the North Sea and the Celtic and Irish Seas) using animal-borne telemetry devices during their first months of independence at sea. Dive duration, depth, bottom time, and benthic diving increased over the first 40 days. The shape and magnitude of changes differed between regions. Females consistently had longer bottom times, and in the Celtic and Irish Seas they used shallower water than males. Regional sex differences suggest that extrinsic factors, such as water depth, contribute to behavioural sexual segregation. We recommend that conservation strategies consider movements of young naïve animals in addition to those of adults to account for developmental behavioural changes.

Simultaneous estimation of diet composition and calibration coefficients with fatty acid signature data

Knowledge of animal diets provides essential insights into their life history and ecology, although diet estimation is challenging and remains an active area of research. Quantitative fatty acid signature analysis (QFASA) has become a popular method of estimating diet composition, especially for marine species. A primary assumption of QFASA is that constants called calibration coefficients, which account for the differential metabolism of individual fatty acids, are known. In practice, however, calibration coefficients are not known, but rather have been estimated in feeding trials with captive animals of a limited number of model species. The impossibility of verifying the accuracy of feeding trial derived calibration coefficients to estimate the diets of wild animals is a foundational problem with QFASA that has generated considerable criticism. We present a new model that allows simultaneous estimation of diet composition and calibration coefficients based only on fatty acid signature samples from wild predators and potential prey. Our model performed almost flawlessly in four tests with constructed examples, estimating both diet proportions and calibration coefficients with essentially no error. We also applied the model to data from Chukchi Sea polar bears, obtaining diet estimates that were more diverse than estimates conditioned on feeding trial calibration coefficients. Our model avoids bias in diet estimates caused by conditioning on inaccurate calibration coefficients, invalidates the primary criticism of QFASA, eliminates the need to conduct feeding trials solely for diet estimation, and consequently expands the utility of fatty acid data to investigate aspects of ecology linked to animal diets.

Navigating uncertain waters: a critical review of inferring foraging behaviour from location and dive data in pinnipeds

In the last thirty years, the emergence and progression of biologging technology has led to great advances in marine predator ecology. Large databases of location and dive observations from biologging devices have been compiled for an increasing number of diving predator species (such as pinnipeds, sea turtles, seabirds and cetaceans), enabling complex questions about animal activity budgets and habitat use to be addressed. Central to answering these questions is our ability to correctly identify and quantify the frequency of essential behaviours, such as foraging. Despite technological advances that have increased the quality and resolution of location and dive data, accurately interpreting behaviour from such data remains a challenge, and analytical methods are only beginning to unlock the full potential of existing datasets. This review evaluates both traditional and emerging methods and presents a starting platform of options for future studies of marine predator foraging ecology, particularly from location and two-dimensional (time-depth) dive data. We outline the different devices and data types available, discuss the limitations and advantages of commonly-used analytical techniques, and highlight key areas for future research. We focus our review on pinnipeds - one of the most studied taxa of marine predators - but offer insights that will be applicable to other air-breathing marine predator tracking studies. We highlight that traditionally-used methods for inferring foraging from location and dive data, such as first-passage time and dive shape analysis, have important caveats and limitations depending on the nature of the data and the research question. We suggest that more holistic statistical techniques, such as state-space models, which can synthesise multiple track, dive and environmental metrics whilst simultaneously accounting for measurement error, offer more robust alternatives. Finally, we identify a need for more research to elucidate the role of physical oceanography, device effects, study animal selection, and developmental stages in predator behaviour and data interpretation.