Key fish species, northern fur seals, Callorhinus ursinus, and fisheries interactions involving walleye pollock, Theragra chalcogramma, in the eastern Bering Sea

The recent decline of the northern fur seal population breeding on the Pribilof Islands has not yet been explained. This study estimates the amounts and sizes of walleye pollock (the fur seal's main prey) removed by predatory fish, fur seals and commercial fisheries in the main fur seal feeding area in the eastern Bering Sea during summer 1985. Fur seals relied primarily on age-1 pollock during 1985, while predatory fish consumed pollock mainly of ages 0-1. The fishery took pollock older than age-3. This indicated no direct effect of fishery removal on fur seal prey abundance, at least during the study period. In the short term, the 1985 pollock fishery may have indirectly affected fur seal prey abundance in a positive manner by removing adult pollock that compete with fur seals for age-1 pollock.     

Inter-island variation in the diet of female northern fur seals (Callorhinus ursinus) in the Bering Sea

The diet of adult female northern fur seals ( Callorhinus ursinus ) is examined through the analysis of faecal material collected during the summer breeding season at three breeding locations in the Bering Sea: St. Paul Island (1988, 1990) and St. George Island (1988, 1990) of the Pribilof Islands Group (USA), and Medny Island (1990) of the Commander Islands Group (Russia). Prey consumption varies annually and accordingly with the physical and biological environment surrounding each island. Juvenile walleye pollock ( Theragra chalcogramma ) is the most common prey of northern fur seals from St. Paul Island; the island is surrounded by a broad neritic environment with widely separated frontal zones and is the greatest distance from the continental shelf-edge. Gonatid squid ( Gonatopsis borealis/Berryteuthis magister and Gonatus madokail Gonatus middendorffi ) were the most common prey of northern fur seals from Medny Island; the island is surrounded by a compressed neritic environment and is adjacent to the continental shelf-edge and the oceanic marine environment. A combination of walleye pollock and gonatid squid is consumed by northern fur seals from St. George Island; the island has a surrounding oceanographic environment intermediate between the other two islands.
Variability in predation on walleye pollock is consistent with fishery information concerning the relative abundance and availability of walleye pollock around St. George and St. Paul Islands. The abundance and availability of these prey resources during the summer breeding season are key factors which influence the health and growth of the northern fur seal populations in the Bering Sea.     

Foraging habitats based on the diet of female northern fur seals (Callorhinus ursinus) on the Pribilof Islands, Alaska

Scats (fecal samples) collected between 1987 and 2000 on northern fur seal Callorhinus ursinus rookeries of St Paul ( n =2968) and St George Islands ( n =1203), Alaska, were used to examine the relationship between breeding sites and food habits of adult female seals. On the basis of the frequency of occurrence (FO) and per cent minimum number of individual prey (%MNI) in scats, juvenile walleye pollock Theragra chalcogramma and gonatid squid Gonatopsis borealis/Berryteuthis magister and Gonatus madokai/Gonatus middendorffi were the dominant prey species consumed overall. Other primary prey (FO>5%) included Pacific sand lance Ammodytes hexapteus , Pacific herring Clupea pallasi , northern smoothtongue Leuroglossus schmidti , Atka mackerel Pleurogrammus monopterygius , Pacific salmon ( Oncorhynchus spp.) and other squid of the Gonatus genus. We identified five rookery complexes from a cluster analysis of the FO of primary prey in scats. Rookery complexes were separated geographically and each was further defined by characteristic patterns in the representation of prey types typically associated with specific hydrographic domains. Diet differences were observed among rookeries on the north and south side of St George Island and on the east, south and south-west side of St Paul Island. The rookery clusters observed in this study provide evidence of resource partitioning among adult female northern fur seals and have important implications for fur seal conservation and management.     

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.     

DENSITY DEPENDENCE IN NORTHERN FUR SEALS (CALLORHINUS URSINUS)

This paper summarizes, updates, and interprets information on density-dependent dynamics of populations of the northern fur seal ( Callorhinus ursinus ). Density-dependent changes observed in these populations have involved various aspects of growth (body length, body weight, tooth weight, and size of other skeletal parts, especially the skull), survival, age at maturation, incidence of disease, and time spent foraging. For the population of northern fur seals on St. Paul Island of the Pribilof Islands, which was observed during a major increase and during two significant declines, density-dependent changes that occurred during the growth of the population were reversed during the declines. The Robben Island population in the western Pacific declined after 1965, and the decline was accompanied by changes similar to those observed during the declines on the Pribilof Islands. Although data are not available for all age and sex classes, it appears that most or all components of the populations exhibit similar changes. The overall implication of these changes is that current populations on Robben Island and the Pribilof Islands are reduced to levels below what could be supported by the resources available in their environments. Density dependence for this species is consistent with that of other large mammals, specifically in that vital rates for fur seals ate related to density in a nonlinear fashion.     

Morphological adaptation of an invasive American mink population in Mediterranean areas of Spain

In this work we studied the morphology of an invasive population of American mink Mustela vison in Catalonia, Mediterranean Spain. Body weight, body length, tail length, hindfoot length and ear length were measured for four age–sex classes: subadult male (n = 17), subadult female (n = 16), adult male (n = 36) and adult female (n = 10). A General Linear Mixed Model was used to test the effect of year, sex, age and age–sex interaction, on each parameter. The morphological results differed from those of other introduced populations because of their different origin and their adaptation to different environments. Differences in sex and age were found, pointing to sexual dimorphism both in adults and subadults. The degree of dimorphism was lower than that of other populations, probably because of a lack of trophic niche separation between male and female mink because in the study area only small prey animals were available.     

Broad thermal capacity facilitates the primarily pelagic existence of northern fur seals (Callorhinus ursinus)

Thermoregulatory capacity may constrain the distribution of marine mammals despite having anatomical and physiological adaptations to compensate for the thermal challenges of an aquatic lifestyle. We tested whether subadult female northern fur seals (Callorhinus ursinus) experience increased thermoregulatory costs in water temperatures potentially encountered during their annual migration in the Bering Sea and North Pacific Ocean. Metabolic rates were measured seasonally in 6 captive female northern fur seals (2.75–3.5 yr old) in ambient air and controlled water temperatures of 2°C, 10°C, and 18°C. Rates of oxygen consumption in ambient air (1°C–18°C) were not related to environmental temperature except below 2.5°C (winter only). However, metabolism was significantly higher during the fall seasonal trials (September–October) compared to other times of year, perhaps due to the costs of molting. The fur seals appeared thermally neutral in all seasons for all water temperatures tested (2°C–18°C) except during the summer when metabolic rates were higher in the 2°C water. Comparing this broad thermal neutral zone to the average sea surface temperatures potentially encountered during annual migrations indicates wild fur seals can likely exploit a large geographic area without added thermal metabolic costs.     

Feeding habits of three seal species at the Danco Coast, Antarctica: a re-assessment

The analysis of prey overlap among Weddell, Antarctic fur and leopard seals was conducted using fecal samples collected at the Danco Coast, Antarctic Peninsula, in 1998 and 2000. The re-occurrence of prey species was moderate in samples collected in 1998, and low in 2000, and reflects resource partitioning among seal species. Prey species that mostly co-occurred in seals’ diet were the Antarctic krill Euphausia superba, bivalves, and the myctophids Gymnoscopelus nicholsi and Electrona antarctica. A dietary similarity index of prey overlap has been calculated and demonstrates evident fluctuations in pairwise comparisons between the seal species. The highest and lowest values of prey overlap were observed between Antarctic fur seals and leopard seals, and between Weddell seals and leopard seals, respectively. Prey overlap between Antarctic fur seals and Weddell seals was moderate in both seasons.     

Otolith size and location in digestive tracts of northern fur seals (Callorhinus ursinus): Implications for dietary interpretations

Walleye pollock (Theragra chalcogramma) otoliths (n= 2,706) recovered from stomachs, small intestines, and colons of 43 northern fur seals (Callorhinus ursinus) were evaluated for size and wear by location in the digestive tract. Pollock fork length was regressed on otolith length after correction for erosion, and age was estimated from the calculated body size. Age‐1+ pollock otoliths (≥6.3‐mm length) were concentrated in stomachs while age‐0 otoliths (≤6.2‐mm length) were concentrated in colons. Less than 10% of otoliths were found in the small intestines. Pollock age decreased with progression along seal gastrointestinal tracts. Otolith quality increased along gastrointestinal tracts in numbers ≥20, which was typical of age‐0 otoliths recovered from colons. Otolith distribution by age and quality along gastrointestinal tracts suggests that small (≤12 cm) schooling prey are consumed in large volume and passed as a bolus rapidly through the digestive tract before significant erosion of bony remains occurs; while larger prey are eaten in smaller volume and subjected to otolith erosion due to longer retention in the stomach. Our results illustrate the importance of multiple sampling strategies to comprehensively represent prey size in pinniped diet.     

Whisker isotopic signature depicts migration patterns and multi-year intra- and inter-individual foraging strategies in fur seals

The movement and dietary history of individuals can be studied using stable isotope records in archival keratinous tissues. Here, we present a chronology of temporally fine-scale data on the trophic niche of otariid seals by measuring the isotopic signature of serially sampled whiskers. Whiskers of male Antarctic fur seals breeding at the Crozet Islands showed synchronous and regular oscillations in both their δ¹³C and δ¹⁵N values that are likely to represent their annual migrations over the long term (mean 4.8 years). At the population level, male Antarctic fur seals showed substantial variation in both δ¹³C and δ¹⁵N values, occupying nearly all the ‘isotopic space’ created by the diversity of potential oceanic habitats (from high Antarctica to the subtropics) and prey (from Antarctic krill to subantarctic and subtropical mesopelagic fishes). At the individual level, whisker isotopic signatures depict a large diversity of foraging strategies. Some seals remained in either subantarctic or Antarctic waters, while the migratory cycle of most animals encompassed a wide latitudinal gradient where they fed on different prey. The isotopic signature of whiskers, therefore, revealed new multi-year foraging strategies of male Antarctic fur seals and is a powerful tool for investigating the ecological niche during cryptic stages of mammals'' life.     

Harp seal foraging behaviour during summer around Svalbard in the northern Barents Sea: diet composition and the selection of prey

The harp seal Pagophilus groenlandicus is a major high trophic level predator in the Barents Sea, and to better understand their function in the Barents Sea ecosystem, we need to understand their foraging behaviour during their most intensive feeding period. We analysed the diet composition and prey preference of 184 harp seals and 94 faeces samples, sampled in the northern Barents Sea (around Svalbard) during the period May–August in 1996, 1997, and 2004–2006. Concurrent with the sampling of seals, prey availability was assessed in one area in 1996 and 1997 and in two areas in 2006 using standard acoustic methods. Our study showed that harp seal diet composition varied significantly both in time (year) and space, and that their diets appeared to be size dependent. Both subadult (<150 cm) and adult seals were associated with pelagic crustaceans (particularly krill), whereas primarily adult seals were associated with fish (capelin, gadoids and flatfish). Krill was the most important prey group (63 %) followed by polar cod (16 %) and other fish species (10 %). The prey preference of harp seals varied in time and space; polar cod was often preferred by the seals whereas krill was commonly consumed in lower proportion than observed in the survey area. Gadoids and capelin had either been exploited in the same or less proportion as observed in the survey sea. This study emphasises the ecological significance of krill as prime food for harp seals during their intensive feeding period in summer.     

Hematology of free-ranging, lactating northern fur seals, Callorhinus ursinus

Thirteen standard hematology values were determined for a healthy and growing population of free-ranging, lactating northern fur seals (Callorhinus ursinus) from Lovushki Island in the Kuril Islands of far-east Russia. Results are presented from 24 females sampled between June and August during the 3-yr period of 2006-08. Hematologic values have been made available for future comparisons with the declining population of northern fur seals on the Pribilof Islands, Alaska, and are compared with published values for other otariid species.     

Feeding ecology of the endangered Valencia toothcarp,Valencia hispanica (Actinopterygii: Valenciidae)

Patterns of food resource utilisation by Valencia toothcarp were studied in a small coastal lagoon in the Iberian Peninsula, its most northern population. Emphasis was placed on feeding activity, diet composition, seasonal and intraspecific variation in diet and feeding strategy. Important features of the overall feeding pattern of the Valencia toothcarp are decreased feeding activity during winter, concentration on benthic invertebrates and narrow dietary breadth, with a focus on gammarid amphipods. The feeding of V. hispanica was independent of seasonal variation in food availability, because its main prey were much the same throughout the year. There were no differences in seasonal niche width and niche overlap between seasons was high. There are also no differences in diets between sexes, although diet composition, niche width and niche overlap differ between juvenile and adult fish. The Valencia toothcarp population from Santes Creus lagoon was formed by specialist individuals, with narrow niche widths, that fed on four preferential prey types, but also consumed some occasional prey.     

Geographical variation in the diet of the Antarctic fur seal<Emphasis Type="Italic"> Arctocephalus gazella</Emphasis>

The diet of non-breeding male Antarctic fur seals Arctocephalus gazella was investigated at different localities of the Antarctic Peninsula (Cierva Point and Hope Bay), South Shetland Islands (Deception Island and Potter Peninsula) and the South Orkney Islands (Laurie Island), by the analysis of 438 scats collected from January to March 2000. The composition of the diet was diverse, with both pelagic and benthic-demersal prey represented in the samples. Antarctic krill Euphausia superba was the most frequent and numerous prey at all the study sites except at Cierva Point, followed by fish, penguins and cephalopods. Antarctic krill also predominated by mass, followed by either fish or penguins. Fish were the second most important prey by mass at the Antarctic Peninsula whereas penguins were the second most important prey by mass at the South Shetland and South Orkney Islands. Among fish, Pleuragramma antarcticum was the most important species in the diet of the Antarctic fur seals at the Antarctic Peninsula whereas Gymnoscopelus nicholsi predominated at the South Shetland and South Orkney Islands. The results are compared with previous studies, and the possibility of implementing monitoring studies on the distribution/abundance of myctophids and P. antarcticum based on the analysis of the diet of the Antarctic fur seal is considered.     

Behavior of Cape fur seals (Arctocephalus pusillus pusillus) in relation to temporal variation in predation risk by white sharks (Carcharodon carcharias) around a seal rookery in False Bay,South Africa

The marked differences in predation risk posed by white sharks (Carcarodon carcarias) at island rookeries of Cape fur seals (Arctocephalus pusillus pusillus) offer a quasi‐experimental design within a natural system for exploring how prey adjust their behavior in response to temporal variation in predation risk. Here we compare movement of juvenile and adult Cape fur seals at a high risk (Seal Island) and low risk (Egg Island) rookery. We further compare juveniles and adults at Seal Island in low and high risk seasons and at low and high risk times of day within those seasons. Adult fur seals at Seal Island avoided traversing the zone of high white shark predation risk during the high risk period (0700–0959) in the season of high risk (winter), but not during the low risk season (summer). By contrast, adult fur seals at Egg Island showed no temporal discretion in either season. Unlike juvenile fur seals at Egg Island, juveniles at Seal Island adjusted their temporal movement patterns to more closely mimic adult seal movement patterns. This suggests that exposure to predators is the primary driver of temporal adjustments to movement by prey species commuting from a central place.     

The Survival Dynamics of Male Northern Fur Seals (<Emphasis Type="Italic">Callorhinus ursinus</Emphasis>) on Tyulenii Island,Sea of Okhotsk,Based on Long-Term Observations

We discuss and improve the methods proposed previously for estimating the survival rate of male northern fur seals representing different age groups. Estimates of the number of pups and bulls per rookery are given, as well as information on the age structure of animals hunted during the coastal harvest. The data were obtained by researchers from the Pacific Research Fisheries Center over 56 years of observation of the fur seal (Callorhinus ursinus) herd on Tyulenii Island, located in the southwestern part of the Sea of Okhotsk, south of Cape Terpenia, 15 km from Sakhalin Island. Lander’s method and its modifications were used for estimating the juvenile survival rate of male fur seals. This methodology was found to have not been working properly for the past several decades (since the end of the 1980s) due to changes in population harvesting. Satisfactory estimates for all characteristics of the male life cycle were obtained. Structural changes in survival ability were revealed as having occurred at the end of the 1980s, i.e., the survival of subadult and adult males increased slightly. New estimates of survival rates allow us to create a model of the dynamics of bull numbers that is in good agreement with that observed, its mean error of approximation equaling 3.2%.     

King-size fast food for Antarctic fur seals

The study of scats of adult male Antarctic fur seals Arctocephalus gazella (AFS) revealed occasional frequent capture of penguins. Although AFS adult males have been occasionally reported to kill king penguins Aptenodytes patagonicus at the shore, here we report the first observations of at-sea predation by AFS on adult king penguins in the Crozet archipelago, southern Indian Ocean. During our 20 days survey, we observed 17 penguins attacked and either severely injured or killed and consumed. Only AFS adult males were seen catching king penguins successfully. Some adult females and sub-adults also attempted to catch penguins, either at sea or in land, and so did subantarctic fur seals Arctocephalus tropicalis individuals. Our results confirm the ability of fur seals to catch and predate large seabirds, even at sea.     

Thermal limits in young northern fur seals,Callorhinus ursinus

The thermoregulatory abilities of northern fur seals (Callorhinus ursinus) during their first two years in the frigid waters of the North Pacific Ocean may limit their geographic distribution and alter the costs for exploiting different species of prey. We determined the thermoneutral zone of six young northern fur seals by measuring their metabolism in ambient air and controlled water temperatures (0°C–12°C) from ages 8 to 24 mo. We found that the ambient air temperatures within our study (overall 1.5°C–23.9°C) did not affect resting metabolic rates. Calculated lower critical temperatures in water varied between 3.9°C and 8.0°C, while an upper critical temperature in water was only discernible during a single set of trials. These thermal responses provide insight into the possible physiological constraints on foraging ecology in young northern fur seals, as well as the potential energetic consequences of ocean climate change and altered prey distributions.     

Does size dimorphism reduce competition between sexes? The diet of male and female pine martens at local and wider geographical scales

Sex-specific niche segregation is often used to explain sexual size dimorphism (SSD). However, whether food niche partitioning between sexes occurs as a case of sexual size dimorphism or by other mechanisms, such as behavioural dimorphism or habitat segregation, remains poorly understood. To evaluate the nature and extent of food-niche differentiation between sexes in a solitary predator I examined variation in the diet of male and female pine martensMartes martes Linnaeus, 1758 in years of high and low rodent abundance. Small mammals were the most important prey for pine martens in years of both low and high rodent abundance (occurring in more than 49% of scats). Birds, invertebrates and plant material were relatively common food items in summer diet, whereas ungulate carcasses were often consumed in autumn—winter. In general, males consumed more ungulate carcasses, plant material, amphibians and reptiles than did females, whereas females preyed more on squirrels and birds than males. There was significant seasonally dependent, between-sex variation in the occurrence of shrews, small rodents, other mammals, birds and invertebrates in marten diet. Whereas the occurrence of bank vole, birds, carcasses and plant material changed between sexes, seasons and years with various rodent abundances, both sexes consumed larger prey and had increased food niche breadth in years of low compared with high rodent abundance. Neither prey size nor food niche breadth were significantly different between males and females. The food-niche overlap between sexes was consistently lower in spring and in years of low rodent abundance. A wider geographical comparison of different marten populations showed that the diet of males and females varied significantly between locations. Females consistently preyed on squirrels and birds, whereas males fed more often on ungulate carcasses and plant material. Local and geographical comparison of male and female diets suggest that food-niche partitioning between male and female pine martens changes across different habitat and food conditions, and is not related to sexual size dimorphism, but rather to behavioural differences between sexes.     

Alaskan marine mammal tissue archival project

A project to establish an archive of Alaskan marine mammal tissues was conceived in 1987 to be a part of the National Biomonitoring Specimen Bank (NBSB). Protocols and field collection of marine mammals, long-term storage, and analysis are summarized in this paper. Instrumental neutron activation analysis has been used for an initial evaluation of trace element content in samples of northern fur seal (Callorhinus ursinus) from the Pribilof Islands. The findings agree with previously observed trace element levels in northern fur seals. The archived specimens can be used in future studies when comparisons of past and present pollution levels are needed.