EVALUATION OF AN EFFECTIVE METHOD TO ESTIMATE AGE OF CAPE FUR SEALS USING GROUND TOOTH SECTIONS

Teeth of known-age Cape fur seals Arctocephalus pusillus pusillus were used to validate age estimated from ground sections. In the canines, dentine growth layer groups (GLGs) reflected age accurately but no reliable readings could be obtained from GLGs in the cementum. Upper canines were the most suitable for age estimation. By contrast, in the postcanines where the cementum is thicker, only GLGs in the cementum could be used for age determination, but not with the same accuracy as for dentine in the canines. Therefore, it is recommended that GLGs in the dentine be used to determine age in the Cape fur seal. However, pulp cavities in canines closed at about 13 yr and consequently GLGs in the cementum of the postcanines should be used where the pulp cavities of canines are closed. Accurate estimation of age is not possible from the dentine of older animals.     

COUNTS OF GROWTH LAYER GROUPS IN CEMENTUM AND DENTINE IN RINGED SEALS (PHOCA HISPIDA)

We compared counts of growth layer groups (GLGs) in the dentine of un-decalcified, unstained cross-sections and in the cementum of decalcified, stained longitudinal sections of canine teeth from 144 ringed seals ( Phoca hispida ). Although there was a statistically significant correlation until approximately 10 GLGs, about 75% of paired readings at ≤ 10 cementum GLGs disagreed. After 10 GLGs, the number of GLGs in the cementum usually was greater. The maximum GLG count in cementum was 33, compared to a maximum in dentine of only 19. Interobserver differences in median counts were not statistically significant using cementum or dentine counts. Regression analysis revealed that for cementum in female seals, readers differed at higher counts (P < 0.05), and for dentine, there was a constant difference of about 0.6 GLGs ( P < 0.05) for male seals and 1.1 GLGs ( P < 0.05) for female seals. Counting GLGs in the cementum of decalcified and stained longitudinal sections provided higher counts and more agreement between readers, and it was the better of the methods examined for ageing ringed seals.     

Prey capture and processing behaviors vary with prey size and shape in Australian and subantarctic fur seals

When hunting at sea, pinnipeds should adapt their foraging behaviors to suit the prey they are targeting. We performed captive feeding trials with two species of otariid seal, Australian fur seals (Arctocephalus pusillus doriferus) and subantarctic fur seals (Arctocephalus tropicalis). This allowed us to record detailed observations of how their foraging behaviors vary when presented with prey items that cover the full range of body shapes and sizes encountered in the wild. Small prey were captured using suction alone, while larger prey items were caught in the teeth using raptorial biting. Small fish and long skinny prey items could then be swallowed whole or processed by shaking, while all prey items with body depths greater than 7.5 cm were processed by shaking at the water's surface. This matched opportunistic observations of feeding in wild Australian fur seals. Use of “shake feeding” as the main prey processing tactic also matches predictions that this method would be one of the only tactics available to aquatic tetrapods that are unable to secure prey using their forelimbs.     

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.     

Interannual variation in pup production and the timing of breeding in benthic foraging Australian fur seals

Large interannual variations in reproductive success caused by fluctuations in oceanography and prey availability are common to many species of air breathing epipelagic predators. In contrast, little is known about variation in benthic foragers such as Australian fur seals ( Arctocephalus pusillus doriferus ). Between 1997 and 2007, pup production was assessed in 9 yr, while the timing of breeding and adult female condition was assessed in 5 yr at Kanowna Island in Bass Strait, southeastern Australia. Pup production was variable (     = 1,726 ± 42, range = 1,386–2,301), but without temporal trend, as was median birth date (     = 23 November ± 1, range = 21–25 November) and pupping synchrony (period of 90% births:     = 28 ± 2 d, range = 23–31 d). Pup production was negatively correlated with median birth date and positively correlated with female condition, winter sea-surface temperature (SST) and zonal wind strength within Bass Strait. Pup production was also negatively correlated with SST in the previous summer within Bass Strait and in the eastern Great Australian Bight upwelling region. The results suggest that the reproductive success of Australian fur seals is influenced by oceanography but less so than in otariids foraging epipelagically in major upwellings. Despite spanning several El Niño events, no correlation between pup production and the Southern Oscillation Index was observed.     

Multi-decadal trends in biomarkers in harp seal teeth from the North Atlantic reveal the influence of prey availability on seal trophic position

Arctic food webs are being impacted by borealisation and environmental change. To quantify the impact of these multiple forcings, it is crucial to accurately determine the temporal change in key ecosystem metrics, such as trophic position of top predators. Here, we measured stable nitrogen isotopes (δ¹⁵N) in amino acids in harp seal teeth from across the North Atlantic spanning a period of 60 years to robustly assess multi-decadal trends in harp seal trophic position, accounting for changes in δ¹⁵N at the base of the food web. We reveal long-term variations in trophic position of harp seals which are likely to reflect fluctuations in prey availability, specifically fish- or invertebrate-dominated diets. We show that the temporal trends in harp seal trophic position differ between the Northwest Atlantic, Greenland Sea and Barents Sea, suggesting divergent changes in each local ecosystem. Our results provide invaluable data for population dynamic and ecotoxicology studies.     

Annual variability in dentin δ15N and δ13C reveal sex differences in weaning age and feeding habits in Risso's dolphins (Grampus griseus)

Teeth of odontocetes accumulate annual dentinal growth layer groups (GLGs) that record isotope ratios, which reflect the time of their synthesis. Collectively, they provide lifetime records of individual feeding patterns from which life history traits can be inferred. We subsampled the prenatal dentin and postnatal GLGs in Risso's dolphins (Grampus griseus) (n = 65) that stranded or were collected as bycatch in Taiwan (1994–2014) and analyzed them for δ¹⁵N and δ¹³C. Age‐specific δ¹⁵N and δ¹³C values were corrected for effects of calendar year, stranding site, C/N, and sex. δ¹⁵N values were higher in prenatal layers (14.94‰ ± 0.74‰) than in adult female GLGs (12.58‰ ± 0.20‰), suggesting fetal enrichment during gestation. Decreasing δ¹⁵N values in early GLGs suggested changes in dietary protein sources during transition to complete weaning. Weaning age was earlier in males (1.09 yr) than in females (1.81 yr). Significant differences in δ¹⁵N values between weaned males and females suggest potential sexual segregation in feeding habits. δ¹³C values increased from the prenatal to the 4th GLG by ~1.0‰, indicative of a diet shift from ¹³C‐depleted milk to prey items. Our results provide novel insights into the sex‐specific ontogenetic changes in feeding patterns and some life history traits of Risso's dolphins.     

Prey selection of subadult male northern fur seals (Callorhinus ursinus) and evidence of dietary niche overlap with adult females during the breeding season

During the breeding season northern fur seals (Callorhinus ursinus) congregate on the Pribilof Islands in large numbers creating the potential for intraspecific competition. Due to the declining trend in the Pribilof Islands population of fur seals, it is important to understand how prey resources are partitioned among the population. Fur seals exhibit a high degree of sexual dimorphism resulting in energetic differences among age and sex classes. Therefore, we hypothesized that subadult male and adult female fur seals would differ in the type and size of prey consumed. We examined the diets of subadult male (age 2–8; mean mass 28–176 kg) and adult female (age ≥ 3 yr; mean mass 13–50 kg) seals on St. Paul Island from 1992 to 2000. Prey remains found in fecal samples were compared using niche overlap indices. There was nearly complete dietary niche overlap between subadult male and adult female fur seals. Walleye pollock (Theragra chalcogramma), Pacific salmon (Oncorhynchus spp.), Pacific herring (Clupeia pallasi), and cephalopods were common prey items found in the diets of both groups. We found differences in the size of pollock consumed and that geographic location of sample collection may be important in determining diet differences. Our results indicate high levels of dietary overlap among subadult male and adult female fur seals.     

Using stable isotopes to understand changes in ringed seal foraging ecology as a response to a warming environment

Trends toward increased temperatures, reduced sea ice extent, and longer open water seasons have resulted in changing Arctic ecosystem dynamics. Expected changes include shifts in distribution and abundance of prey species for seabirds and marine mammals. Using stable isotope analysis, we studied spatial and interannual variation in ringed seal (Pusa hispida) feeding ecology in Hudson Bay in relation to environmental variables, between 2003 and 2010. Ringed seal muscle and hair samples collected from Arviat and Sanikiluaq, Nunavut, were analyzed for stable isotope ratios of nitrogen (δ¹⁵N) and carbon (δ¹³C). Seals from western Hudson Bay (Arviat) had higher δ¹⁵N and lower δ¹³C than seals from eastern Hudson Bay (Sanikiluaq), and stable isotope ratios varied interannually within each region. Peak δ¹⁵N occurred in years with spring air temperatures between approximately ?5°C and ?2°C. This temperature range was characteristic of warm years in western Hudson Bay and cool years in eastern Hudson Bay. We hypothesize that the high δ¹⁵N observed in ringed seals is indicative of greater importance of capelin (Mallotus villosus) in ringed seal diet. A comparison of ringed seal isotopic niche widths indicated greater dietary differences between western and eastern Hudson Bay with warming, suggesting a possible ecological divergence related to climate change.     

Accuracy and precision of age determination using growth layer groups for California sea lions (Zalophus californianus) with known ages

Age determination from counts of growth layer groups (GLGs) in tooth dentine is a common method for aging marine mammals. Using known‐aged animals, we validated this method for acid etched teeth of California sea lions (CSLs), Zalophus californianus. Between 1991 and 2013, the upper left canine (n = 33) was collected opportunistically during necropsy from animals tagged or branded as pups that later died. Overall, 55%–61% of age estimates by GLG counting were within 1 yr of the known‐age in the sample of 1–30‐yr‐old CSLs. Accuracy of age estimates was found to be dependent on age of the CSLs, however. 71%–79% of age estimates were within 1 yr of the known‐age in CSLs <10 yr old. These findings support the validity of counting GLGs to estimate age for CSLs <10 yr old to within 1 yr of accuracy.     

Age estimation and growth layer patterns in teeth of crabeater seals: A comparison of techniques

Age estimation of marine mammals provides important information about ecological and life history parameters. Counting growth layer groups (GLGs) in the dentine and cementum of teeth is the most common technique for age estimation in pinnipeds. In this study, we used acid-etched canines (n = 38) and decalcified stained postcanine sections (n = 40) to calibrate readings in mummified crabeater seals (Lobodon carcinophaga). A subsample of this group received a prior cleaning treatment of boiling the teeth with hydrogen peroxide (H₂O₂). Ages ranged from 0 to 31 years. Dentine from canines and cementum from postcanines showed higher estimated ages (maximum 31 years) than dentine from postcanines (maximum 24 years), mainly in those teeth that did not receive prior treatment. Boiling with H₂O₂ has shown to affect cementum structure and, thus, results in underestimated ages, but dentine did not seem to be affected. The results presented here showed that ages can be estimated for the crabeater seal regardless of which tooth/tissue or method is used, at least for seals <13 years old. However, nontreated, stained thin sections of postcanine teeth are recommended because more reliable age estimations in older crabeater seals are obtained.     

The diet of Antarctic fur seals,Arctocephalus gazella,during the summer-autumn period at Mossman Peninsula,Laurie Island (South Orkneys)

Summary From mid January to April 1988 a total of 105 faecal samples of non breeding male fur seals, Arctocephalus gazella, was collected at Mossman Peninsula, Laurie Island. Examination of scats containing prey remains showed that fur seals fed mainly on krill, occurring in 100% of samples but fish were also a common food item (78.8%). Myctophids constituted more than 90% of fish taken by fur seals. Squid beaks occurred in 34.3% of scats, being more common in the autumn period.     

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.     

The diet of Antarctic fur seals, Arctocephalus gazella, at King George Island, during the summer–autumn period

The diet of non-breeding male Antarctic fur seals, Arctocephalus gazella, was investigated at Stranger Point, King George Island, by scat analysis from February to April 1996. Overall, krill and fish were the most frequent prey, occurring in an average of 97% and 69% of samples (n=128), followed by cephalopods (12%). Myctophids constituted almost 90% of the fish predated, with Electrona antarctica and Gymnoscopelus nicholsi being the most abundant and frequent species consumed. All fish taxa identified were krill-feeding species suggesting that seals foraged primarily on krill and opportunistically on fish species associated with krill swarms. A seasonal change observed in the relative proportions of the different fish prey taxa indicates that fur seals spent more time foraging over the shelf in summer and off the shelf in autumn. During the study period, commercial fishing in the area was not based upon any of the fish identified in this study.     

Internal physiology of live krill revealed using new aquaria techniques and mixed optical microscopy and optical coherence tomography (OCT) imaging techniques

The Guadalupe fur seal (Arctocephalus townsendi) is a specialist predator feeding on prey present in one trophic level. Data related to the diet of the Guadalupe fur seal are few. It is still unknown where most of the individuals forage or the composition of their diet. On Isla Guadalupe, the San Benito Archipelago and the Farallon Islands, fur seals primarily feed on pelagic and coastal squids. However, differences between colonies were found probably caused by differences in diversity and abundance of prey species over the continental shelf and the pelagic environment, and maybe due to the plasticity of the species in their foraging behavior. Diet composition of the Guadalupe fur seal might reflect adaptations to local and temporal environmental conditions. The aim of this work was to consider historical information, add new information, identify main prey species, and determine where in the marine regions the Guadalupe fur seals feed.     

Lunar cycles in diel prey migrations exert a stronger effect on the diving of juveniles than adult Galápagos fur seals.

In our study of the development of diving in Galápagos fur seals, we analysed changes in diving activity and body mass trends over the lunar cycle. Based on previously observed lunar cycles in colony attendance patterns, we hypothesized a greater impact of prey migrations of deep scattering layer organisms on younger fur seals. Using electronic dive recorders, we determined that seals dived less and deeper on moonlit nights than at new moon, and incurred body mass losses. These changes in foraging over the lunar cycle correlate with the suppression of the vertical migration of prey by lunar light. All effects were more pronounced in juveniles than adult females, with greater relative mass loss during full moon, which must (i) negatively affect long-term juvenile growth rates, (ii) lengthen periods of maternal dependence, and (iii) contribute to the lowest reproductive rate reported for seals. This underlines the importance of studying ontogeny in order to understand life histories, and for determining the susceptibility of animal populations to fluctuations in food availability.     

Penguins, fur seals, and fishing: prey requirements and potential competition in the South Shetland Islands, Antarctica

Antarctic and sub-Antarctic seabirds, marine mammals, and human fisheries concentrate their foraging efforts on a single species, Antarctic krill (Euphausiasuperba). Because these predators may have a significant effect on krill abundance, we estimated the energy and prey requirements of Adelie (Pygoscelisadeliae), chinstrap (Pygoscelisantarctica), and gentoo (Pygoscelispapua) penguins and female Antarctic fur seals (Arctocephalusgazella) breeding on the South Shetland Islands, Antarctica and compared these estimates with catch statistics from the Antarctic krill fishery. Published data on field metabolic rate, population size, diet, prey energy content, and metabolic efficiency were used to estimate prey requirements of these breeding, adult, land-based predators and their dependent offspring. Due to their large population size, chinstrap penguins were the most significant krill predators during the period examined, consuming an estimated 7.8 × 10⁸ kg krill, followed by Adelie penguins (3.1 × 10⁷ kg), gentoo penguins (1.2 × 10⁷ kg), and Antarctic fur seals (3.6 × 10⁶ kg). Total consumption of all land-based predators on the South Shetland Islands was estimated at 8.3 × 10⁸ kg krill. The commercial krill fishery harvest in the South Shetland Island region (1.0 × 10⁸ kg) was approximately 12% of this. Commercial harvest coincides seasonally and spatially with peak penguin and fur seal prey demands, and may affect prey availability to penguins and fur seals. This differs from the conclusions of Ichii et al. who asserted that the potential for competition between South Shetland predators and the commercial krill fishery is low. Received: 26 August 1997 / Accepted: 16 December 1997     

Feeding ecology of phocid seals and some walrus in the Alaskan and Canadian Arctic as determined by stomach contents and stable isotope analysis

Feeding habits of ringed (Phoca hispida), bearded (Erignathus barbatus), spotted (Phoca largha) and ribbon (Phoca fasciata) seals and walrus (Odobenus rosmarus) were studied using stomach contents and stable carbon and nitrogen isotopes. Bearded seals fed benthically, primarily crustaceans and mollusks. Both zooplankton and fish were significant prey for ringed seals, while fish was principal spotted seal prey. Few gastric contents were available from ribbon seals. δ¹⁵N was positively correlated with age in ribbon seals and δ¹³C was positively correlated with age in ringed and ribbon seals. δ¹⁵N was highest in spotted seals, in agreement with their fish-dominated diet. δ¹⁵N was not different between Alaskan-harvested ringed and bearded seals, while δ¹⁵N was lowest in ribbon seals and walrus. Carbon-13 was most enriched in bearded seals and walrus reflecting benthic ecosystem use. Canadian ringed seals were depleted in ¹³C compared to Alaskan pinnipeds, likely because of Beaufort Sea versus Chukchi and Bering seas influence.     

Seasonal diving behaviour in lactating subantarctic fur seals on Amsterdam Island

Diving behaviour was investigated in female subantarctic fur seals (Arctocephalus tropicalis) breeding on Amsterdam Island, Indian Ocean. Data were collected using electronic Time Depth Recorders on 19 seals during their first foraging trip after parturition in December, foraging trips later in summer, and during winter. Subantarctic fur seals at Amsterdam Island are nocturnal, shallow divers. Ninety-nine percent of recorded dives occurred at night. The diel dive pattern and changes in dive parameters throughout the night suggest that fur seals follow the nycthemeral migrations of their main prey. Seasonal changes in diving behaviour amounted to the fur seals performing progressively deeper and longer dives from their first foraging trip through winter. Dive depth and dive duration increased from the first trip after parturition (16.6 ± 0.5 m and 62.1 ± 1.6 s respectively, n=1000) to summer (19.0 ± 0.4 m and 65 ± 1 s, respectively, n=2000) through winter (29.0 ± 1.0 m and 91.2 ± 2.2 s, respectively, n=800). In summer, subantarctic fur seals increased the proportion of time spent at the bottom during dives of between 10 and 20 m, apparently searching for prey when descending to these depths, which corresponded to the oceanic mixed layer. In winter, fur seals behaved similarly when diving between 20 and 50 m, suggesting that the most profitable depths for feeding moved down during the study period. Most of the dives did not exceed the physiological limits of individuals. Although dive frequency did not vary (10 dives/h of night), the vertical travel distance and the time spent diving increased throughout the study period, while the post-dive interval decreased, indicating that subantarctic fur seals showed a greater diving effort in winter, compared to earlier seasons. Accepted: 1 August 1999     

Changes in diets of individual Baltic ringed seals (Phoca hispida botnica) during their breeding season inferred from stable isotope analysis of multiple tissues

The stable isotope ratios (δ¹³C and δ¹⁵N) of three tissues with different metabolic rates (plasma, liver, and muscle) were used to investigate temporal variation in diet among nine individual Baltic ringed seals (Phoca hispida botnica Gmelin) from the Bothnian Bay, northeast Baltic Sea. The isotope values from plasma should reflect the most recent diet, values from liver the diet of the past weeks prior to sampling, and values from muscle should integrate diet over almost the entire breeding season of the ringed seals. In general, δ¹³C values of liver were more enriched in ¹³C than were those of either muscle or plasma, suggesting that the diet of the seals may have included a higher proportion of ¹³C‐enriched benthic prey in April. Females showed more variable δ¹³C values than males, suggesting possible gender differences in diet or in foraging locations. The differences that were apparent between females possibly reflect individual variation in the onset and duration of parturition and lactation, both of which likely restrict female foraging. Previous data from parasite infections and from alimentary tract contents of the same seals were linked to the isotope data to assist in drawing inferences about changes in the diets of individual seals.