Experimental evidence of seawater drinking in juvenile hooded (Cystophora cristata) and harp seals (Phoca groenlandica)

This study was undertaken to measure whether young harp seals (Phoca groenlandica) and hooded seals (Cystophora cristata) drink seawater and, if so, to investigate how the excess salt load is handled. Blood and urine samples were collected from hooded seal pups (n=3) and harp seal pups (n=3) after 2 weeks of freshwater exposure, at intervals during 3 weeks of seawater exposure and, finally, after 2 weeks of re-exposure to fresh water. Total water turnover, as measured by injection of tritiated water, was 2200 ml · day⁻¹ and 3300 ml · day⁻¹ in hooded seals and harp seals, respectively. The extent of mariposia was taken as the difference between total water turnover and influx of water through food (free and metabolic water) and respiratory water exchange. Seawater drinking amounted to 14% and 27% of total water turnover (r_H2O) for the hooded seals and harp seals, respectively. Further evidence of mariposia was obtained from an increase in the excretion rate of the urine osmolytes Na⁺, Cl⁻ and Mg²⁺, during the period of seawater exposure. It is concluded that water influx due to seawater drinking can not be excluded as a source of error when estimating food consumption of free-ranging harp seals and hooded seals, by use of labeled water techniques. Accepted: 11 May 2000     

Variation in blubber fatty acid composition among marine mammals in the Canadian Arctic

The composition of predator adipose stores can provide important insights into foraging patterns and the ecological relationships among species. We determined the fatty acid (FA) composition of 843 blubber samples from 80 bearded seals (Erignathus barbatus), 33 harbor seals (Phoca vitulina), 239 harp seals (Pagophilus groenlandicus), 32 hooded seals (Cystophora cristata), 281 ringed seals (Phoca hispida), 53 walruses (Odobenus rosmarus rosmarus), 105 beluga whales (Delphinapterus leucas), and 20 narwhals (Monodon monoceros) across the Canadian Arctic to examine patterns of variability among and within species. FA signatures accurately distinguished phocid seals, walruses, and whales. Belugas and narwhals had the most similar FA signatures of any two species, suggesting substantial overlap in their diets, especially in the narwhal‐wintering area off eastern Baffin Island. Among phocid seals, harp and hooded seals had the most similar FA signatures. Bearded seals were most similar to walruses, which was consistent with the benthic feeding habits of both species. Within species, geographic differences in FA signatures were found over both large (>4,000 km) and small (<100 km) spatial scales. Overall, within‐species differences were smaller than among‐species differences. In general, FA signature patterns were consistent with previous studies of the ecology and diets of arctic marine mammals.     

Investigation of mercury concentrations in fur of phocid seals using stable isotopes as tracers of trophic levels and geographical regions

Recent studies have shown that the complementary analysis of mercury (Hg) concentrations and stable isotopic ratios of nitrogen (δ¹⁵N) and carbon (δ¹³C) can be useful for investigating the trophic influence on the Hg exposure and accumulation in marine top predators. In this study, we propose to evaluate the interspecies variability of Hg concentrations in phocids from polar areas and to compare Hg bioaccumulation between both hemispheres. Mercury concentrations, δ¹⁵N and δ¹³C were measured in fur from 85 individuals representing 7 phocidae species, a Ross seal (Ommatophoca rossii), Weddell seals (Leptonychotes weddellii), crabeater seals (Lobodon carcinophagus), harbour seals (Phoca vitulina), grey seals (Halichoerus grypus), ringed seals (Pusa hispida) and a bearded seal (Erignathus barbatus), from Greenland, Denmark and Antarctica. Our results showed a positive correlation between Hg concentrations and δ¹⁵N values among all individuals. Seals from the Northern ecosystems displayed greater Hg concentrations, δ¹⁵N and δ¹³C values than those from the Southern waters. Those geographical differences in Hg and stable isotopes values were likely due to higher environmental Hg concentrations and somewhat greater number of steps in Arctic food webs. Moreover, dissimilarities in feeding habits among species were shown through δ¹⁵N and δ¹³C analysis, resulting in an important interspecific variation in fur Hg concentrations. A trophic segregation was observed between crabeater seals and the other species, resulting from the very specific diet of krill of this species and leading to the lowest observed Hg concentrations.     

Size and distribution of oxygen stores in harp and hooded seals from birth to maturity

Pinnipeds rely primarily on oxygen stores in blood and muscles to support aerobic diving; therefore rapid development of body oxygen stores (TBO₂) is crucial for pups to transition from nursing to independent foraging. Here, we investigate TBO₂ development in 45 harp (Pagophilus groenlandicus) and 46 hooded (Cystophora cristata) seals ranging in age from neonates to adult females. We found that hooded seal adults have the largest TBO₂ stores yet reported (89.5 ml kg⁻¹), while harp seal adults have values more similar to other phocids (71.6 ml kg⁻¹). In adults, large TBO₂ stores resulted from large blood volume (harp169, hood 194 ml kg⁻¹) and high muscle Mb content (harp 86.0, hood 94.8 mg g⁻¹). In contrast, pups of both species had significantly lower mass-specific TBO₂ stores than adults, and stores declined rather than increased during the nursing period. This decline was due to a reduction in mass-specific blood volume and the absence of an increase in the low Mb levels (harp 21.0, hood 31.5 mg g⁻¹). Comparisons with other phocid species suggests that the pattern of blood and muscle development in the pre- and post-natal periods varies with terrestrial period, and that muscle maturation rates may influence the length of the postweaning fast. However, final maturation of TBO₂ stores does not take place until after foraging begins.     

Large increases of harp seals (Phoca groenlandica) and hooded seals (Cystophora cristata) on Sable Island, Nova Scotia, since 1995

Beach surveys for harp (Phoca groenlandica) and hooded (Cystophora cristata) seals documented a dramatic increase in their numbers on Sable Island in the mid-1990s. From late 1994 to 1998, 1,191 harp and 870 hooded seals, mostly young animals, were recorded on the island whereas, in the 1980s, no more than 5 animals of both species were observed each year. Of the 2,061 harp and hooded seals examined, 41.7% were found alive, 26.7% were killed by sharks, and 31.6% were found dead but intact. This increase in numbers of harp and hooded seals on Sable Island, which is south of their historic northern range, is consistent with the recent increase of extralimital occurrences of these species along the east coast of North America. However, the large number of seals recorded in this study provides more information on their demography than has previously been possible.     

THE TROPHIC ROLE OF MARINE MAMMALS IN THE NORTHERN GULF OF ST. LAWRENCE

The trophic role of apex predators was evaluated in the northern Gulf of St. Lawrence ecosystem. An Ecopath model was developed for the period 1985–1987 prior to the collapse of commercially exploited demersal fish stocks in this area. Marine mammal trophic levels were estimated by the model at 4.1 for cetaceans, 4.4 for harp seals (Pagophilus groenlandicus), 4.7 for hooded seals (Cystophora cristata), 4.5 for gray seals (Halichoerus grypus), and 4.3 for harbor seals (Phoca vitulina). Harp seals were the third most important predator on vertebrate prey following large Atlantic cod (Gadus morhua) and redfish (Sebastes spp.). Different seal species preyed on different levels of the food chain. Harp seals preyed on most trophic groups, whereas larger seals, such as gray seals and hooded seals, mainly consumed higher trophic levels. The model suggested that apex predators had a negative effect on their dominant prey, the higher trophic level fish, but an indirect positive feedback on the prey of their preferred prey, mainly American plaice (Hippoglossoides platessoides), flounders, skates, and benthic invertebrates. Our results suggest that both marine mammals and fisheries had an impact on the trophic structure.     

Histological,histochemical, and ultrastructural investigations on the gastrointestinal system of antarctic seals: Weddell seal (Leptonychotes weddellii) and crabeater seal (Lobodon carcinophagus)

The morphology of the principal sections of the gastrointestinal system of two Antarctic seals with different dietary habits, namely, the Weddell seal (Leptonychotes weddellii) and the crabeater seal (Lobodon carcinophagus), has been investigated. Histologically examined by light microscopy, the tissue layers of the gastrointestinal tract of both seals are almost identical to those observed in most other mammals and no major differences in principle organization could be found between the two seal species. The ultrastructure of the gastric and intestinal epithelial cells has been examined and is also closely comparable to that of these cells in other mammals; however, Paneth cells have not been found in our material. In general, therefore, adaptations of the gastrointestinal tract to the aquatic environment or the diet are not obvious at the morphological levels of organization studied. Histochemical differences are found between the two closely related species; mucins of the surface epithelium in the stomach of Weddell seals are highly sulfated, while those in the crabeater seal are not. Mucous neck cells in Weddell seals contain acid mucosubstances, while those of crabeater seals contain neutral ones. Goblet cells in the small and large intestine in Weddell seals contain both neutral and acid mucosubstances. Both mucin types are detected in the crabeater seal; however, the mucins of the colon in the crabeater seal are more highly sulfated than those in the Weddell seal. The ratio of globet cells to enterocytes in the large intestine of crabeater seals is higher than that in Weddell seals. © 1995 Wiley-Liss, Inc.     

Diving patterns of a Ross seal (Ommatophoca rossii ) near the eastern coast of the Antarctic Peninsula

In January 1987 we documented the diving patterns of a female Ross seal (Ommatophoca rossii) in the marginal pack-ice zone near the eastern coast of the Antarctic Peninsula for 2 days using a microprocessor-based time-depth recorder. The seal hauled out during the day and dived continually when in the water at night. Dives averaged 110 m deep and 6.4 min long; the deepest dive was 212 m and the longest 9.8 min. Dives were deepest near twilight and shallowest at night; this pattern suggests that the seal's prey, presumably mid-water squid and fish, may have been making vertical migrations or changing predator-avoidance behavior in response to diel light patterns. The dives of this Ross seal were substantially deeper, on average, than those of crabeater seals (Lobodon carcinophagus), which forage in the same areas on Antarctic krill (Euphausia superba). Received: 15 August 1996 / Accepted: 22 February 1997     

Movements and diving of adult ringed seals (Phoca hispida) in Svalbard

Seven post-moulting adult ringed seals (Phoca hispida) were equipped with Satellite Linked Dive Recorders in Svalbard in July 1996 to determine if ringed seals conduct long-distance post-moulting feeding excursions, and to obtain details of their diving behaviour. The mean duration of tags was 206 days (range 103–325). Two seals swam 400 km north to the drifting pack ice (82°N). The rest undertook more local movements. Forty-eight percent of all dives were shallower than 20 m and 90% were shallower than 100 m. Ninety-five percent of all dive durations were shorter than 10 min, and 99.5% were shorter than 15 min. This study has shown that adult ringed seals undertake varying patterns of post-moulting excursions. Accepted: 1 April 2000     

Seawater drinking restores water balance in dehydrated harp seals

The purpose of this study was to answer the question of whether dehydrated harp seals (Phoca groenlandica) are able to obtain a net gain of water from the intake of seawater. Following 24 h of fasting, three subadult female harp seals were dehydrated by intravenous administration of the osmotic diuretic, mannitol. After another 24 h of fasting, the seals were given 1,000 ml seawater via a stomach tube. Urine and blood were collected for measurement of osmolality and osmolytes, while total body water (TBW) was determined by injections of tritiated water. In all seals, the maximum urinary concentrations of Na⁺ and Cl⁻ were higher than in seawater, reaching 540 and 620 mM, respectively, compared to 444 and 535 mM in seawater. In another experiment, the seals were given ad lib access to seawater for 48 h after mannitol-induced hyper-osmotic dehydration. In animals without access to seawater, the mean blood osmolality increased from 331 to 363 mOsm kg⁻¹ during dehydration. In contrast, the blood osmolality, hematocrit and TBW returned to normal when the seals were permitted ad lib access to seawater after dehydration. In conclusion, this study shows that harp seals have the capacity to gain net water from mariposa (voluntarily drinking seawater) and are able to restore water balance after profound dehydration by drinking seawater.     

Diving behaviour of hooded seals (Cystophora cristata) in the Greenland and Norwegian Seas

Satellite-linked dive recorders were used to collect data on depths and durations of ∼120,000 dives by 16 hooded seals (Cystophora cristata). Following tagging after moult (four males, eight females) and breeding (four females) off east Greenland, seals dispersed widely in the northeast Atlantic during 172 ± 97 days (mean satellite-linked dive recorder lifetime ± SD). Meso/bathypelagic dives of 5- to 25-min duration to 100–600 m dominated (75%), but some very deep (≥1016 m) and long (>52 min) dives occurred. Diving in open ocean was continuous, with an estimated 90.7±0.8% (mean±SE) of time spent submerged. The proportion of time spent submerged was similar during night and day, but dives during the day were generally deeper and longer (P < 0.05) than during the night. Also, dives in winter were deeper and longer than in summer. Published data on the distribution of likely prey suggest that Greenland halibut (Reinhardtius hippoglossoides), redfish (Sebastes spp.), polar cod (Boreogadus saida), herring (Clupea harengus), squid (Gonatus fabricii) and blue whiting (Micromesistius poutassou) are important prey of hooded seals. Accepted: 22 January 1999     

Inter-annual consistency in the fluctuating energy requirements of captive harp seals Phoca groenlandica

To examine seasonal and inter-annual shifts in the energy requirements of captive harp seals (Phoca groenlandica), metabolic rate and body temperature were repeatedly measured over the annual cycle. Seasonal shifts were evident in both parameters, with spring and summer (April to September) peaks decreasing throughout fall and winter (October to March). Seasonal changes in oxygen consumption concur with earlier published reports and data presented here validate these trends inter-annually. The standard metabolic rates (SMRs) of all seals were lower than predicted during fall/winter, but were indistinguishable from expected rates during spring/summer. Although individual variation in metabolic rate was largely independent of changes in both total body mass and predicted total body fat over the year, such variation was more closely related to changes in predicted lean body mass. Both deep rectal and core body temperatures co-varied with metabolic rate, perhaps indicating a metabolic defense of fat for thermoregulation. The implications of these shifting basal requirements are considered in the light of calculating the impact of the harp seal herd on fisheries resources in the Northwest Atlantic. Received: 18 December 1996 / Accepted: 30 April 1997     

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     

Volume capacity and contraction control of the seal spleen

Cabanac, Arnaud, Lars P. Folkow, and Arnoldus Schytte Blix.Volume capacity and contraction control of the seal spleen. J. Appl. Physiol. 82(6):1989-1994, 1997.Volume changes in the spleens of hooded seals(Cystophora cristata) and harp seals(Phoca groenlandica) were measuredplethysmographically in vitro in response to epinephrine,norepinephrine, isoprenaline, phentolamine, and acetylcholine. Dilated spleens contracted forcefullywithin 1-3 min of -adrenoceptor activation with 1.0-5.0µg epinephrine/kg body mass, whereas stimulation of -adrenoceptorsand cholinergic receptors had little effect. The mass of dilated hoodedseal spleens corresponded to 2-4%(n = 7) of body mass, with volume (V;ml) relating to body mass (M; kg) as follows: V = 12.0M + 910 (r² = 0.96, n = 4). Thus the spleen of a 250-kghooded seal maximally expels 3.9 liters, or 13%, of its estimatedtotal blood volume. Average hematocrit in splenic venous outflow fromdilated spleens was 90 ± 3% (n = 3) in hooded seals and 85% (n = 2) inharp seals. From these data we have estimated that the aerobic divinglimit of a 250-kg hooded seal increases only 105 s, at the most, if complete emptying of the spleen occurs during diving, while the corresponding estimate for a 112-kg harp seal is 80 s.

     

Genetically effective population sizes of Antarctic seals estimated from nuclear genes

We analyzed eight nuclear microsatellite loci in three species of Antarctic seals; Weddell seal (Leptonychotes weddellii; mean N = 163), crabeater seal (Lobodon carcinophaga; 138) and Ross seal (Ommatophoca rossii; 35). We estimated genetic diversity (Θ) and effective population size (N _E) for each species. Autosomal microsatellite based N _E estimates were 151,200 for Weddell seals, 880,200 for crabeater seals, and 254,500 for Ross seals. We screened one X-linked microsatellite (Lw18), which yielded similar N _E estimates to the autosomal loci for all species except the Ross seals, where it was considerably larger (~103 times). Microsatellite N _E estimates were comparable with previously published N _E estimates from mitochondrial DNA, but both are substantially lower than direct estimates of population size in all species except the Ross seals. The ratio of maternally versus biparentally derived estimates of N _E for Ross seals was not consistent with the hypothesis that they are a polygynous species. We found no sign of a recent, sustained genetic bottleneck in any of the species.     

Dinucleotide microsatellite markers from the Antarctic seals and their use in other Pinnipeds

Twenty‐four microsatellite loci were isolated from three species of Antarctic seals (Subfamily Monachinae, Tribe Lobodontini). Eleven loci were cloned from Weddell seal, Leptonychotes weddellii, seven from leopard seal, Hydrurga leptonyx, and six from crabeater seal, Lobodon carcinophagus. Variability was assessed in Weddell seals collected in McMurdo Sound, leopard seals from Bird Island, South Georgia, and crabeater seals sampled in the eastern Ross Sea. All loci were variable in the three species used for cloning and 22 of these loci amplified variable products in the Ross seal, Ommatophoca rossii. Cross‐species amplification was largely successful, with an average of 19 loci amplifying products in other phocids.     

Light-and electron microscopical observations on the terminal airways and the alveolar parenchyma of the antarctic seals Lobodon carcinophagus and Leptonychotes weddelli

Summary The terminal airways of two antarctic seals (Leptonychotes weddelli, Lobodon carcinophagus) are composed of typical small bronchi and bronchioles the initial segment of which contains cells probably representing Clara cells. The respiratory bronchioles are of considerable length. Their wall contains a highly developed system of spirally arranged bundles of smooth muscle cells. This is interpreted to represent the main means which by being closed before diving prevents the reabsorption of nitrogen while returning to the surface. The amount of smooth muscles evidently is greater in the deep diving Weddell seal than in the crabeater seal. The pneumocytes II occur both within the respiratory bronchioles and in the alveoli, their number seems to be relatively high in both species. The diameter of thin parts of the blood-air barrier in both species is 0.3–0.4 m (0.19–0.22 m in terrestrial mammals). The alveolar septa contain myofibroblasts and one layer of capillaries. The connective tissue of both seals lung is highly developed forming a dense, strong meshwork of septa and a thick pleura visceralis. The septa contain bundles of smooth muscle cells and extensive lymphatic vessels. Due to its particularly thick septa the lobulaton of the lung tissue of the Weddell seal is more obvious than in the crabeater seal, however, in both species the amount of connective tissue in the interlobular septa and the pleura visceralis is greater than in terrestrial carnivores.     

Health status of harp seals (Phoca groenlandica) and hooded seals (Cystophora cristata) on Sable Island,Nova Scotia,Canada, concurrent with their expanding range

Beach surveys for harp (Phoca groenlaandica) and hooded (Cystophora cristata) seals documented a dramatic increase in their numbers on Sable Island in mid 1990s. In the 1980s, no more than five animals of both species were observed on this island each year, however, during late 1994 to 1998, 1,191 harp and 870 hooded seals, mostly young animals, were recorded. Although some of these seals had been killed by sharks, most (roughly 75%) were found alive or as intact carcasses on the beach, and some of the live seals were later found dead. Emaciation/starvation was considered the primary cause of death in seals that were not obviously killed by sharks. Factors that may have compounded this poor body condition included gastric impaction with abnormal ingesta, hemorrhagic diathesis possibly induced by parasitic migration and secondary vasculitis, and stomatitis (in hooded seals only). Some harp and hooded seals expanding their range in recent years may be unable to feed successfully, although the reasons for this are unclear.     

Development of myoglobin concentration and acid buffering capacity in harp (<Emphasis Type="Italic">Pagophilus groenlandicus</Emphasis>) and hooded (<Emphasis Type="Italic">Cystophora cristata</Emphasis>) seals from birth to maturity

Pinnipeds rely on muscle oxygen stores to help support aerobic diving, therefore muscle maturation may influence the behavioral ecology of young pinnipeds. To investigate the pattern of muscle development, myoglobin concentration ([Mb]) and acid buffering ability (β) was measured in ten muscles from 23 harp and 40 hooded seals of various ages. Adult [Mb] ranged from 28–97 to 35–104 mg g tissue⁻¹ in harp and hooded seals, respectively, with values increasing from the cervical, non-swimming muscles to the main swimming muscles of the lumbar region. Neonatal and weaned pup muscles exhibited lower (~30% adult values) and less variable [Mb] across the body than adults. In contrast, adult β showed little regional variation (60–90 slykes), while high pup values (~75% adult values) indicate significant in utero development. These findings suggest that intra-uterine conditions are sufficiently hypoxic to stimulate prenatal β development, but that [Mb] development requires additional postnatal signal such as exercise, and/or growth factors. However, because of limited development in both β and [Mb] during the nursing period, pups are weaned with muscles with lower aerobic and anaerobic capacities than those of adults.     

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.