Stomach contents of the Antarctic Silverfish Pleuragramma antarcticum from the southern and eastern Weddell Sea (Antarctica)

Summary The stomach contents of 142 Pleuragramma antarcticum from the southernmost part of the Weddell Sea (Gould Bay: 77°19S) and from the eastern coast (72°–74°S) were analyzed. The fish were collected in February 1983 and stored frozen. Size range of the investigated specimens was from 6.4 cm SL (7.3 cm TL) to 21.9 cm SL (24.1 cm TL). The fish were caught by bottom trawl (Gould Bay) and Agassiz Trawl and pelagic RMT-m net in the upper 300 m layer at the eastern coast, respectively. Thirty-six stomachs were empty or contained sand only. The most important prey in terms of biomass were euphausiids, which occurred in 49 stomachs at a mean number of 2. Their dry weight was estimated to be 15–50 times that of the next important food items, which were copepods, gastropods and gammarids. Krill (Euphausia superba) constituted up to 14% of the euphausiids in the diet of fish caught in the eastern Weddell Sea. The rest was usually made up by Euphausia crystallorophias. This species was taken with preference even when the abundance of E. superba in accompanying plankton catches was 16 times higher. In the Gould Bay, gastropods were frequently eaten despite high numbers of copepods in the plankton, whereas in the eastern Weddell Sea, copepods were abundant both in plankton and in the stomach content. The almost complete absence of the early stages of fish in the diet of Pleuragramma antarcticum in the southern and eastern Weddell Sea is due to a pronounced vertical segregation of fish of different sizes. This distribution pattern is thought to be an adaptive mechanism to avoid intraspecific predation, as 90% of the ichthyoplankton in that area is Pleuragramma antarcticum.     

Fish prey of southern elephant seals, <Emphasis Type="Italic">Mirounga leonina</Emphasis>, at King George Island

Between November and February in the years 1993/1994 to 1999/2000, 153 southern elephant seals, Mirounga leonina, were stomach lavaged at King George Island. Fish occurred in 16 of 108 stomachs containing food items. Myctophids were the dominant fish prey, contributing 76.6% of the 145 sagittal otoliths found. The most important prey species was Gymnoscopelus nicholsi, which constituted 69% of the otoliths and occurred in 75% of stomach samples. Next in importance was the nototheniid Pleuragramma antarcticum, which occurred in 31.3% of samples and represented 11.7% in numbers. We suggest that while myctophids may be the dominant fish prey of elephant seals in areas close to King George Island, they are probably replaced by P. antarcticum as seals migrate towards higher latitudes where this fish is extremely abundant.     

The diet of the Weddell Seal Leptonychotes weddellii at the Danco Coast, Antarctic Peninsula

The diet of the Weddell Seal Leptonychotes weddellii at the Danco Coast, Antarctic Peninsula, was investigated by the analysis of 105 and 39 faeces collected at beaches surrounding Cierva Point during January–March 1998 and 2000, respectively. The diet was diverse and composed of both pelagic and benthic-demersal organisms. Fish, followed by cephalopods, constituted the bulk of the diet by number and mass. Among fish, Pleuragramma antarcticum, Chaenodraco wilsoni and Gobionotothen gibberifrons were the dominant fish by mass in 1998, whereas Chionodraco rastrospinosus, P. antarcticum and C. wilsoni predominated by mass in 2000. The contribution of channichthyid species to the diet of seals at the Danco Coast was higher than previously reported. Besides those species, the myctophid Electrona antarctica was also an important prey by number in the diet of seals in both summer seasons. The results are compared with information from other study areas and the possibility of using information on the diet of this seal as a gross indicator of fish availability/distribution is considered.     

DIET OF WEDDELL SEALS LEPTONYCHOTES WEDDELLII AT VESTKAPP, EASTERN WEDDELL SEA (ANTARCTICA), IN RELATION TO LOCAL FOOD SUPPLY

Thirteen Weddell seals ( Leptonychotes weddellii ) were collected at Vestkapp, eastern Weddell Sea coast, in austral spring 1986. All stomachs contained partially digested food. The mean wet weight of stomach contents was 7.5 kg, 3.3% of the. mean body weight of the collected seals. Twelve fish species and three cephalopod species were identified from 372 left otoliths and 25 lower beaks, representing 58.4% of 679 total prey items obtained. Composition by number of total prey was: Chionodraco myersi (15.8%), Trematomus eulepidotus (10.0%), Pagetopsis maculatus (9.7%), Racovitzia glacialis (9.6%) and Cryodraco antarcticus (4.1%). Otoliths of the seven other fish species and beaks of the three cephalopod species together represented 9.1% of total prey numbers. The pooled wet weights calculated from 13 prey species (regressions for two octopod species were not available) amounted to 43.5 kg food mass and represented 44.7% of the combined food mass in all stomachs. Composition by mass was: C. myersi (44.5%), T. eulepidotus (19.8%), squid Psychroteuthis glacialis (8.5%), P. maculates (7.9%), C. antarcticus (7.1%) and R. glacialis (6.2%). The remaining 7 fish species together represented 5.8% by mass. Temporal variation in food availability was apparent. Midwater fish Pleuragramma antarcticum was the staple food of Weddell seals from the same area during the 1985 summer, whereas it was absent in the samples taken in spring 1986. Estimates of fish biomass from net hauls demonstrate a highly variable availability of pelagic food resources for top predators in the Vestkapp area.     

The diet of Weddell seals in McMurdo Sound, Antarctica as determined from scat collections and stable isotope analysis

The diet of adult and juvenile Weddell seals (Leptonychotes weddellii) in McMurdo Sound, Antarctica, was determined from both scat and stable isotope analyses, to ascertain if foraging behavior varied with age, season, or diving pattern. Scats were collected over 6 years and recovered hard parts identified. Stable carbon and nitrogen isotope values were determined for seal blood samples and potential prey items and used to identify primary prey species and assess trophic interactions. Pleuragramma antarcticum remains were recovered from between 70 and 100% of the scats, and there was little evidence for inter-annual or age-specific variation in foraging behavior. However, stable isotope and dive data analyses indicated that while most seals foraged predominantly on pelagic fish and squid, some juveniles concentrated on shallow benthic Trematomus spp. Combining these three methods permitted firm conclusions about diet and foraging behavior to be drawn. Received: 10 June 1997 / Accepted: 8 November 1997     

Pleuragramma antarcticum (Pisces,Nototheniidae) as food for other fishes in McMurdo Sound,Antarctica

Summary Nine spcies of notothenioid fishes were captured near the southern limit of their range in ice covered McMurdo Sound, Antarctica. Stomach contents were examined using an occurrence method. Fishes were present in the diets of 8 of 9 species;Dissostichus mawsoni andGymnodraco acuticeps were predominantly piscivorous.Pleuragramma antarcticum was the most common prey fish consumed, being present in stomachs of 4 of 8 species.Pleuragramma was also partially piscivorous (22%) as well as cannibalistic (13%). As evidenced by their presence in the stomach contents of other fishes, all life history stages from 20 mm SL postlarvae to 160 mm SL adultPleuragramma were represented under the ice of the Sound.Pleuragramma was also a major food item for Weddell seals, birds and possibly invertebrates. As a widely distributed species in the pelagic zone,Pleuragramma may be an ecological substitute for euphausiids in the food web of the Sound.     

Abundance,horizontal and vertical distribution of fish in eastern Weddell Sea micronekton

Summary The spatial distribution and species composition of high-Antarctic ichthyonekton was investigated during the EPOS 3 cruise by RV Polarstern in the eastern Weddell Sea during January–February 1989. A multiple rectangular midwater trawl was used to collect samples from the surface to near the sea floor at 11 stations along a 245 kra transect off Halley Bay. Early larval stages of 18 species, representing about 24% of the known Weddell Sea ichthyofauna, were present in the water column. The Antarctic silver-fish, Pleuragramma antarcticum, over-whelmingly dominated the catches comprising 84.5% of the 5022 specimens caught. The abundance of this species markedly increased towards the offshore end of the transect with the highest numbers occurring near the shelf-break front associated with the westerly current of the southern limb of the Weddell Gyre. The increased abundance of P. antarcticum in continental slope waters was attributed to deflection of the East Weddell Coastal Current beyond the shelf/slope break by fringing ice shelves. Most larval and juvenile fish were found in the seasonally warmed upper 0–70 m layer of the Antarctic Surface Water where conditions occurred that appeared to be favourable to both feeding and growth. Cluster analysis indicated that inner-, central-and outer-shelf assemblages were represented and that the species composition was most effectively described by reference to water mass and depth.     

Notes on winter feeding of crabeater and leopard seals near the Antarctic Peninsula

Summary Stomach contents of crabeater (Lobodon carcinophagus) and leopard (Hydrurga leptonyx) seals collected in the pack ice west of the antarctic Peninsula in August–September 1985 were analyzed. Food remains were found in 7 of 56 crabeater seals and 5 of 29 leopard seals. The primary foods were krill (Euphausia superba) which occurred in all 12 stomachs, and fish (Pleuragramma antarcticum) which occurred in 3. Eleven of the seals with food in their stomachs were collected in the southern portion of Bismark Strait. The incidence of feeding seemed highest in pregnant females. These results, and comparisons with previous collections, suggest that krill were not abundant or widely distributed in the area at the time the seals were collected. The sizes of krill eaten by crabeater and leopard seals were very similar, and were significantly larger than krill found in 2 samples taken by midwater trawls in nearby open water.     

Summer phytoplankton distributions in the Weddell Sea

Summary The quantitative composition of phytoplankton was studied along a transect of 14 hydrographic stations, between the southern coast of the Weddell Sea and the Antarctic Peninsula, during the austral summer of 1984–1985. The most apparent feature of the phytoplankton distribution was the presence of a bloom of Phaeocystis at a frontal zone over the shelf break, and the marked contrast between a southern region, with high phytoplankton biomass, and a poorer region north of the shelf break. The most widely distributed diatom genus was Nitzschia (Fragilariopsis section). The phytoplankton assemblage of the southern region included the silicoflagellate Distephanus speculum, the diatom Rhizosolenia alata and several heterotrophic dinoflagellates such as Protoperidinium antarcticum and P. applanatum. The northern assemblage could be characterized by the abundance of flagellates and small dinoflagellates, and by diatoms such as Chaetoceros criophilum, Corethron criophilum Nitzschia kerguelensis and other Nitzschia species of the Fragilariopsis section.     

An apparent population decrease,or change in distribution,of Weddell seals along the Victoria Land coast

Ground counts during 1959–1968 compared with counts using high resolution (0.6 m²) satellite imagery during 2008–2012 indicated many fewer Weddell seals (Leptonychotes weddellii) at two major molting areas in the western Ross Sea: Edisto Inlet‐Moubray Bay, northern Victoria Land, and McMurdo Sound, southern Victoria Land. Breeding seals have largely disappeared from Edisto‐Moubray, though the breeding population in McMurdo Sound appears to have recovered from harvest in the 1960s. The timing of decline, or perhaps spreading (lower numbers of seals in more places), is unknown but appears unrelated to changes in sea ice conditions. We analyzed both historic and satellite‐derived ice data confirming a large expansion of pack ice mostly offshore of the Ross Sea, and not over the continental shelf (main Weddell seal habitat), and a thinning of fast ice along Victoria Land (conceivably beneficial to seals). Timing of fast ice presence and extent in coves and bays along Victoria Land, remains the same. The reduction in numbers is consistent with an altered food web, the reasons for which are complex. In the context of a recent industrial fishery targeting a seal prey species, a large‐scale seal monitoring program is required to increase understanding of seal population changes.     

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.     

Blood rheology of Weddell seals and bowhead whales

Red blood cell (RBC) aggregation and blood viscosity are important determinants of in vivo blood flow dynamics and, in marine mammals, these parameters may impact diving physiology by altering blood oxygen delivery during the diving response. Weddell seals are superb divers and exhibit age-related patterns in blood oxygen chemistry and diving ability. By contrast, bowhead whales are not long duration divers, and little is known of their blood properties relative to diving. The present study was designed to compare rheological characteristics of blood from Weddell seal pups, Weddell seal adults, and from adult bowhead whales: blood viscosity and RBC aggregation in plasma and in polymer solutions (i.e., RBC "aggregability") were measured. Salient findings included: (1) significant 4- to 8-fold greater aggregation in blood from adult seals compared with pups and human subjects; (2) 2-to 8-fold greater aggregation in bowhead whale blood compared with human blood; (3) compared to human red cells, enhanced RBC aggregability of RBC from adult seals and whales as determined by their greater aggregation in polymer solutions; (4) increasing RBC aggregation and aggregability of seal pup blood over a seven day period following birth; (5) significantly greater blood viscosity for adult seals compared with pups at both native and standardized hematocrits. These results indicate that, for both species, hemorheological parameters differ markedly from those of humans, and suggest progressive changes with seal age; the physiological implications of these differences have yet to be fully defined.     

Satellite imagery can be used to detect variation in abundance of Weddell seals (Leptonychotes weddellii) in Erebus Bay, Antarctica

The Weddell seal population in Erebus Bay, Antarctica, represents one of the best-studied marine mammal populations in the world, providing an ideal test for the efficacy of satellite imagery to inform about seal abundance and population trends. Using high-resolution (0.6 m) satellite imagery, we compared counts from imagery to ground counts of adult Weddell seals and determined temporal trends in Erebus Bay during November 2004–2006 and 2009, and December 2007. Seals were counted from QuickBird-2 and WorldView-1 images, and these counts were compared with ground counts at overlapping locations within Erebus Bay during the same time. Counts were compared across years and within individual haul-out locations. We counted a total of 1,000 adult Weddell seals from five images across all years (for a total of 21 satellite-to-ground count comparisons), approximately 72% of the total counted on the ground at overlapping locations. We accurately detected an increase in abundance during 2004–2009. There was a strong, positive correlation (r = 0.98, df = 3, P < 0.003) between ground counts and counts derived from the imagery. The correlation between counts at individual haul-out locations was also strong (r = 0.80, df = 19, P < 0.001). Detection rates ranged from 30 to 88%. Overall, our results showed the utility of high-resolution imagery to provide an accurate way to detect the presence and variation in abundance of Weddell seals. Our methods may be applied to other species in polar regions, such as walruses or polar bears, particularly in areas where little is known about population status.     

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.     

Fish diet of the Antarctic fur seal,Arctocephalus gazella,in summer,at Stranger Point,King George Island,South Shetland Islands

 In February 1992, 34 faecal samples from non-breeding male Antarctic fur seals, Arctocephalus gazella, were collected at Stranger Point, King George Island, South Shetlands. Fish constituted an important part of the diet, occurring in 90% of those scats containing prey remains. From 1162 otoliths found in the faeces, 1086 were identified to at least family level. Myctophids and nototheniids represented together almost 90% of the fish eaten. The dominant species were Gymnoscopelus nicholsi, Pleuragramma antarcticum and Electrona antarctica, contributing 33.3%, 30.8% and 12.0% of the otoliths respectively. The standard length of these three species was estimated from otoliths with little or no signs of erosion. This study showed that fur seals fed mainly on pelagic fish species that are often associated with krill. These findings are corroborated by fur seal diving patterns. Received: 7 October 1994/Accepted: 5 October 1995     

Brain size in neonatal and adult Weddell seals: Costs and consequences of having a large brain

Little is known about the ontogeny of brain size in pinnipeds despite potential functional implications of brain substrate (glucose, oxygen) requirements for diving, fasting, growth, and lactation strategies. We measured brain mass (brM) and cranial capacity (CC) in newborn and adult Weddell seals. Neonatal Weddell seals had brM that represented ~70% of adult brM. Weddell seals have the largest neonatal brain, proportional to adult brain, reported for any mammal to date, which is remarkable considering the relatively small size of Weddell seal pups at birth (6%–7% of maternal body mass) compared to neonates of other highly precocial mammals. Provision of sufficient glucose to maintain the large, well‐developed brain of the neonatal Weddell seal has a nontrivial metabolic cost to both pup and mother. We therefore hypothesize that this phenomenon must have functional significance, such as allowing pups to acquire complex under‐ice navigation skills during the period of maternal attendance.     

Some anatomical aspects of the cardiovascular system of antarctic seals and their possible functional significance in diving

The hearts and ascending aortae of 11 Weddell seals, Leptonychotes weddelli, three adult Crabeater seals, Lobodon carcinophagus, two adult Ross seals, Ommatophoca rossi, and one adult Leopard seal, Hydrurga leptonyx, were examined for comparison with terrestrial forms. The Weddell seal specimens were from animals ranging in age from midterm in fetal development to mature adults. All specimens were collected in 1971, 1972, and 1973, from McMurdo Sound and the Ross Sea, Antarctica. The phocid hearts were characteristically broader and flatter than those of other carnivore families and they tended toward bifid apices. The heart form indices (height/circumference) averaged 31.5 compared to 39.0 for felids. The right ventricular chambers of the Antarctic seals were found to average longer in Weddells and narrower in all, than those reported for four other carnivore families. An elastic enlargement was present in the ascending aortae of all seals. The largest diameter of the aortic bulb averaged 25.5 mm more than the base of the aorta in the adult Weddell seals which represented an increase of 72.5% over the base. It is suggested that the general heart form, and especially the ascending aortae, are anatomical adaptations to diving. The compressed heart makes possible unimpaired function when the chest is compressed during deep dives. The aortic bulb maintains mean arterial blood pressure and perfusion of the brain and cardiac tissue during diving bradycardia.     

Food and feeding ecology of emperor penguins in the eastern Weddell Sea

Summary The diet of the emperor penguin Aptenodytes forsteri in the eastern Weddell Sea, Antarctica was studied during October and November 1986 by stomach content analysis. Emperor penguins fed mainly on Antarctic krill Euphausia superba, Antarctic silverfish Pleuragramma antarcticum and squid Psychroteuthis glacialis. Benthic prey was not found. The prey composition suggests two different feeding strategies, shallow dives exploring the rugged underside of sea ice where krill is taken, and deep dives when mesopelagic fish and squid are consumed. Chicks were fed on average every 1.44 days.     

Zooplankton communities in the Southern Weddell Sea (Antartica)

Summary Standardized abundances of 40 frequently occurring macrozooplankton taxa collected by double-oblique bongo net hauls between the surface and 200 m depth were submitted to an agglomerative hierarchical cluster analysis, to characterize surface zooplankton communities in the southern and eastern Weddell Sea. The sensitivity of the analysis concerning reduction of the number of considered taxa (especially stages of species) was tested. Dominant taxa in the entire area over the whole period were copepods Metridia gerlachei and Calanoides acutus. Calanus propinquus, Appendicularia spp. and calyptopis-1 larvae of krill, Euphausia superba were abundant at 73°S/19°W. Euphausia crystallorophias and larval fish Pleuragramma antarcticum dominated at 77°S/40°W on the southern shelf. The cluster analysis revealed marked similarities between the southern shelf community and the January community further to the northeast. E. crystallorophias, and larval Pleuragramma antarcticum are thought to be transported in surface layers of the coastal current to the southwest. Oceanic species such as Thysanoessa sp. and E. superba are probably not transported in the same way. Possible mechanisms for the maintenance of the community structures are discussed. A simplified method of characterizing communities, based on analysis of euphausiids, is presented.     

Albino Weddell seal at Cape Shirreff,Livingston Island,Antarctica

To our knowledge, this paper is the first record/report of a juvenile light-coloured Weddell seal (Leptonychotes weddellii) at Cape Shirreff, Livingston Island in January 1998, determining that it was an albino individual. Based on available literature, three cases of albino seals have been reported exclusively for Harbour seal pups, and no albino has been reported for Antarctic pagophilic true seals. Therefore, this is the first confirmed case of albinism in Antarctic pagophilic true seals species, indicating that this phenomenon is indeed of a rare occurrence.