Crabeater Seals Lobodon Carcinophagus Research Articles

Sea-ice and macrozooplankton distribution as determinants of top predator community structure in Antarctic winter

The Antarctic Peninsula marine ecosystem is highly productive, with large populations of commercially and ecologically important species including Antarctic krill Euphausia superba, Adélie penguins Pygoscelis adeliae, and crabeater seals Lobodon carcinophagus. The ecology of the peninsula is rapidly changing due to accelerating climate change and fishing pressure. Systematic ecosystem surveys have focused on austral spring and summer, leaving an information gap on winter ecosystem dynamics. Using data from 5 consecutive ecosystem surveys, we quantified the composition and distribution of winter predator communities and investigated the physical and biological influences on community structure. Seabirds and marine mammals clustered into 3 communities: an ice-associated community represented by Adélie penguins and crabeater seals; a diverse marginal ice zone community dominated by fur seals and several species of seabirds including 3 petrels, kelp gulls Larus dominicanus, and Antarctic terns Sterna vittata; and an open water community consisting of southern fulmars Fulmarus glacialoides and 4 species of petrels. These communities were distributed along an environmental gradient ranging from ice-covered, cold, saline water to ice-free, warmer, and fresher water with greater chlorophyll concentrations. Predator communities were also associated with different communities of macrozooplankton: ice-associated predators with an extremely diverse assemblage of typically mesopelagic zooplankton; marginal ice zone predators with a community of large-bodied euphausiids (E. superba, E. crystallorophias); and open water predators with a community of small-bodied euphausiids (Thysanoessa macrura). Our synthesis of integrated winter predator and macrozooplankton communities relative to sea-ice concentration provides reference points for future ecosystem assessments within this rapidly changing region.

Satellite‐linked dive recorders provide insights into the reproductive strategies of crabeater seals (Lobodon carcinophagus)

AbstractInsights into the reproductive strategies of crabeater seals Lobodon carcinophagus were obtained from satellite‐linked dive recorders deployed during the breeding season. All of the three females and seven males on which instruments were deployed hauled out continuously for extended periods of up to 24 days. The duration of extended haulout is consistent with a lactation period of around 3 weeks. In two cases where dive recorders were deployed on a male and female in the same group, the two animals stayed together on the ice continuously and entered the water at exactly the same time when extended haulout ceased. In both cases the male and female became geographically separated within a short time after entering the water, indicating that the association did not last beyond the period on the ice. Some males had multiple periods of extended haulout, suggesting they may try to find more than one female with which to mate. An optimal reproductive strategy with regard to the time spent on the ice may balance the conflicting needs of a short time imposed by the availability of breeding habitat and a long time to increase the reproductive success of males and females by finding more, or choosing between, mates.

The effect of density‐dependent catastrophes on population persistence time

Summary There has been increasing recognition that catastrophes are an important factor in modelling threatened populations. However, density dependence has generally been omitted from models of threatened populations on the assumption that this omission yields conservative predictions. We explore the significance of including density‐dependent catastrophes in models of threatened populations. Using an analytical model, we show that density‐dependent catastrophes have a significant effect on population persistence, decreasing mean persistence time at large population sizes and causing a relative increase at intermediate sizes. We illustrate our results with empirical data from a disease outbreak in crabeater seals Lobodon carcinophagus and show that intermediate population sizes have the longest predicted persistence times. The pattern we found is qualitatively different from previous results on persistence time based on density‐independent models, in which persistence time increases with population size to an asymptote. Synthesis and applications. This study has important implications for the conservation of species that may experience density‐dependent catastrophes, such as disease outbreaks or starvation. Our results indicate that small and intermediate sized populations may contribute disproportionately to species persistence. Thus populations that have been dismissed as ‘marginal’ may actually be important for conservation. In addition, culling may increase the persistence of populations that experience density‐dependent catastrophes.

Size and growth of the crabeater seal Lobodon carcinophagus (Mammalia: Carnivora)

AbstractGrowth in length and weight of a large sample of crabeater seals Lobodon carcinophagus collected over the period 1967–78 is reported. Growth in dorsal standard length of 1146 seals (490 males, 656 females) from 0.5 to 20 years of age was similar to the pattern seen in other phocid seals. Length was plotted against age and Gompertz curves fitted to the data. Growth rates were estimated from the derived equations: males, y= 231.9 − 58.7 e−0.55t; females, y= 236.8 − 63.6 e−0.44t. The asymptote is at approximate length 234 cm and age 10 years in both males and females. Body weights were recorded for 249 seals aged from 0.5 to 20 years, of which reliable age estimates for 230 (100 males, 130 females) were available. Rates of growth in body weight were estimated from the derived equations: males, y= 200.0 − 94.1 e−0.48t; females y= 212.4 − 116.2 e−0.40t. The best estimate of body weight from linear body measurements is obtainable from a predictive equation relating body weight, length and axillary girth, determined by maximum likelihood: W = 0.000046 LG2 (W, body weight; L, dorsal standard length; G, axillary girth). There was no significant difference between the sexes. Seasonal variations in body condition could not be determined because all animals were sampled in February and March. However, inter‐year differences in recovery of body condition of adults, following the demands of breeding and moulting in the spring and summer, were observed. The data represent a unique time series covering the 1960s and 1970s. In terms of global climate change they should provide a valuable baseline for future studies.

Long-term monitoring of Antarctic pinnipeds in Admiralty Bay (South Shetlands, Antarctica)

Year-round monitoring of five Antarctic pinnipeds was conducted in Admiralty Bay from 1988 up to 2000. Two breeding species: southern elephant sealsMirounga leonina (Linnaeus, 1758) and Weddell sealsLeptonychotes weddellii (Lesson, 1826), were present throughout the year. Three other species: crabeater seals Lobodon carcinophagus (Hobron and Jacquinot, 1842), leopard sealsHydrurga leptonyx (Blainville, 1820), and Antarctic fur sealsArctocephalus gazella (Peters, 1875) visited the area only for short periods. During this study, the abundance of elephant seals was stable, whereas those of Weddell and crabeater seals declined. Leopard seals numbers fluctuated irregularly. We detected a possible immigration from South Georgia: of a stable magnitude for elephant seals, and of variable magnitude, depending on food accessibility, for Antarctic fur seals. We found a strong recurrence of the spatial distributions of elephant, Weddell, and Antarctic fur seals in the 13 oases on the shore of Admiralty Bay. Annual distribution patterns were characteristic for each species. The innermost beaches were used predominantly by the animals during their annual fasts: the breeding and the moulting seasons.

Age estimation in crabeater seals (Lobodon carcinophagus)

AbstractThe age of crabeater seals Lobodon carcinophagus was first estimated from the teeth by counting layers in the dentine in transverse sections of the canine teeth. Reliable estimation of age from canine teeth in animals older than c. 10 years is not possible because of root closure at a relatively early age. The present investigation compared various methods of estimating age by examination of tooth sections from a large sample of crabeater seals from Marguerite Bay, Antarctic Peninsula. The most reliable estimations were shown to be obtained by counting layers in the cementum of the third postcanine tooth. Ages of 1233 animals (58% female, 42% male) ranged from 0.5 to 39 years. It is suggested that time‐ and age‐specific historical material is potentially relevant for contemporary growth and life‐history studies as well as for trophic relations or concentrations of pollutants in animal tissues.

Spring distribution and ecological role of seabirds and marine mammals in the Weddell Sea, Antarctica

The early spring distribution of seabirds, pinnipeds and cetaceans was quantitatively determined in the northern Weddell Sea during the EPOS 1 cruise of the icebreaking RV Polarstern. Two hundred and ninety-one half hour counts were made in the Antarctic region between October 18 and November 16, 1988 (+94 counts in sub-antarctic and sub-tropical water). The bird populations were dominated by the Adelie Penguin Pygoscelis adeliae, which represented 90% of the birds counted in the closed pack ice (CPI), with a mean density of 31 penguins per km2. Crabeater seal Lobodon carcinophagus, the most common pinniped, had a mean density of 1.2/km2 in the CPI with local concentrations reaching 14/km2. Expressed as biomass, mean values of about 90 kg/km2 were found in the whole Antarctic zone for seabirds, 50 for seals, and 35 for baleen cetaceans. Densities were higher in the CPI: 140, 180 and 115 kg/km2 respectively, were found for birds, pinnipeds and baleen whales, and lowest in open water (8 kg/km2 for the birds, 0.2 for the pinnipeds and no whales). A minimum value of food intake by seabirds and marine mammals was estimated to be 2.7 mg C/m2/day for the Antarctic zone (4.1 in the CPI). Taking into account that pinnipeds density and cetacean food uptake are underestimated, and that Antarctic seabirds consume much more fish than generally suspected, a minimum krill production of 16 mg C/m2/day is proposed for the Antarctic zone: 22.5 in the CPI, 3 in open water, and intermediate values in the marginal ice zones.

OBSERVATIONS ON TERRESTRIAL LOCOMOTION IN ANTARCTIC SEALS

Observations are reported on the modes of terrestrial locomotion among Antarctic members of the Phocidae. An account is given of the sinuous (or “swimming”) type of progression found most developed in the crabeater seal Lobodon carcinophagus (Hombron and Jacquinot), and of its occurrence in the leopard seal Hydrurga leptonyx (Blainville). This type of movement has not been seen in the Weddell seal Leptonychotes weddelli Lesson, where the undulatory (caterpillar‐like) method is the normal locomotory pattern on land. The fore‐flippers are not used by the adult Weddell seal in terrestrial locomotion, but the pup does make some use of the fore‐flippers during the first 10 to 15 days of postnatal life. The locomotory patterns used by the elephant seal Mirounga leonina Linn, and the probable methods used by the Ross seal Ommatophoca rossi Gray, are noted. The observation that the harp seal Pagophilus groenlandicus (Erxleben) shows the sinuous type of movement under stress is recorded.

A MASS DYING OF CRABEATER SEALS, LOBODON CARCINOPHAGUS (GRAY)

Summary.In 1955 some 3000 Crabeater Seals Lobodon carcinophagus wintered on the sea‐ice of the Crown Prince Gustav Channel. The seals, most of which were young, were ten times more numerous in 1955 than is normal in winter. Most of the animals died in the spring from a contagious disease whose effect was probably increased by crowding and partial starvation. Its exact nature was not elucidated, but a virus infection is likely. The average mortality was 85 per cent but in places the figure was 97 per cent.Although the population was observed before, during and after the normal pupping season, no live births but only abortions were recorded. Some abortions were found three weeks before any adult mortality or any sign of disease. It is unlikely that the epidemic had any appreciable effect on the total number of Crabeater Seals in Antarctica.The disease did not spread to the intermingled Weddell Seals Leptonychotes weddelli; diseased meat was eaten by dogs and by man with no ill effects.