Lobodon carcinophaga (Hombrot & Jacquinot, 1842)

Species details

Distinguishing Characters

The key distinguishing characteristics of crabeater seals is their relatively uniform colour, as they lack the prominent spots and streaks of Weddell, Ross and leopard seals. They are generally smaller, more slender and lighter in colour than elephant seals which may also be found in the pack-ice. They have a blunt, square shaped shapes snout in comparison to other seals, and very distinctive multi-cusped teeth.

Size

The global population size of crabeater seals and its long term trends are unclear. This uncertainly is due to extreme difficulty of conducting synoptic circumpolar surveys in the pack ice regions used by the seals. Estimates from the 1970 and 1980s put the global population at around 30 million seals, but these were revised down to 12 million as data improved in the 1990s. It has also been suggested the crabeater seal population increased throughout the 20th century as a consequence of increased krill availability arising from the decline in whale numbers. Despite some demographic data supporting this idea, there are no systematic survey data from before whaling to test the hypothesis. SCAR coordinated a international pack ice seal survey in 1999-2001 which estimated the population of eastern Antarctica (64–150°E, one quarter of the continental coastline) to be 914,200 seals (95% confidence limits: 698,600–1,302,000). Differences in the methodologies between this and earlier surveys in the region, prevented an assessment of trends, and there is still no revised global estimate for population size for this species.

Reference

Burns JM, Costa DP, Fedak MA, Hindell MA, Bradshaw CJA, Gales NJ, McDonald B, Trumble SJ, Crocker DE (2004) Winter habitat use and foraging behavior of crabeater seals along the Western Antarctic Peninsula. Deep-Sea Research Part II 51: 2279-2303. Burns JM, Hindell MA, Bradshaw CJA, Costa DP (2008) Fine-scale habitat selection of crabeater seals as determined by diving behavior. Deep-Sea Research Part II 55: 500-514. Gales NJ, Fraser WR, Costa DP, Southwell C (2004) Do crabeater seals forage cooperatively? Deep-Sea Research Part II 51: 2305-2310. Huckstadt LA, Burns JM, Koch PL, McDonald BI, Crocker DE, Costa DP (2012) Diet of a specialist in a changing environment: the crabeater seal along the western Antarctic Peninsula. Marine Ecology-Progress Series 455: 287-301. Laws RM, Baird A, Bryden MM (2003) Breeding season and embryonic diapause in crabeater seals (Lobodon carcinophagus). Reproduction 126: 365-370. Laws RM, Baird A, Bryden MM (2003) Size and growth of the crabeater seal Lobodon carcinophagus (Mammalia : Carnivora). Journal of Zoology 259: 103-108. Southwell C (2005) Optimising the timing of visual surveys of crabeater seal abundance: haulout behaviour as a consideration. Wildlife Research 32: 333-338. Southwell C, Paxton CGM, Borchers D, Boveng P, De la Mare W (2008) Taking account of dependent species in management of the Southern Ocean krill fishery: estimating crabeater seal abundance off east Antarctica. Journal of Applied Ecology 45: 622-631. Wall SM, Bradshaw CJA, Southwell CJ, Gales NJ, Hindell MA (2007) Crabeater seal diving behaviour in eastern Antarctica. Marine Ecology-Progress Series 337: 265-277.

Species distribution

Distribution info

Crabeater seals are found almost entirely in the Antarctic pack-ice, with only occasional vagrants hauling out on sub-Antarctic islands north of the polar front, or even more rarely on the coast of Australia, New Zealand, Africa and South America. Within the pack-ice, their distribution seems to be largely determined by that of the primary prey, Antarctic krill. Tracking studies have shown that in the West Antarctic Peninsula they occur on the continental shelf, while in Eastern Antarctica highest densities are associated with the continental shelf break or the marginal ice zone.

Depth

Crabeater seals typically dive deeper during the day than at night as they follow the vertical migration of their prey, but the overall dive depths vary considerably regionally. In the West Antarctic Peninsula, mean day time dives depths are 158 m compared to 73 m during the night. This is consistent with the seals using krill swarms compressed along the ocean floor during the day, and at night foraging on krill that are more dispersed throughout the water column. In eastern Antarctica, dives are very much shallower, generally less than 20 m. It is important to note that the West Antarctic Peninsula studies were conducted in late autumn and winter while the Eastern Antarctic studies were in spring and summer so these differences may be due to some extent to seasonal differences in the behaviour of krill. The deepest dive recorded for a crabeater seal is 664 m and the longest 23.6 min.

Ecology

Crabeater seals may be one of the most abundant large mammals in the world, but paradoxically relatively little is known about their basic biology. Mortality in the first year of life is as high as 80%, largely due to predation by leopard seals. Other sources of mortality are killer whales, which have been seen hunting together to take adult crabeater seals off ice floes. They typically live for 20-25 years, but can live for up to 40 years.
There is a strong diel cycle to the seals haul-out behaviour. On average adult seals spend 20-30% of each day hauled out on ice floes, but this varies both with the time of day and with the time of year. On the WAP, the seals hauled more at night and focused foraging activity during the daylight hours. By the end of August the seals switch to hauling out during the day and foraging at night, which is also the pattern in the Ross Sea and EA. The change in haul out behaviour most likely reflects changes in the behaviour of their prey and the strategies that the seals use to catch their prey.

Habitat

Crabeater seals are regarded as pack-ice obligate, using the floes as a substrate for breeding and resting, and foraging for krill in the waters beneath them.

Feeding Behaviour

They are regarded as having a highly specialised diet, with most studies indicating that 85-90% of the diet is Antarctic krill, with the remainder being made up of fish and other invertebrates. The seals diving behaviour reflects the vertical distribution of krill.

Reproductive

Crabeater seals breed in the austral spring with pups born in September and October. Females breed on the pack-ice, giving birth to single pup weighing 20-35 kg. The pups wean in approximately 3 weeks by which time they weigh 60-80 kg. Their mothers fast during lactation spending all of their time attending to their pup. A single male will associate himself with the mother pup pair remaining with them throughout the lactation period and vigorously defending them from other males. They are regarded as serially monogamous, because once the female has come into oestrus and been mated by the male, he will leave to seek another mother-pup pair. Outside the breeding season crabeater seals are largely solitary, although occasionally they can form aggregations of over 100 seals which appear to diving and feeding synchronously.