Whales and Seals
Whales and seals are the two groups of marine mammals to be found in the Southern Ocean where they are an important part of the marine ecosystem.
Whales and seals of the Southern Ocean have been severely exploited by man in the past, but are now mostly protected. Some seals and whales have had dramatic population increases in recent decades, though others remain greatly reduced compared to pre-hunting levels. The removal of millions of krill-eating whales and seals had a marked effect on the Antarctic marine ecosystem, and it may be centuries before a new equilibrium is reached.
There are two natural groups of whales — toothed and baleen. As their name implies, toothed whales have teeth; they mostly eat squid and fish, and take their prey one at a time. The largest toothed whale is the sperm whale (Physeter macrocephalus) of Moby Dick fame, and the group includes more than 60 other species, varying in size down to porpoises, one thousand of which would equal the body weight of a single sperm whale.
Between these two extremes are more than 20 species of beaked whale, and many dolphins, of which the largest is ironically known as the killer whale (Orcinus orca) and preys on fish, penguins, seals and whales. The baleen whales, such as the blue whale (Balaenoptera musculus) and humpback (Megaptera novaeangliae), have fibrous plates of baleen instead of teeth, which they use to strain plankton and small fish from seawater. The most numerous baleen whale is the minke whale, a species that spends much of its time in the Antarctic near the ice-edge.
Most, perhaps all, Southern Ocean whales are migratory, heading to warmer waters during the Antarctic winter. Calves are born in these more hospitable seas, as they would struggle to survive in polar waters during their first few months. The whales return south in the austral spring, following the receding ice edge, an area of high biological productivity which provides a rich feeding ground.
There are also two natural groups of seals — true seals and eared seals, the latter comprising fur seals and sea-lions. In fact all seals have ears, but ‘eared’ seals differ in having a small external ear rather than just a small aperture on the side of the head. Depending on species, seals feed on fish and squid and/or krill. The leopard seal (Hydrurga leptonyx) is also a predator of penguins and other seals. Thick layers of blubber beneath the skin of both whales and seals, act as a food reserve and insulation. All Antarctic seals also have a layer of fur, giving additional insulation when they are hauled out on land or ice.
Seals leave the water to breed, rest and moult. Of the six Antarctic species, four are ice habitat specialists, breeding on the sea ice in spring. Leopard (Hydrurga leptonyx) and Ross seals (Ommatophoca rossii) tend to be solitary, whereas Weddell (Leptonychotes weddellii) and Crabeater seals (Lobodon carcinophagus) form loose breeding aggregations.
Antarctic fur seals (Arctocephalus gazella) and elephant seals (Mirounga leonina) are both found north of the pack-ice zone and breed in dense colonies on beaches. Here, dominant males (bulls) maintain harems of females (cows) in territories. In constantly defending these, bulls will not forage at sea, relying instead on blubber reserves laid down in the previous winter. All seals breed annually and the timing of pup production and associated behaviour is linked to habitat and ecology. Mating occurs after pupping, though a fertilised egg will not implant in the uterus until later in the year.