Seals

Fourteen species of seals are found in or adjacent to the Antarctic Sector of the Pacific. Breeding colonies occur on the coasts of southern New Zealand, southern Chile, the Subantartic islands, and on the ice itself. It was the Subantarctic and temperate seal colonies that attracted the 18th century sealers, and so started the massive depletion of marine mammal stocks in the Southern Ocean. Many Subantarctic islands were discovered by sealing expeditions.

Between the late 1700s and 1830, many seals were taken from the islands to the south of New Zealand, as well as from the Antarctic Peninsula, with the result that by 1830 the fur seal (hunted for its soft fur) and the elephant seal (oil) were so rare that the industry ceased (Chapman, 1893). By 1870, fur seals had recovered enough to allow a sealing industry to begin again in the Subantarctic islands, but this was short-lived, and by 1894 a total ban on catching was in force in that region (Csordas and Ingham, 1965).

In 1971-72, the U.S.S.R. took 1,000 crabeater seals from Antarctica as an experiment, but did not develop an industry on that species.

The biology of Antarctic seals has been summarized by Laws (1981, 1984), papers in Ridgway and Harrison (1981), and King (1983), and the interactions between man and seals by Bonner (1982). Daniel and Baker (1986) provided an identification guide to seals of the New Zealand region, which includes species found in the Ross Sea. Laws (1981) referred to the Antarctic seals as those that breed on land, and those that breed on the ice. The fur seals, sea lions, and elephant seal fall into the land-breeding category, and the leopard, Ross, crab-eater, and Weddell seals into the ice-breeding group.

Fur Seals

The Southern Hemisphere fur seals all belong to the same genus, Arotocephalus. The Antarctic and Subantarctic fur seals, primarily southern Atlantic and Indian Ocean species, come within the geographical scope of this account as breeding colonies only at Macquarie Island (Shaughnessy and Fletcher, 1987) and as stragglers to the Chilean coast (Guerra and Torres, 1987) and (the Subantarctic fur seal only) to southwestern New Zealand (Csordas, 1962) and the Antipodes Islands (Taylor, 1979).

Four other species of fur seals are found mainly in the South Pacific region: the Australian, New Zealand, Juan Fernandez, and South American fur seals. The Australian fur seal is found on the southeastern Australian coast, including the islands of Bass Strait and Tasmania north of 45°S (Warneke and Shaughnessy, 1985). The New Zealand fur seal also occurs on the coast of southern Australia, and at Macquarie Island, but its main populations are in the New Zealand region including the Subantarctic islands south to Campbell Island (Mattlin, 1987). The Juan Fernandez fur seal occurs only at that group of islands and the San Felix Group slightly further north off the coast of Chile whereas, on the mainland, the South American fur seal ranges from near Cape Horn along the coast of Chile to southern Peru (Guerra and Torres, 1987; Majluf, 1987).

Total population size

Trend

Antarctic fur seal 1,100,000

Subantarctic fur seal 300,000

New Zealand fur seal 50,000

Australian fur seal 25,000

South American fur seal 500,000

Juan Fernandez fur seal 6,000

rapid increase rapid increase increasing stable increasing increasing

The above figures show the total population sizes and trends for Southern Hemisphere fur seals (from Croxall and Gentry, 1987, and King, 1988). The rapid increase in populations of the Antarctic fur seal since 1956 (when monitoring began) has been attributed to decreased competition for food (krill) with the now depleted baleen whales (Bonner, 1982).

Sea Lions

Three species of sea lions are found in the South Pacific: Hooker's sea lion, the

Australian sea lion, and the South American sea lion. Hooker's sea lion has breeding colonies on the islands to the south of New Zealand, primarily Auckland Islands and Campbell Island (Fig. 9.10). Some breeding also occurs on the Snares Islands, and occasional stragglers, mostly young males, reach Macquarie Island and the New Zealand mainland (King, 1983). Hooker's sea lion was exploited commercially in the early 1800s, and had almost vanished by 1830. The population has built up slowly since then, and is now estimated to be over 6,000 animals. It is currently under some threat as a by-catch in the squid trawl fishery near the Auckland Islands (Cawthorn et al., 1985).

The Australian sea lion has a restricted distribution amongst the offshore islands of southern and western Australia east to about 140°W, so is just on the edge of the region covered in this volume. The total population is thought to be between 3,000 and 5,000 seals.

The South American sea lion has a wide distribution along both coasts of southern South America, reaching northern Peru in the west. It is common throughout the Chilean archipelagoes from Tierra del Fuego to Chiloe Island (pers. observ.), and breeding colonies are known as far north as Islas Lobos de Tierra at 6°30'S. The Peruvian population was thought to be about 20,000 and increasing in 1968 (Grimwood, 1969). The South American sea lion was exploited once the eastern Pacific was opened to ships by the discovery of the Straits of Magellan in the sixteenth century. Sea lions are protected in both Peru and Chile, but some are still killed by fishermen who farm salmon and suffer predation on

Fig. 9.10. Female Hooker's sea lions at the Auckland Islands : a species threatened by commercial fishing (Photo : National Museum of N.Z.).

the salmon by sea lions, or who use the seal flesh for bait in the king-crab fishery of southern Chile (pers. observ.).

Elephant Seal

The southern elephant seal's main breeding populations occur on the circumpolar Subantarctic islands, which, in the Antarctic Sector of the Pacific, are Mac-quarie Island, Campbell Island, and the Auckland Islands. The largest population is on Macquarie Island which is estimated at 136,000 animals (McCann, 1985). Southern elephant seals were extensively hunted in the early 1800s, and the populations were severely reduced in most places. Sealing had ceased in the Antarctic Sector of the Pacific by early this century, and it was thought that the elephant seals had regained their former population level by 1950 (Carrick et al., 1962). However, recent studies at Macquarie Island by Hindell and Burton (1988b) indicated a 40-50% decrease there over the past 36 years. The reasons for this decline are not known, but some of the potential underlying causes suggested are natural fluctuations in population size generated by such intrinsic factors as density dependent pup mortality, increased predation, depletion of resources and environmental changes. Elephant seals regularly reach the coasts of New Zealand and Tasmania (Fig. 9.11) and, although one has been recorded from Peru, none has yet been found in Chilean waters (King, 1983). The biology of southern elephant seals is reviewed by Ling and Bryden (1981).

Fig. 9.11. A male elephant seal, a regular visitor to New Zealand shores, but whose numbers are decreasing in the Southern Ocean (Photo : National Museum of N.Z.).

The Ice-breeding Seals

The leopard seal is found in the circumpolar pack ice in summer and north through the Subantarctic islands occasionally to the coasts of New Zealand, southern Australia, and Tierra del Fuego in winter. It is a solitary animal, but the total population could be quite large (about 220,000) (Gilbert and Erickson, 1977). Because of its solitary habit, the leopard seal has not been subject to exploitation. The Ross seal also lives in the ice, apparently preferring consolidated, heavy ice rather than open pack ice in the Ross Sea (Hofman, 1975). It does not often move north from Antarctica during the winter, there being only a few records from Australia and New Zealand. In Antarctic seas, the Ross seal is commonest in the Ross Sea and King Haakon VII Sea. Like the leopard seal, it seems to be largely solitary. Population estimates by Hofman et al. (1973) were in the order of 100,000-650,000 animals but Ray (1981) believed that there are still insufficient sightings of the Ross seal to make any reasonable estimates of its numbers.

The crabeater seal lives on the drifting pack ice surrounding Antarctica (Kooyman, 1981a). It is regarded as the most abundant seal in the world, with population estimates ranging up to 75 million animals (Erickson et al., 1971). However, more recent and accurate estimates by Gilbert and Erickson (1977) put the numbers at about 15 million. It is especially abundant in the waters west of the Antarctic Peninsula and in the southern part of the Ross Sea. A few stragglers have reached the coasts of New Zealand and Australia. The crabeater actually feeds on krill rather than crabs, having teeth specialized for straining small crustaceans from the water. It is thought that, like the Antarctic fur seal, stocks of crabeaters have increased substantially this century as a result of the greater availability of krill following the demise of Antarctic whales (Laws, 1977). This seal has not been significantly exploited. The mummified seals known from the inland Dry Valleys of Victoria Land, Antarctica, are mostly crabeaters.

The Weddell seal is unusual because it spends its entire life more or less literally in Antarctic waters, feeding under the pack ice, and breeding there. In the spring and summer, they can be seen lying languorously about on the ice surface. Like the other ice-breeders, the Weddell seal is sometimes seen at the Subantarctic islands and, more rarely, in New Zealand and southern Australia. Stirling (1971) and Kooyman (1981b) reviewed the knowledge of this species of well-studied seal, and Tedman (1985) listed important references to more recent work. Total population estimates vary from 250,000 to 750,000 animals, with about 50,000 in the western part of the Ross Sea (Laws, 1971; De Master, 1979). Weddell seals were hunted briefly in 1821, but otherwise they have not attracted commercial attention.

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