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Compiled by Leon P. Zann
Great Barrier Reef Marine Park Authority, Townsville Queensland
Department of the Environment, Sport and Territories, Canberra (1995)
ISBN 0 642 17391 5
The tropical dugong is the only fully herbivorous marine mammal and the only Sirenian (sea cow) to occur in Australia. It is extinct or near extinct in most of its former range which extended from East Africa to South East Asia and the Western Pacific. Northern Australia has the last significant populations (estimated to be over 80,000) in the world. Large, long-lived mammals, dugongs become sexually mature at around 10 years and calve every three to five years, making them vulnerable to overhunting.(18)
Major concerns are possible overhunting of Torres Strait populations, mortalities in fish gillnets and shark nets, and loss of seagrass habitat. A major mortality occurred in Hervey Bay (Qld) in 1992 following die-off of seagrasses. The dugong is listed by the IUCN as 'Vulnerable to extinction' but it is not listed under the Commonwealth Endangered Species Protection Act.(18)
Three species of eared seals breed in mainland Australian waters: the endemic Australian sea lion, Australian fur seal and New Zealand fur seal(18). The Australian sea lion has recently been listed as 'Rare' by the IUCN(84). Five species of true seals and two other species of fur seals breed in Australia's subantarctic islands and the Antarctic Territories(18).
Australia's seals were seriously overhunted last century. They are now fully protected and some populations appear to be increasing. Major human threats include entanglement in fishing nets and ocean litter, oil pollution, and disturbances by visitors. Fur seals are still occasionally illegally killed for lobster bait, and around fish farms for 'stealing' fish. Development of predator-resistant cages has reduced the latter problem.(18)
Entanglement in nets and plastic box straps remains a major problem. About 2% of seals at haul out or resting sites in Tasmania are entangled in net fragments and other plastic litter at any time(18),(53),(54). It is thought that a significant number of more badly tangled seals must drown before reaching haul out sites. In 1990 an oil spill in Western Australia affected a number of New Zealand fur seal pups(39).
Eight species of baleen whales and 35 species of toothed whales, porpoises and dolphins are found in Australian waters, although none are endemic. This is almost 60% of the world's total cetacean species.(18)
All whales and dolphins are large, to extremely large mammals. Because they are slow to mature and calve at intervals of several years, they have slow rates of natural population increase and are highly vulnerable to overexploitation.(18)
Until recently, hunting was the major impact on whales and several species were driven to near extinction. Australian breeding populations of southern right whales were depleted by 1845. Their population has slowly increased from small remnants totalling a hundred or so, to between 500 and 800. Australian breeding populations of humpback whales were depleted by 1963. Numbers are now recovering and there are now thought to be up to 4,000 breeding in Australian waters. In the Antarctic, overhunting has endangered the blue whale, perhaps the largest animal which has ever existed, and severely depleted other baleen species.(18)
Gillnets, shark nets set off bathing beaches, discarded fishing nets, and ingestion of plastic litter are considered threats to cetaceans within Australia. It is estimated that during the 1980s almost 14,000 dolphins were drowned in Taiwanese shark gillnets off northern Australia but this fishery is now closed(18). The use of long driftnets (sometimes referred to as the 'walls of death') which caused substantial mortalities of cetaceans is now banned under the Convention for the Prohibition of Fishing with Long Driftnets in the South Pacific and the UN global moratorium on their use(84). However, considerable numbers of cetaceans are still caught in protective shark nets off bathing beaches. For example around 520 dolphins were caught in shark nets in Queensland between 1967 and 1988. Contamination of cetaceans by organochlorine pesticides and poly-chlorinated biphenyls (PCBs) is regarded as a significant worldwide threat.(18),(75)
Figure 36: The humpback whale migrates northwards in winter to calve. It feeds in Antarctic waters.
Figure 37: The southern right whale migrates northwards to southern Australia to calve.
Whaling has been replaced by the new industry of 'whale watching'. Because of concerns that boats, aircraft and divers may affect whale behaviour, regulations govern the distances that observers may approach whales.(18)
The southern right whale, humpback whale and blue whale are listed as endangered species under the Commonwealth Endangered Species Protection Act. Internationally, Australia has been very active in whale management through the International Whaling Commission and the recent establishment of the Southern Ocean Sanctuary.(18),(60)