The Australian Antarctic Territory: Marine ecosystems
Independent report to the Australian Government Minister for the Environment and Heritage
Beeton RJS (Bob), Buckley Kristal I, Jones Gary J, Morgan Denise, Reichelt Russell E, Trewin Dennis
(2006 Australian State of the Environment Committee), 2006
The ecosystems of the Southern Ocean have been subjected to human pressure for more than 200 years. Historically, sealing, whaling and fishing have had significant effects on populations of marine species. Climate change is an additional pressure that may affect the rates of upwelling of nutrients and the melting rate of the pack ice in spring. Increases in levels of carbon dioxide may increase the acidity of the ocean and affect the health and normal function of marine organisms.
Seal populations have not yet recovered from the eighteenth century exploitation, but there is some evidence of improvement. Land-breeding seals have increased from the low levels at Heard, McDonald and Macquarie Islands in the nineteenth century, but the status and trends of ice-breeding seals are far less certain due to difficulties in surveying. Whales were severely over-harvested in the middle of the twentieth century and although some species, such as the humpback whale (Megaptera novaengliae) are recovering, the status of others, such as blue whales (Balaenoptera musculus), is uncertain. The minke whale (Balaenoptera acutorostrata) has been harvested less than other species but there is still debate over its numbers.
Fish have been harvested commercially in the Southern Ocean since the 1960s and many stocks have never recovered from the initial phase of overexploitation (see ‘Fishing’). Current finfish fisheries have suffered greatly from illegal, unregulated and unreported fishing. While fisheries in the Antarctic region today are focused on Patagonian Toothfish (Dissostichus eleginoides), Mackerel Icefish (Champsocephalus gunnari) and Antarctic Krill (Euphausia superba), the focus may shift to other species in the future. The position of Antarctic Krill in many critical food webs suggests that their increased utilisation could be a significant issue and so, despite their relative abundance, catch limits will need to take this into account.
Antarctic waters are highly productive, with a complex plankton-driven food chain . The zooplankton composition changed in the year 2000, accompanied by a substantial change to the composition of smaller zooplankton. It is too early yet to determine if this is a permanent regime shift or part of a cyclic pattern.
Many of the flighted seabird colonies are in remote areas and their status is largely unknown. Of the large number of flighted seabirds in the Antarctic, Heard Island cormorants and Macquarie Island shags are listed as vulnerable. Of the seabirds that breed at Macquarie Island, albatrosses and petrels are probably the most threatened birds worldwide. The Wandering Albatross (Diomedea exulans) is the most critical, with less than 15 breeding pairs. The numbers of Southern Giant Petrels (Macronectes giganteus) at Macquarie Island have doubled since the 1970s and appear to be stable. More generally, Giant Petrel (Macronectes species) populations appear to have decreased since their discovery some 40 years ago (Woehler et al 2003).
Due to the overlap of commercial fishing areas with foraging ranges of seabirds, the two interact directly. It is believed to be here that most of the mortality among seabirds occurs, particularly various species of albatrosses and medium-sized petrels, as birds drown either by swallowing baited hooks or by being entangled in fishing gear.
Of the 17 species of penguin world-wide, seven are known to breed regularly in, or occasionally use, the ice-free areas of the Australian Antarctic Territory, including the islands, and forage in the oceans surrounding these areas. Estimates of populations from about three decades of research show trends for two species at three sites. Gentoo Penguin (Pygoscelis papua) breeding populations at Heard Island were estimated at 10 000 pairs in 1950 and 16 600 pairs in 1987. Breeding populations of King Penguins (Aptenodytes patagonicus) are difficult to estimate, but at Heard Island, data collected between 1963 and 1993 suggest that the population is doubling every five years (Woehler 2006). At Macquarie Island, the population has recovered rapidly from harvesting in the last century and is thought to be still expanding. Royal Penguins (Eudyptes schlegeli) have been estimated at approximately 850 000 breeding pairs at Macquarie Island, but no information on trends is available.