A problem of plenty
AN EXPLOSION in the population of Antarctic fur seals has caused wide- spread changes to many coastal, terrestrial and freshwater ecosystems in the northern maritime Antarctic islands and on the west coast of the Antarctic Peninsula. Dominic A Hodgeson and Nadine M Johnston of the British Antarctic Survey, Cambridge, conducted studies based on used seal hairs found in lake sediment cores from one maritime Antarctic island as a historical record of seal populations (Nature, Vol 387,No 6628).
This enabled them to study the possible causes of the increasing numbers of visiting Antarctic fur seals, and has provided a historical framework from which to evaluate conservation plans to minimise the adverse effects of seals at sites of particular ecological importance. The procedure of examining sediment cores for the presence of seal hairs to evaluate the seal population is quite accurate as the evaluation since 1977 corresponds to the census data available. So it can be assumed to safely assess the seal populations for the periods for whiclif data is unavailable.
Though the Antarctic seal was hunted to near extinction during the nineteenth and early twentieth century, their numbers have been increasing recently. From less than 100 seals that visited Signy Island in the South Orkney Islands from the main breeding beaches in South Georgia in 1976, their numbers have gone up to almost 20,500 in 1994. This large increase has caused extensive destruction of vegetation, soil erosion and the eutrophication (depletion of the oxygen content in a lake due to extraordinary growth in organic and mineral nutrients) of freshwater lakes on coast-lines where the seals haul out. The protocol on environment protection was adopted by the Antarctic Treaty Nations in 1991 and it is expected to enter into full international force in the near future. Given the commitment of the Treaty Nations to limit the adverse effects on Antarctica, the current fur seal population raises three important questions. First, is the increase the result of human or natural influences? Second, does the increase fit in the range of normal population variability during the past several thousand years? Third, how might any control measures that are deemed necessary be implemented within the Antarctic Treaty system?
As the study of ieal hair found in lake sediment cores from Signy Island make it possible to study the timing of the Antarctic fur seal population explosion in relation to possible causal factors. Geophysical and biological palaeoclimate indicators in sediment cores show that the population explosion is unlikely to have been influenced by changing natural ecological conditions. However the evidence does suggest a link between seal populations and the activities of the whaling industry.
A short sediment core showed that seals visited the island before commercial sealing began, but declined in numbers during the sealing periods in the nearby South Shetland Islands between 1820 and 1870s. As sealers withdrew from the South Shetland Islands there seemed to be brief increase in the abundance of visiting seals, but regional sealing based in South Georgia continued to reduce the population near to extinction. There is no evidence that fur seals visited Signy Island between 1950 and the late 1970s, and it was not till 1977 that the summer influx of seals began to increase rapidly. One possible cause of this increase might be the greater than 90 per cent reduction in the number of baleen whales in the Southern Ocean by the whaling industry since 1922. This seems to have resulted in an abundance of the Antarctic krill (on which the whales feed) which has subsequently been available to seals.
The'number of seals hairs presently being deposited in the sediments is 78- 94 per cent greater than at any during the past 6,570 years (as measured from the radiocarbon system of dating). This implies that the present number of visiting seals exceeds the range of natural variability. At present there is no evidence of any particular species or community type being endangered by the seals, but in the northern maritime Antarctic islands and on the west coast of the Antarctic peninsula, changes to lowlands and freshwater ecosystems in the past two decades is significant and are apparently spreading.
There is a growing need for the Antarctic Treaty Nations to consider how ecologically important sites might be protected. Though this can be done by preventing the seals from gaining access to areas of particular ecological or scientific importance through the means of wire mesh and electrical fences, such methods might prove to be controversial. Besides, they might be unnecessary when regulations set to manage stocks of whales, as ascribed in the International Whaling Commission, and stocks of fish and krill, as governed by the Convention on the Conserption of Antarctic Marine Living Resources, begin to exert a regulatory influence on seal populations in the future.
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