The increasing population of Antarctic fur seals threatens the ecology of the region
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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