Geoscience Reference
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Figure 6.11
Leopard seals are important
predators, eating penguins, fish and krill, and
have been known to kill other seals, especially
young crabeaters. (Credit: Peter Rejcek, NSF)
hundred each year. Because minke whales remain within the pack ice for much
of the year they are dif
cult to count from ships and there is therefore considerable
debate over how large their population is. Current non-lethal research on Southern
Ocean whales is concentrating on their role in the ecosystem, with some studies
tracking whale migrations using electronic tags, others using molecular techniques
to identify food from faeces samples and to examine population structure, and
others looking at populations by identifying their underwater
.
The true top predators of the Antarctic region, apart from humans of course,
are the orcas, the leopard seals and seabirds such as giant petrels and skuas. These
vertebrates prey on other large animals of the Southern Ocean, orcas taking a range of
prey items from
'
songs
'
fish to the young of great whales, and leopard seals concentrating
on smaller food items such as penguins, krill and
fish. The predatory birds haunt
penguin rookeries and other bird colonies in search of eggs and chicks.
The open ocean
Once out of the reach of winter pack ice, the marine ecosystem begins to
resemble the oceanic ecosystems of other parts of the globe. The vast open ocean
is stormy and relatively unproductive, despite there being signi
cant quantities of
nutrients. The food chains here support populations of microscopic animals and
plants as well as smaller invertebrates, but these are scant pickings for the larger
species such as whales and seabirds. Dotted around the Southern Ocean are islands
or island groups, and, where the circumpolar current
flows over the shelves
surrounding these islands, it brings up deep nutrients that stimulate localised
blooms that stream out around the islands, especially in the lee, making them
biological hot spots. The islands also provide rare and globally important breeding
places for seals and seabirds which can often be crowded into vast colonies, fed
by the productive waters surrounding them. A further measure of the exceptional
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