Sperm whales

From: Phillip J. Clapham (pclapham@whsun1.wh.whoi.edu)
Date: Mon Oct 08 2001 - 09:23:41 EDT

Whether sperm whales are endangered is a very good question. The answer is
probably not, but this all comes down to a question of how many sperm
whales there are, and how their populations are structured. The population
structure issue is important because if all of the whales in one ocean
belong to a single population that mixes freely, from a conservation point
of view that's very different from a situation in which there are several
isolated populations. Just to explain this, take the gray whale in the
North Pacific... the eastern population is doing very well, with more than
26,000 animals. However, there is also a western population that is
critically endangered with maybe only 100 whals remaining. If these two
populations mixed freely across the North Pacific, then it wouldnt matter
that there were only 100 in the west, because - if they were just part of a
much larger, mixed population - others would replace them easily. But
they're not: the western gray whale is a separate population, and if they
become extinct there will not be replacement from the larger eastern stock,
and gray whales will disappear from the western North Pacific altogether.
OK, back to sperm whales. They're very difficult to count because, of all
the large whales, they tend to live in remote areas which make the business
of getting out there to count them very very difficult. Not only that, but
they dive for much longer than other big whales, so they're frequently not
at the surface to see when you're doing surveys.
The general feeling is that sperm whales are fairly plentiful in most areas
(even though they are listed as an endangered species). BUT, much of the
commercial whaling on this species in the 20th century was targeted at
males, because males are the only ones who travel to high latitudes.
400,000 sperm whales were killed in the Southern Hemisphere in the 20th
century, and the majority were males. This is important, because of the
way that sperm whales mate and breed. It used to be thought that killing
big males didn't matter because, as long as there were other males around,
breeding would continue. Work by Dr Hal Whitehead and colleagues suggests
that this is not the case; specifically that female sperm whales simply
won't accept and mate with younger males. As a result, the birth rate of
some populations may have been reduced by the effects of whaling.
By the way, even though we don't know enough about sperm whales in the
North Pacific, the Japanese are now killing them again as part of their
spurious campaign to "prove" that whales compete with humans for
commercially valuable fish and squid. They say that they need to "study
the role of the sperm whale in the ecosystem" - as if killing 400,000 of
them didn't already tell us that sperm whales eat squid...
Hope this helps!
Phil Clapham

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