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10 Jan 07
Climate food threat to porpoises
Climate change is disturbing the food supply of the harbour porpoise, one of Britain's sea mammals, research shows.
Scientists found that the number of sandeels available each spring for porpoises to eat has declined sharply, and more of the porpoises are starving. The mammals need food frequently to survive, researchers say, and are vulnerable to shortfalls in their prey. The problem, reported in the Royal Society's journal Biology Letters, is also affecting some British sea birds.
Sandeels are rich in oil, which make them a vital food for many birds and mammals. "The energy content per gram of tissue is the key factor," observed Colin MacLeod from Aberdeen University. "And harbour porpoises are quite small, so if they don't get enough food, what happens is the blubber layer gets too thin and they die of hypothermia," he told the BBC News website.
The research shows that sandeels have been hard to find for harbour porpoises in the Scottish North Sea in recent years. The team's data comes from harbour porpoises which have been found dead on shore, and autopsied to determine the cause of death. Autopsies frequently include examination of stomach contents.
Between the years of 1993 and 2001, only 5% of the animals dying in springtime died of starvation; in 2002 and 2003, the figure was 33%. The proportion of sandeels found in their stomachs declined sharply between the two periods.
In an average March between 1993 and 2001, sandeels made up about 60% of the average harbour porpoise's diet; in 2002 and 2003, it was only 7%. The same trend has been seen for the month of May, though not April.
"The biology of sandeels is quite complicated," said Dr MacLeod. "They spend a lot of the year including the winter buried in sand; and what seems to be happening in recent years is that they are coming out of the sand into the water for much less of the time, and it's only then that the porpoises can eat them."
This behavioural change is believed to be linked to a loss of plankton, their key food, as seas warm, though overfishing is also a factor. The sandeel decline has caused major problems for sea birds such as kittiwakes, arctic terns, guillemots and puffins, and fishing quotas have been cut drastically in an attempt to stem the tide.
The researchers observe that if water temperatures continue to rise and plankton decline accordingly, then a further reduction in the availability of sandeels can be expected.
Related articles on Global: marine issues and dolphins
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