for National Geographic News
Porpoises are starving to death in European waters because of fish shortages linked to ocean warming, according to a new study that warns of mounting fatalities if sea temperatures continue to rise.
An investigation of harbor porpoises stranded along the coast of Scotland suggests that the marine mammals are dying because the fish that the porpoises eat, called sand eels, are in short supply.
Dwindling sand eel stocks in the North Sea have already been blamed for poor breeding success among seabirds. This latest research suggests that marine mammals are also affected (see United Kingdom map).
A Scottish team from Aberdeen University and the Scottish Agricultural College found that the number of harbor porpoises dying from starvation rose to 33 percent in 2002 and 2003—up from 5 percent in previous years.
The study, reported in the journal Biology Letters, was based on autopsies of beached harbor porpoises, Europe's smallest whale.
The porpoises rely heavily on sand eels, which make up to 80 percent of the mammals' diet in the spring, said Aberdeen University's Colin MacLeod, who led the study.
"We didn't really find other species of fish in their stomachs," he said. "If the sand eels aren't there, then there isn't any alternative food for them."
Spring is a vital feeding time for harbor porpoises, MacLeod explained. The animals have just come through the winter, and the waters of the North Sea are at their coldest.
"If they're not feeding, they have to use the energy in their blubber, which means the blubber layer gets much thinner," MacLeod added.
This can lead to hypothermia, which was found to be the actual cause of death in the starved porpoises.
"The reason the porpoises die from hypothermia is because they're so small," the marine zoologist said.
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