Porpoises Starving in Europe Due to Ocean Warming

January 18, 2007

James Owenfor National Geographic News

January 10, 2007

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."

Less Blubber

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.

They lose body heat a lot more quickly if they don't have sufficient blubber."

Previous studies have suggested that sand eels in the North Sea are harmed by rising sea temperatures, because the cold-water plankton they eat are becoming scarcer.

North Sea temperatures have risen by around 1.8 degrees Fahrenheit (1 degree Celsius) over the past 25 years and are predicted to increase by between 0.4 and 0.9 degree Fahrenheit (0.2 and 0.5 degree Celsius) per decade for the foreseeable future, MacLeod said.

The study team warned that predicted temperature rises and their impact on sand eels could have devastating consequences for porpoises.

The situation may already have worsened since the study was conducted, MacLeod noted.

"There appeared to be more animals that were dying of starvation last spring. Locally, in Aberdeen, we have quite a lot that wash up on the beach."

(See a National Geographic magazine feature on rescuing harbor porpoises.)

While some marine species can move north with rising sea temperatures, MacLeod said harbor porpoises, a shallow-water species, only keep to depths of less than 650 feet (200 meters) in the North Sea.

"They don't really have an option of continually moving further and further north, because they're not a deep-water animal," he said.

Whales and Dolphins

The study team said its findings suggest that other whales, dolphins, and porpoises could be similarly at risk.

For instance, the minke whale, another species that feeds on sand eels, appears to have shifted its distribution dramatically, with sightings off western Scotland down between 80 and 90 percent in the past two summers.

"Sand eels are very much the basis of the food chain, certainly around Scotland," MacLeod said. "Anything that affects them is going to affect everything."

Emily Lewis-Brown, marine climate change officer for conservation group WWF-UK, said the effects of dwindling sand eel stocks first became apparent in other North Sea wildlife in 2003, when thousands of chicks starved to death in Scottish seabird colonies.

She noted that the plankton that sand eels eat are being replaced by warm-water species that are not only less nutritious but also bloom at a different time of year. This in turn affects animals that time their reproduction to coincide with peak numbers of sand eels.

"Everything links together in a really neat chain, but parts of this chain are now moving in time and space, so they are peaking earlier or later, or they're moving north," Lewis-Brown said.

"Different parts of the food chain are now mismatched."

She added that reports of underweight harbor porpoises are becoming more common in U.K. waters and that there is evidence that dolphins are now killing the smaller porpoises as the two species compete for dwindling food supplies.

Overfishing is to blame as well as climate change, Lewis-Brown said.

"We need to manage our fisheries better. A whole ecosystem that's under pressure from overfishing has got no chance of dealing with climate change."