May 12, 2005— Almost two-thirds of the bottom-dwelling fish species studied in the North Sea are showing signs of heading for cooler climates as the water temperatures warm, a survey says.

If the exodus continues, said scientists conducting the survey, the North Sea could lose some economically important species over the next 45 years.

The species that are moving north the fastest appear to be those that are smaller and have shorter life cycles, said Allison Perry, an ecologist and doctoral student at the University of East Anglia in Norwich, England.

Perry is the lead author on a research paper on the matter appearing in the May 12 issue of Science Express.

"The species that are responding have young that respond (to temperature changes) that much faster," said Perry.

That's because small fish — either juveniles or adults — are usually more sensitive to temperature changes, she explained.

So, in effect, the smaller, quick-living fish may be acting like the canary in the proverbial coal mine when it comes to climate changes in the sea.

The northern shift of southern species into the North Sea also signals warming water, she said. For example, bib, a fish that is not currently of much commercial value, is moving north. Still, it will be encountering larger fish that move north more slowly, with unpredictable results.

"The fact that other species are moving in and others are moving out raises questions about predator-prey relationships," Perry said.

"The analysis is quite good," said biological oceanographer David Checkley of the Scripps Institution of Oceanography at the University of California, San Diego.

Checkley is also the chief editor of the journal Fisheries Oceanography. "It's consistent with what is known about the North Sea."

Previous research has shown, for instance, that cool-water-loving plankton in the North Sea has shifted north, Checkley said.

The data used for the study come from a rare annual scientific survey of bottom fish — both of commercial and noncommercial value — in the North Sea, conducted every year since 1977.

"In order to perform analysis like this requires science-based data from long periods of time," said Checkley.

Using data gathered from commercial fishing operations doesn't provide as reliable a picture of what's happening, he said.

On the other hand, what's learned has great importance to fisheries. If sea temperatures continue to increase at the rates predicted by many climate models, Perry said, some commercially important species like cod and sole could move out of the North Sea entirely by the year 2050.