WASHINGTON — Rules that allow only the catching of larger fish may encourage their replacement with slower growing, more timid varieties.

That, at least, is the concern of researchers who studied test populations in two artificial lakes and report their findings in this week's edition of Proceedings of the National Academy of Sciences.

Peter A. Biro of the department of environmental science at the University of Technology in Sydney, Australia, explained that it's the fast-growing more aggressive fish that tend to get caught, removing them from the breeding pool.

That leaves reproduction up to slower-growing fish who are more timid, he explained in an interview via e-mail.

"This will cause evolution to slower growth rates and slow the rate of recovery for fished populations, and could explain why fisheries tend not to rebound in the manner we expect after we reduce harvest or close a fishery," he said.

"What surprised me was how fast it occurred," Biro said. He said the largest catch occurred on the first day of fishing.

Biro and his colleague, John R. Post, stocked two lakes in western Canada with different types of rainbow trout — one type was known to be aggressive in seeking food and to grow rapidly, while the other grew more slowly and tended to take fewer risks in foraging.

They set gillnets in the ponds over five days, moving them each day, and caught 50 percent of the stocked fast-growing fish but just 30 percent of the more cautious ones.

"Fish that are highly active and bold tend to bump into these nets more often and are less likely to avoid them," he explained. And increased activity is necessary to get enough food for rapid growth.

John Waldman, an aquatic biologist at Queens College in New York, called the report important.

"Harvest of fishes is probably the most profound impact mankind is having on the sea, yet we rarely succeed in even the basics of achieving long-term sustainability of important commercial species," said Waldman, who was not part of the research team.

The report shows that "differences in 'boldness,' which are positively correlated with grow rate, render bold individuals more vulnerable to harvest, thereby adding an important and, till now, unconsidered direct effect to the known indirect effect" of fishing, Waldman said.

"The implication for managers is that the continued reproduction of a meaningful portion of fast growing individuals is likely even more important than previously recognized," he said.

The research was supported by the University of Technology Sydney and the National Science and Engineering Research Council of Canada.

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