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Some of the world's commercial fish stocks are approaching a population collapse because of overfishing, but a new study says fish can recover if fishermen will give them a chance.

Jeffrey A. Hutchings of Dalhousie University in Halifax, Nova Scotia, co-author of a study published recently in the journal Science, said overharvesting can so diminish a population that reproduction cannot keep up with natural loss, no matter what limits are set.It happened in Lake Erie with the blue pike, now thought to be extinct. And it happened to the passenger pigeon, a vanished bird that once darkened the North American sky with its numbers. And, said Hutchings, it could happen with the cod, herring, turbot, sturgeon, haddock and salmon, all dangerously depleted fish stocks that are the main source of protein for millions of people worldwide.

"It has been demonstrated that a species can disappear if the density becomes so low that it can't keep up with natural loss," said Hutchings. Eventually, he said, a fish population becomes so short of spawners that mates are scarce and the whole reproduction cycle of a species is disrupted.

"After a certain level, the population is doomed to extinction," he said.

In the past 30 years, many of the world's fish stocks have gone into a population crash due to overfishing. Hutchings said biologists have been worried that some of the species will never recover.

But the study in Science indicates it is not too late for most of the major fish species.

Hutchings and three other researchers found that 125 of the commercial species would bounce back if fishing pressure was reduced and the fish were given a chance to reproduce. The chances for survival, however, are less certain for three species: the spring spawning Icelandic herring, and salmon from the Sashin Creek and Prince William Sound areas of Alaska.

Some experts hailed the study as good news that also carries a warn-ing.

"An implication of their work is that there is little evidence to indicate fish stocks won't recover once you stop fishing them," John Beddington of Imperial College, London, said in Science.

Alec MacCall of the U.S. National Marine Fisheries Service said in Science that the study "places the burden right back on the fisheries manager."

Hutchings said Norway proved that cod stocks can recover. He said the fish population crashed because of overfishing off Norway in the 1980s. The country put strict limits on the catch and now the cod has recovered enough to support a reduced level of harvesting.

Fishermen in the Georges Banks in the North Atlantic typically deplete cod stocks there by more than 60 percent annually, a rate far above the sustainable level, said Hutchings. If fish harvesting were limited to 20 percent, the cod would recover and fishermen could harvest at that reduced level virtually forever. But if the fish continue to be overexploited, he said, the species could vanish.

Science is the journal of the American Association for the Advancement of Science.