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Overfishing has dramatic impact on fish breeding

PARIS (AFP) - Excessive commercial fishing can encourage a genetic bias in fish breeding, wielding a potentially major impact on stocks, according to a study into Atlantic cod, once a dietary staple in northeastern Canada and Western Europe.
Norwegian and Canadian marine biologists found that disastrous overfishing off southern Labrador and eastern Newfoundland caused a big change in the age at which the female Atlantic cod (Gadus morhua) reached sexual maturity.

In 1980, when the fish population was abundant, female cod generally became fertile at six years of age; in 1987, when stocks started to plummet, sexual maturity was at five years, and those fish were smaller than their predecessors.

The reason, say the authors: fish which were larger and whose maturity occurred later in life were more likely to be caught before they could reproduce.

That gap in the population was a boon for fish which were smaller and which were genetically programmed to breed earlier. They became the dominant numbers in the fishery.

The finding is significant, because it shows overfishing creates evolutionary pressures -- influences that work against, or in favour of, specific genetic traits on fish stocks, and which have an enduring impact.

Man-made bias is a well-known consequence of agriculture, and indeed farmers often use it deliberately in order to favour new races of animals or species of plants that are more productive than others.

The study, published on Thursday in the British weekly science journal Nature, is led by Esben Olsen, at the University of Oslo.

The authors are cautious about whether wrecked fish populations may bounce back faster if sexual maturity occurs earlier.

"In the late 1980s and early 1990s, northern cod underwent one of the worst collapses in the history of fisheries," Olsen's team says, pointing out that cod numbers in those zones fell by more than 99 percent compared with three decades earlier.

"The Canadian government closed the directed fishing for northern cod in July 1992, but even after a decade-long offshore moratorium, population sizes remain historically low."