Shrinking fish can get big again, but such reversal may take time: study suggests managers should consider fishing's effects.

People can reverse evolution when it comes to the effects of fishing on sea creature size, researchers say. Just don't hold your breath.

The common practice of catching only the bigger fish in a population becomes an evolutionary pressure for later generations to stay small and grow slow, says fsheries scientist David Conover of Stony Brook University in New York.

He and his colleagues now report a lab experiment on silverside fish that stops the shrinkage trend and nudges the population's fish back toward larger sizes. Changing the direction of the selective pressure by eliminating the "take the big ones" rule brought a noticeable size increase within five generations, Conover and his colleagues report in a study released March 3 in Proceedings of the Royal Society B.

"It's the first experimental demonstration of reversing fisheries-induced evolution," Conover says.

Fisheries biologist Mikko Heino of the University of Bergen in Norway calls the work very interesting and important. The results support predictions that any recovery will be slower than the evolutionary effects of fishing, he says.

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Theorists have debated whether populations could recover from the size effects of fishing, Conover says. One argument states that natural selection might be too mild to push back hard enough to restore size on a human timescale.

To experimentally address the question, Conover and his colleagues caught hundreds of silversides (Menidia menidia) in Great South Bay, N.Y.

The researchers divided the bait fish into six populations. For two groups, the team annually removed the large individuals, taking all but the smallest 10 percent. The team targeted small fish in two of the other groups, and removed fish randomly from the remaining groups. After five years, about five generations, the catch-big populations had a smaller average size than the comparison groups.

For the next five years, Conover and his colleagues kept the populations but changed the rules. The team removed fish randomly from all groups.

At the end of that five years, the shrinking trend had reversed, and adults in the catch-big populations had regained half their original length, on average. But the recovery was slow, Conover says. He calculates that if the silversides maintain their current pace, they will need a total of 12 years to return to their former size.

Just how Conover and his colleagues' research would apply to ocean fish in the wild is "an open question" Heino says.

But Conover says the best path is to avoid the shrinkage to begin with or stop it as soon as possible.

"That's the biggest implication--you'd better hurry," says fisheries management specialist Ellen K. Pikitch, who directs the Institute for Ocean Conservation Science at Stony Brook, which provided funding for the 10-year research project.