By Paul Rincon
BBC News science reporter, in Washington DC
The shrinking size of fish due to their overexploitation has dire consequences for the recovery of depleted stocks, scientists have claimed.
Large-scale fishing effort is having a huge impact
Fishing drives natural selection for smaller fish that grow more slowly and have reduced reproductive potential.
These changes are genetic and therefore hard to reverse, scuttling the renewal of dwindling fish populations.
Details of the research were discussed on Saturday at a major science conference in Washington DC.
"Most fisheries are collapsing and many are on the brink of potentially irreversible loss," said Jeremy Jackson, of the Scripps Institution of Oceanography in California.
"There are massive evolutionary shifts going on in the remnant populations of fish. Large fish with huge reproductive potential are being replaced by smaller fish with diminished reproductive potential," he told the annual meeting of the American Association for the Advancement of Science (AAAS).
David Conover, of Stony Brook University in New York found a two-fold change in productivity in Atlantic silversides within just four generations of fish.
"By selectively harvesting the largest fish, we end up changing the whole biology - not only growth rates, but egg size, fecundity, feeding behaviour," he said.
"The scary part is that when we stopped size-selective harvest, the biology didn't change back, it was permanent."
Research by Dr Conover and other groups in Norway, Canada and Austria also found declines in the reproductive potential of fish populations.
Work by Steven Berkeley, of the University of California-Santa Cruz, shows that older and therefore bigger female Pacific rockfish produce exponentially more eggs than younger, smaller females and their larvae have a greatly increased chance of survival.
"What we need to provide is some refuge from fishing so that the genes for larger, faster-growing fish have some sanctuary from fishing," said Dr Berkeley.
Andy Rosenberg, of the University of New Hampshire, said that the way fish resources were currently being managed was prolonging the period of recovery for stocks.
"The longer we ignore these fine-scale processes, the longer it will take for that recovery. And it's not a simple linear relationship - you can cause massive damage in a very short period of time and it can take them much longer to recover," he said.
Cod off the coast of Newfoundland - once one of the largest fish populations in the world - have suffered a 99% decline since the 1960s.
Research shows that changes in size and age at maturity caused by just 30-50 years of fishing have reduced the chance of cod's recovery by 25-30%.