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Scientists have warned that a fishing rethink is needed after finding that catches of big fish trigger a rapid change in the gene pool of fish stocks.

Researchers at Bangor University say they found that over-harvesting larger fish leads to a population of smaller fish that are less fertile.

The research suggest that the change happens within a few generations.

The scientists say the findings could have a massive impact for the future of global fishing policies.

"Our findings have major implications for the sustainability of harvested populations," said Prof Gary Carvalho, of Bangor University's School of Biological Sciences.

He said a "shift in the genetic make-up of harvested fish to smaller less fertile individuals" would be "serious global consequences for the environment and for global fishing industry".

"We would urge the scientific community, policy makers and managers to consider the capacity of harvested stocks to adapt to, and recover from, harvesting and predation."

The research is published on Monday by the in the United States.

The north Wales scientists collaborated with research teams at University of East Anglia, the University of the West Indies and the Max-Planck-Institute for Developmental Biology.

In lab tests, they studied a model tropical fish, the Trinidadian guppy, and managed to examine changes in the DNA of genes as the fish became fewer and smaller.

The scientists stated that they had proved for the first time that changes in the body size and maturation of the fish is "an evolutionary response to over-fishing".

"What was seen under laboratory conditions has probably already taken place in any number of commercial fishing grounds," warned the researchers.

The teams also suggested that the time for fish stocks to recover from the changes to their DNA and return to larger fish specimens will take five to 10 times longer than anticipated - if the DNA change can be reversed at all.

"This means that current estimates for how quickly commercial fisheries will recover from declines and over-fishing are probably far too optimistic," they added.

"In terms of fish as a food source, not only do such genetically-based shifts lead to the need to harvest more smaller individual fish for the same tonnage, but this also has repercussions for the wider marine community and environment."

Much of the work was conducted by Serinde van Wijk, studying under a Bangor University-funded doctorate.

"Our attempts to conserve fish communities by regulating the size of fish that can be fished for, and by removing specifically the larger fish, may have had opposite effects to those intended," she said.

"As well as losing the capacity to produce large sized and productive fish, specific fish populations may also be at risk of losing other specific adaptations by selective fishing, such as adaptations to particular location characteristics, like colder water or migration routes.

"The loss of these genetic 'types' may mean that populations may not be able to recover completely or at all."