Undesirable genetic changes are taking place in fish stocks as a result of commercial exploitation, marine biologists warned yesterday. The evolutionary pressure is producing smaller fish that grow more slowly and reproduce less successfully than their ancestors did under similar conditions.

"These evolutionary changes have been largely overlooked until now in the debate about the management of marine resources," Ulf Dieckmann of the Institute for Applied Systems Analysis in Austria told the EuroScience forum. "The changes are a cause of grave concern because they will be very hard to reverse."