Fishery changed cod breeding: study

Overfishing of mature cod has caused the fish to change their breeding tactics, a new study suggests.

Researchers from Austria, Norway and the federal Fisheries Department in Newfoundland and Labrador studied the collapse of the Atlantic cod fishery.

• INDEPTH: Newfoundland's cod fishery

They found in the mid-1980s, the average Atlantic cod began reproducing by age six. By the mid-1990s, it was happening a year earlier.

When humans target the larger fish, only smaller and younger fish are left behind to reproduce, which favours those who breed earlier, the researchers said.

When fish reproduce at earlier ages, they tend to grow less quickly and produce fewer eggs. Biologists call these changes in life history.

Until now, it has been difficult for scientists to tell non-genetic changes in the physical or behavioural characteristics from genetic ones.

Cod fishery collapse

In 2003, the Committee on the Status of Endangered Wildlife in Canada reported Atlantic cod stocks fell 99.9 per cent from the 1960s to the early 1990s. The Canadian government closed the northern cod fishery in 1992, but stocks remain historically low.

Ebsen Olsen of the International Institute for Applied Systems Analysis in Laxenburg, Austria and his colleagues used a new statistical model to tease out the age and size changes.

"Our study provides new evidence that intense fishing may indeed lead to rapid evolution of key life-history traits in harvested populations," the researchers wrote in Thursday's issue of the journal Nature.

In a commentary accompanying the study, biology professor Jeffrey Hutchings of Dalhousie University in Halifax said the study shows "the potential for fishing to generate evolutionary change within harvested populations can no longer be seriously discounted."