Hungry humans have altered the course of evolution for Canadian cod, according to a new study. The researchers conclude that the cod's recent shift toward breeding earlier in life results from a genetic change brought on by overfishing.
Cod off the coast of Labrador and Newfoundland have been heavily fished in the 20th century. In the 1980s and 1990s populations plummeted, and in 1992, Canada nixed all cod fishing in the region. The ban persists, but fish populations have not recovered. As scientists tried to figure out what went wrong, they discovered hints that the average age of reproduction had dropped in the 1980s. This phenomenon had been spotted in other commercial fisheries, but scientists weren't sure of the cause. Some argued that the loss of so many fish simply made life for the survivors easier: Less crowding meant more food, which enabled the fish to grow bigger and mature more quickly. Others suggested that the shift reflected an evolutionary adaptation: Cod that bred earlier were more likely to pass on their genes before ending up on someone's dinner plate.
The new study supports this second scenario. Researchers led by evolutionary ecologists Esben Olsen and Ulf Dieckmann of the International Institute for Applied Systems Analysis in Laxenburg, Austria, and fisheries scientists from Canada's Northwest Atlantic Fisheries Center examined data on the size, age, and sexual maturity of 10,778 female cod caught off the coast of Newfoundland and Labrador between 1977 and 2002. Over that time period, fish began to mature--and therefore presumably breed--at an earlier age, the team reports in the 29 April issue of Nature. Because this trend was evident prior to the fishery's crash, the team suggests that paying attention to age and size at maturity may be a red flag for fisheries managers that the population is in trouble.
The findings do indeed support the idea that the shift in the cod's breeding habits are due to a genetic change, says fisheries scientist Jeff Hutchings of Dalhousie University in Halifax. If the shift were due to greater availability of food, the researchers should have seen a dramatic increase in growth rates, he says. The fact that they didn't seems to rule out faster growth as the cause of the shift toward earlier breeding.