Evolution in Action, Cod Shrink in Response to Overfishing

| Tue Feb. 6, 2007 6:39 PM EST

Creationists might not like it, but a study reported on the AAAS (American Association for the Advancement of Science) website details how cod in Canada's Gulf of St. Lawrence have evolved into smaller fish since overfishing in the 1960s selectively reduced their heftier ancestors.

For Atlantic cod, overfishing is the bad gift that keeps on giving. Once a mainstay of fishing fleets, cod began to thin out in the 1960s. Today, their numbers--and the fish themselves--remain small, despite a moratorium on fishing established in 1993. Now, a study of the Gulf of St. Lawrence in Canada might explain why. Researchers report that because the largest and fastest-growing fish were harvested, cod have evolved to grow slowly--an adaptation that haunts them to this day.

The average size of young adult cod has decreased by about 20% in the last 3 decades. Lab experiments have shown that harvesting mainly large fish will cause average size to shrink. But in the wild, other factors can also influence size, such as temperature and population density.

Big fish have been declining in number and size worldwide, as reported in "The Fate of the Ocean," (Mother Jones Mar/Apr 2006). Douglas Swain, fisheries biologist at the Gulf Fisheries Center in Moncton, Canada, and colleagues, examined the data on fishing intensity, cod population, fish size, and environmental variables from 1977 to 1997. They found that temperatures were warm and prey was abundant, variables that should have stimulated growth. Instead, the fish got smaller. The average length of 4-year-old cod correlated with the size selection exerted on their parents. The authors suggest that recent generations inherited their small size from small parents, because most of the larger fish were captured by human fishers.

This makes sense, Swain says; slow-growing fish would have an advantage, as they have a greater chance of reproducing before they're caught in nets.

Evolution is alive, well, crafty, and hopefully faster than us.