Net decrease.
Trawling for big cod has led to slim pickings, researchers say.
Credit: Jeffrey L. Rotman / CORBIS
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 (ScienceNOW, 5 July 2004). But in the wild, other factors can also influence size, such as temperature and population density.
To control for these variables, Douglas Swain, a fisheries biologist at the Gulf Fisheries Center in Moncton, Canada, and colleagues looked back over data on fishing intensity, cod population, fish size, and environmental variables from 1977 to 1997. Temperatures were warm, which should have stimulated growth, and prey was abundant. So why didn't the cod recover to full size? The team found that the change in average length of 4-year-old cod correlated with the size selection exerted on their parents--which suggests that younger generations inherited their small size from small parents, because larger fish had been caught. 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.
"It is the best demonstration that the growth rates of fish themselves have been reduced in this stock," says David Conover of Stony Brook University in New York. "This nails it." But not everyone is convinced. "I am a real skeptic of this result," says Ray Hilborn of the University of Washington, Seattle. Hilborn has analyzed 73 fish stocks and found no relationship between fishing intensity and growth rate. Swain says that the impact could vary depending on fishing methods and the nature of the fish stock. Although the strong selection pressure of fishing can quickly slow down growth rate, he says, it will likely take much longer for nature to speed it up again.
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