Fishers like to catch big fish, and over time this strategy can select for genetically smaller fish in a population. But a new study suggests that this downward spiral is reversible. If fishers harvest at random, big fish can stage a comeback.
Most fisheries target only the larger fish in a population. Trawl nets allow the smallest fish to escape, and recreational fishers usually throw back the little guys. In the case of Canadian cod, harvested to collapse in the 1980s, the survivors are smaller than they were historically. Such trends worry fisheries scientists, because larger fish tend to be healthier and more fecund than smaller ones.
Researchers have wondered whether the declining average size they see in harvested fish populations is due to environmental factors or to genetic changes in the harvested populations. In 2002, David Conover, a marine ecologist at the Stony Brook University in New York state, and his colleagues showed that part of the reason is genetic. In laboratory studies, his team removed 90% of the largest fish in two groups of silversides (Menidia menidia)--a small, commercially harvested fish found off the northeastern U.S.--over 4 years or four generations. As the bigger, faster-growing fish were removed from the population, those with genetically slower growth rates grew to dominate, driving down the average size.
Conover decided to see whether it was possible to reverse the process. Turning back to the shrunken silversides, his team again removed 90% of the fish from each generation--this time choosing randomly rather than picking the big ones. Over six generations, the fish regained about 50% of their length, the team reports 4 March in the Proceedings of the Royal Society B.
According to Conover, the results mean that "if we stop fishing in a population that's been tremendously downsized, ... we should expect that large size will gradually return." With his silversides, he reckons this would happen within 12 generations or roughly 12 years. But in other commercial species, salmon for instance, generation time is longer, and the same process would take decades, Conover says.
"The positive message is that reversal is possible and pretty rapid," says Christian Jørgensen, an evolutionary ecologist at the University of Bergen in Norway. But Anna Kuparinen, a biometrician at the University of Helsinki, says that real-word fisheries aren't as efficient at finding every single large fish as the laboratory study, so the selective pressures there would not be as great. Because of that, she says, it's hard to say for sure how a wild population would respond to similar changes in harvest.