Fish have been harvested so vigorously around the world that a resource that once seemed limitless is now threatened on a global scale, and new research indicates that severely depleted stocks may be only the tip of the iceberg. There is reason to believe that fishing practices have actually changed the gene pool for numerous species, leaving them less likely to survive and poorly equipped to rebound from overfishing.
In one troubling study that covered six generations of a small anchovylike fish, researchers at Stony Brook University in New York documented dramatic evolutionary changes. Later generations were smaller, produced fewer eggs and were less willing to forage for food.
"It's a very pronounced effect, and these changes leave the species at a disadvantage in its natural environment," says Matthew R. Walsh, lead author of a study that will be published in the February issue of Ecology Letters.
Although that research occurred in a laboratory setting, similar changes are probably taking place among many species in the wild, says Walsh, who is now at the University of California at Riverside.
The findings could partly explain a mystery that has bedeviled marine biologists for years. Some species that have been overfished don't return to their previous population levels, even after fishing has been curbed.
The driving force behind these evolutionary changes may be the simple fact that no fisherman wants to return home with a dinky fish. It's the big guys they're after, the lords of the ocean that can put up a fight and decorate that empty spot over the fireplace.
That holds true for commercial as well as sports fishers, because fishing strategies, and even fishing regulations, are biased toward harvesting the largest members of the species and leaving the smaller fish for another day.
Over time, the big fish are withdrawn from the gene pool, and year after year the fish get a little smaller.
"The fishermen have been saying that for years," says Walsh.
But in an effort to reach beyond anecdotal evidence, Walsh and several colleagues set up an ambitious experiment at Stony Brook. They chose the Atlantic silverside as their study subject, because that species lives only about a year, so it was possible to study several generations in a just a few years.
About 700 silversides were collected in Great South Bay, N.Y., on May 5, 1998, and put into two separate tanks. The two tanks were kept identical in every way possible, except for one difference. Smaller fish were harvested in one tank, and larger fish were harvested in the other.
Removing the largest fish over several generations gradually caused what scientists call a Darwinian debt. Far from survival of the fittest, the fish in that tank showed a number of changes that clearly were not for the better. They became smaller, had fewer surviving offspring and even showed behavioral changes. They foraged and ate less, for example, and seemed more fearful of predators.
The researchers were particularly concerned about the size of the eggs. Under the best of circumstances, less than one-10th of 1 percent of the egg larvae would be expected to produce fish that will reach maturity. That enormous failure rate could be even larger if the eggs are smaller, so survival of the population is further jeopardized.