Is Overfishing Changing the Gene Pool?

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Over time, the big fish are withdrawn from the gene pool, and year after year the fish get a little smaller.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.

The findings challenge the long-held belief that if a few million fish are left, even after overfishing, that will be enough for a species to replenish itself. In a separate study three years ago, researchers at the University of Washington were surprised when they studied a population of New Zealand snapper that had been fished down to about 3 million.

That should have allowed for recovery, but the researchers found that only one in 10,000 fish was capable of breeding, so the genetic diversity of the population depended on as little as a few hundred fish.

Reduced diversity leaves the species more susceptible to toxins, and less likely to survive much less rebound.

So it would seem that longer periods of reduced or banned fishing would be required for overfished stocks to be rehabilitated.

But the Stony Brook researchers themselves point out that what happens in the lab is not directly parallel to what happens in the real world. Most commercial fish species live much longer than a year, and have overlapping generations, so "similar responses in wild fisheries are unlikely to be as rapid," they report.

Still, there is that anecdotal evidence that is hard to ignore. My home is in the heart of Alaska's king salmon runs, and each fall the cultural event of the year takes place. The Great Northern Salmon Derby pits fisher against fisher, friend against friend, husband against wife, in the annual effort to land the biggest king salmon.

Just a few years ago, that required a fish that would weigh in at between 40 and 50 pounds, and sometimes even more.

In recent years, however, the winner has weighed a relatively puny 30 to 35 pounds.

Of course, many factors may figure in to that. But ask any fisher around here and he, or she, will tell you the fish are smaller now.

And if the findings at Stony Brook apply to the real world, that may be because the big fish are harvested before they reach the spawning grounds, and thus their genes are lost. So it may be many, many generations before the big kings return, if indeed they ever do.