POSTED 5 JULY 2002
Courtesy David Conover
The strategy of protecting the young from the net and hook is widely used to protect slow-growing fish like bass, salmon, flounder and trout in fresh and salt water alike (small fish like sardines are regulated differently).
Now comes a study indicating that the tactic may backfire. Instead of ensuring a large, healthy catch, harvesting the large fish may cause genetic change favoring slow growth.
The research was done by David Conover, professor of marine science at the State University of New York at Stony Brook, and his graduate student, Stephan Munch. It is published in the current issue of Science.
The researchers grew a bunch of Atlantic silversides, a fast-growing little fish that seldom ends up on a dinner plate. Once a year, the researchers removed 90 percent of the individual fish and allowed the surviving 10 percent to reproduce.
The culling tactic was different in each of three tanks:
Among the first generation of offspring, the large-harvest technique did result in a greater harvest of fish in terms of mass and average size. But things quickly began to fall apart. By the fourth generation, only half as much biomass was being taken from the large-harvest tank as the small-harvest tank. The average large-harvest fish weighed only half as much.
Getting it bass-ackwards...
Because the fish lived in identical tanks, Conover is confident in attributing the size changes to genetic change. "Given that they are in a common environment," he says, "the difference in phenotype [physical form] is almost certainly caused by a difference in genotype."
The situation reminds us of artificial evolution -- the changes in farm animals that result when farmers breed the best animals. Artificial selection was a foundation for Charles Darwin as he developed his ideas about evolution through natural selection.
Just a fish story?
Although the extreme tactics do not reflect reality in the ocean, Conover explains that they do reflect financial and academic realities: "We chose the experimental design to make sure we could get effects that were measurable in the lifetime of the grant."
While he's provided proof of the principle that taking the largest fish can cause genetic change, in the ocean, he concedes "it's more complicated."
Spare the rod and spoil the fish?
A variation of that tactic has been used in certain areas, says Jack Mattice, director of the New York Sea Grant program, which funded Conover and Munch's research.
The new "slot limits" protect fish of reproductive length. "There is a slot in the middle where you must put the fish back," Mattice says. Fish unfortunate enough to be longer or shorter are eligible for marinating.
A second suggestion, protected marine reserves, is intended to protect ecosystems and fish biodiversity rather than commercial fisheries. But if they can sustain more of the gene pool, reserves might slow the genetic effects of removing larger fish. President Bill Clinton set up a large reserve in the western Hawaiian Islands, and California is in the process of setting up marine reserves.
However, Mattice points out that because the primary goal is saving fish habitat, it's unclear whether marine reserves will influence the genetics of fish outside their borders.
One line of prediction is safe: Before the scales of fishery management can be tipped in a new direction, the arguments will get tangled. Scientifically speaking, researchers may have some alluring arguments, but the net result is their notions may sink unless they hook commercial and recreational fishers.
-- David Tenenbaum