Mixed Schools a Must for Fish?

A matter of scale. Selective catches can cause rapid genetic shifts.

Fish markets teem with neatly iced schools of similarly sized fish. The marked uniformity is often the result of two forces: customer demand for pan-sized portions and fishing regulations that limit harvests to older fish to preserve populations. But a new study suggests selective culls could permanently alter the genetic makeup of wild fish stocks.

Scientists have already suggested that some fish populations are evolving rapidly in response to heavy fishing. Several cod and salmon stocks, for instance, appear to have shifted to smaller, earlier maturing fish as fishers systematically removed larger and older specimens. But wild populations can be difficult to study, so fishing's genetic impacts have remained in dispute.

To get a clearer view, ecologist David Conover of the State University of New York, Stony Brook, and graduate student Stephan Munch moved to the lab. Four years ago, using eggs collected from wild stocks, they hatched six captive populations of Atlantic silversides. Then they went fishing. From each school of 1000, they removed 90%. In two of the tanks, they took the largest fish; in two others, the smallest; and in the remaining control tanks, the harvest was random. After allowing each school to rebound to its original size, they repeated the process for four generations, charting how the size, weight, and growth rates of the populations changed over time.

The results were dramatic. Population characteristics in the random-catch tanks, as expected, stayed relatively even. But the balance disappeared with other methods. Taking the bigger fish produced a catch that was initially heavier than in controls. By the fourth generation, however, the catch was substantially smaller than in controls. In contrast, taking the smaller fish eventually produced a heavier population, the pair reports in the 5 July issue of Science. Conover and Munch argue that similar forces are at work in the wild, and they recommend establishing more reserves that are off-limits to anglers and regulations that protect larger fish as well as smaller ones.

Several fisheries scientists, including Felicia Coleman of Florida State University in Tallahassee, say those ideas are on target. But others, including Ray Hilborn of the University of Washington, Seattle, say the experiment is far too limited to support major management changes. “All they have done is show that growth rates are heritable; what they haven't done is see what the impact of this would be on a realistic fishery,” says Hilborn.

Related sites
Conover Fish Ecology Laboratory
Atlantic silversides

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