Shrinking at Sea: Harvesting drives evolution toward smaller fishes

Fish are becoming smaller and growing more slowly in response to pressures introduced by fishing, scientists say. That shift, which new data suggest is hard to undo, creates populations of fish that are poor at reproducing and inefficient at bulking up.

Commercial fishing is generating a “Darwinian debt,” in the form of less valuable fishes, that could take generations to pay off, says Ulf Dieckmann of the International Institute for Applied Systems Analysis in Laxenburg, Austria.

People have long noted a downward trend in the average size of fish of many species. That’s not surprising. Small fish are more likely than large ones to slither from nets. Also, laws intended to protect juveniles often require that fish smaller than a certain size be returned to the sea.

Fast-growing fish therefore get “penalized” evolutionarily because they quickly become large enough to get caught, says David O. Conover of the State University of New York at Stony Brook. In recent years, he and other marine scientists have shown that fishing can induce inherited changes that cut the average growth rate, maturing time, and size in fish from generation to generation.

In six aquarium tanks, Conover grew successive generations of Atlantic silversides. To mimic commercial fishing in two of the tanks, he periodically removed the largest fish. With each generation, the fish in those tanks grew more slowly.

Conover observed an opposite growth trend in tanks from which he took the smallest fish, and he saw no change in growth rates in tanks subjected to random harvesting.

Small, slow-growing fish burn most of the calories they consume and generate little edible tissue. Relative to larger fish, they also produce smaller, more vulnerable eggs and less-hardy offspring, Conover found.

In his newest work, Conover simulated a halt in fishing to see whether wild stocks might recover their former growth patterns. Preliminary data from two lab-grown generations suggest that such recovery is slower than the initial fishing-induced changes, he says.

This implies that fish populations that have historically been overharvested could face lingering effects, even after fisheries scale back their harvests, says Mikko Heino of the Institute of Marine Research in Bergen, Norway.

In separate research, Dieckmann and Heino compared data collected as far back as 1932 and recent statistics on size and growth in wild stocks of Atlantic cod, herring, and other fish. All the examined species show a shift toward earlier maturation and smaller adult size, the researchers report.

Conover suggests one tactic to counteract such trends: Require that fleets throw back the largest fish as well as the smallest ones, thereby preserving fast-growing fish in any population.

Both studies were presented on Feb. 18 at the annual meeting of the American Association for the Advancement of Science in Washington, D.C.