Point Of No Return

Point of No Return

EVIDENCE IS MOUNTING that fish populations won’t necessarily recover even if overfishing stops. Fishing may be such a powerful evolutionary force that we are running up a Darwinian debt for future generations.


By Natasha Loder
July-September 2005 (Vol. 6, No. 3)

Contrary to the opinion of a number of the citizens of Kansas, evolution is not merely a “nice theory” but rather a demonstrable fact. Evolution is all around us. In our hospitals, bacteria have become so difficult to control that fatal postoperative infections are now common. Insects in agricultural fields develop resistance to pesticides. And those unfortunate enough to have been infected with HIV know that their bodies have become evolutionary battlefields—with HIV constantly mutating and evolving resistance to antiviral drugs.

Against this background several years ago, Stanford biologist Stephen Palumbi issued a warning in the journal Science (1) that humans are dramatically accelerating evolutionary change in other species and that this is costing at least US$33-50 billion a year in the United States. Most of this cost stems from the hospitalization of people whose diseases have become resistant to treatment, but large costs also emerge when pests or disease organisms escape chemical control.

Overlooking the consequences of evolution can be costly for conservation as well. In recent years, evidence has been accumulating that evolution is having an impact on the productivity of commercial fisheries. Although research is only starting to quantify the extent of the problem, the prospect of evolutionary effects in fisheries is to be expected. Consider, for example, a farmer who, from year to year, grew seeds from only the smallest, weakest plants in the field. He would hardly be surprised to find his crops growing successively smaller and feebler as the years went on. Good farmers grow seeds from the largest or most productive plants and thus maximize their yields. Oddly, this is almost the reverse of what happens in commercial fisheries: every year, nets scoop up the largest fish and leave the smallest behind.

In February this year, fisheries scientists met in Washington, D.C., at the annual meeting of the American Association for the Advancement of Science (AAAS) to discuss the emergence of “evolutionary fisheries science”—and to lament the fact that, until now, the evolutionary dimensions of fisheries have been overlooked. They also gathered together some of the best evidence available on the subject. Their models suggest that each year in which current levels of exploitation continue will require several years of evolutionary recovery. This results in a “Darwinian debt” to be paid by future generations.


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Fisheries science has managed to embrace some evolutionary ideas. There is certainly an appreciation of the need for conservation of unique gene pools and of genetic diversity within populations. Nevertheless, David Conover, a professor at the Marine Sciences Research Center at Stony Brook University in New York warns that fishery managers treat variation in size as having “no genetic basis or evolutionary consequences at all.” This is odd because the signs of size change in some important fish stocks are already apparent. For example, in the 1940s, cod in the northeast Arctic had an average size of 95 cm. Today they average only 65 cm. And average size and age of fish at maturation have been decreasing for decades in many commercially exploited fish stocks.

Until recently, it could have been argued that the lack of interest in the possibility of evolutionary change in commercial fisheries arose from the difficulty in proving that it was happening. Size changes in fish might equally be due to environmental effects on fisheries—after all, when fish are being harvested, the community they are part of is also changing. Food availability, temperature, and population density do not remain the same. According to Conover, many fisheries scientists predict that, as fish density declines due to harvesting, fish will grow faster because there is less competition for food. On the other hand, commercial fisheries are subject to strong size-selective mortality, and it would be highly surprising if this were not reflected in their genetics. “One has to take the position that, despite all other organisms in the world being subject to the rules of natural selection and evolution, somehow fisheries are not,” says Conover.

So Conover and Stony Brook colleague Stephan Munch began asking some fundamental questions. They wondered why, despite mounting evidence of rapid life history evolution in fish, current models and management plans for sustainable fishing ignore the Darwinian consequences of selective harvesting. This might be due in part to a lack of proof that size-selective mortality actually causes genetic changes.

They realized the key to proving this connection was to separate genetic from environmental changes. If fish were smaller merely because of current environmental conditions, they would bounce back quite quickly from these smaller sizes after conditions were changed. However, if size-selective mortality were exerting genetic pressure, this could be far harder to reverse and thus have long-term implications for fisheries. What’s more, some of the affected traits, such as growth and sexual maturation, are closely connected to productivity.

So the pair set up an experiment. They subjected captive populations of Atlantic silversides (Menidia menidia) to variation in size-selective harvesting. The Atlantic silverside is a long and slender fish with two dorsal fins and a rounded white belly and is common along the eastern coast of North America. Conover and Munch designed a series of selection experiments in which 90 percent of the silversides were removed. In one experiment the largest fish were taken, in another the smallest, and in a third experiment fish were culled randomly.

The results, published in the journal Science in 2002 (2), turn conventional fisheries management thinking on its head. In most commercial fisheries, fish are removed on the basis of size. There are minimum, not maximum, size limits. But Conover and Munch’s results show that this approach may have results that are exactly the opposite of what is intended. Within only four generations, taking out larger fish produced a smaller and less fertile population that also converted food into flesh less efficiently. In contrast, catching smaller fish increased the average size of fourth-generation fish to nearly double that of fish in populations where only the larger fish had been taken. In the short term, catching the largest fish gave the highest yield and mean weight of fish. But this effect was temporary. After four generations, removing the smaller individuals gave nearly twice the yield of the populations from which larger individuals had been harvested. One reason for this greater productivity was that the larger adults had a greater reproductive potential. Another was that, as expected, removing small individuals selected for fast growth.

Additional evidence is accumulating—from modeling, lab experiments, and studies of wild populations—that the heavy exploitation of fish stocks is indeed causing them to undergo genetic change. Richard Law, an evolutionary biologist at the University of York in the U.K., believes that the effect of evolution on yield has the potential to be quite large (3). Some simple calculations, he says, on the life history of the Northeast Arctic cod suggest that selection due to fishing mortality could halve the yield of the fishery, with the exact reduction in yield depending on the levels of fishing mortality applied on the spawning and feeding grounds. These results should certainly give commercial fisheries managers something to think about.


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