Lost Balance
Unnatural selection: Fish growing up fast
Fishing for a change
If we humans are good at hunting, we excel at fishing. As we vacuum up stupendous numbers of fish from oceans, rivers and lakes, the nature of the ones that get away is changing at an astonishing rate. In particular, the targeting of big animals drives the evolution of smaller fish or ones that become sexually mature at a younger age, or both. Many fish populations are changing dramatically, with average size shrinking by 20 per cent and average life histories 25 per cent shorter (Proceedings of the National Academy of Sciences, vol 106, p 952). Harvested species show the most abrupt trait changes ever observed in wild populations, Michael Kinnison of the University of Maine and colleagues reported recently. Such changes have been documented in many places, including: pilchard off West Africa; American plaice off Newfoundland; Atlantic salmon in Canada and the UK; herring and grayling off Norway; chum salmon in Japan; sole, haddock and European plaice in the North Sea; whitefish in Alberta; sockeye salmon in Alaska; chum salmon, coho salmon and pink salmon in British Columbia; shellfish in California and cod just about everywhere. What's more, this long list includes only the cases where rigorous studies have been done. It's likely such changes are occurring in every population where large fish are targeted. So are these changes really the result of evolution, or merely a temporary response to environmental pressures rather than the result of genetic changes? "It is very likely a mixture of the two," says Kinnison. "Evolution is a substantial component." In several cases, for instance, researchers have shown once fish reach a certain size and age, they are more likely to become sexually mature than previous generations. This rules out most environmental explanations, such as that the fish are maturing at an earlier age because they are finding more food and growing faster. There is also no doubt about the plausibility of such rapid evolution. A decade-long study of Atlantic silversides kept in tanks has shown that intense targeting of large individuals can halve average size in just four generations. Actually tracing the genetic changes involved has not been feasible until recently, but it has now been done for Atlantic cod off Iceland. Cod living with one variant of the Pan I gene live in shallow inshore waters, while those with another variant live in deeper waters further offshore. Because fishing boats target cod in shallow coastal waters, the shallow-living variant is rapidly becoming less common. Some of the management measures are making matters worse. The deep-living cod do move to shallow coastal areas to spawn, but these areas are closed to fishing during spawning, so the shallow-living fish always bear the brunt of fishing. In the worst case scenario, shallow-living fish could disappear in 15 years. That could lead to the collapse of the fishery, because deep-living cod are much harder and thus costlier to catch.
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