Captive Breeding May Threaten Wild Salmon

Captive salmon breeding programs, which rear the offspring of wild salmon in hatcheries and then release them into rivers, may be harming wild populations by reducing the salmon's egg size, scientists report, in the 14 March issue of Science.

In a relatively short period of time, natural selection can drive the evolution of traits that are beneficial in a captive environment but harmful in the wild, according to Daniel D. Heath and colleagues. Scientists have determined that a trade-off between egg size and egg number generally exists for egg-laying animals. That is, larger offspring have a better chance of survival than smaller offspring, but females can lay more eggs overall when offspring are small.

The authors analyzed a population of chinook salmon, reared in captivity, and found that egg size decreased over four generations. This trend was presumably because, with juvenile mortality intentionally minimized, selection for large eggs decreased as well. Heath and his colleagues also examined average egg size over 20 years in four river populations of salmon whose numbers had been supplemented by captive breeding. They found significant egg size declines in the two populations that were supplemented more heavily. They propose therefore, that the effects of captive breeding on egg size might be minimized through modified breeding practices.

—Kathy Wren

18 March 2003