Salmon hatcheries are one of the oldest examples of captive breeding, a strategy intended to shore up ailing species. But a study in the 14 March issue of Science suggests that the strategy may be backfiring. It results in smaller eggs and other genetic changes that could actually harm wild populations.
In hatcheries throughout the Pacific Northwest, salmon that were caught while returning from the ocean are bred. This scheme was originally intended to put more fish onto dinner plates, but it gradually took on the added goal of preserving species from the threat of dams and other river alterations. Scientists have long suspected that hatchery fish, when released, could harm the species by introducing traits that are beneficial in captivity but harmful in the wild. Similar worries surround other captive-breeding programs.
Conservation geneticist Daniel Heath of the University of Windsor in Canada and colleagues studied farmed Chinook salmon eggs in a British Columbia hatchery from 1996 to 2001. In the wild, large eggs are more likely to survive than small eggs. That's still true in the hatchery, but the pressure to produce jumbo eggs is not as great, the researchers discovered. So females that lay a larger number of smaller eggs end up with more surviving offspring. This explains the decline in the hatchery population's average egg size from 0.27 grams in 1988 to 0.20 grams in 2001, they say. The study also compared 20 years of data from four natural streams in British Columbia. Egg size declined only in the two streams that had been heavily supplemented with hatchery salmon.
"This is the first example that I know of that shows that evolution does happen in a captive population," Heath says. "It goes beyond salmon. This is something that is potentially important for any conservation program that includes captive rearing."
Fish ecologist Ian Fleming of Oregon State University's experiment station in Newport says he believes the hatchery population is evolving, but he is not convinced that released fish are what have changed the wild populations. Nevertheless, the study casts serious doubt on the value of hatcheries for conservation, he says: "It's been going on for more than a century in the U.S., and just now has there been a push to find out if supplementation has been beneficial."