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Preserving Salmon Biodiversity

The number of Pacific salmon has declined dramatically. But the loss of genetic diversity may be a bigger problem

Phillip Levin, Michael Schiewe

Each year, countless salmon migrate from the rivers and streams along the western coasts of Canada and the U.S. to the Pacific, while at the same time others leave the ocean and return to freshwater to spawn a new generation. This ritual has been going on for many millennia. But more than a century ago, the number of salmon returning from the sea began to fall dramatically in the Pacific Northwest. The decline accelerated in the 1970s, and by the 1990s the U.S. Endangered Species Act listed 26 kinds of salmon as endangered or threatened.

Were those worries overblown? Not at all. Pacific salmon have now been extirpated from nearly 40 percent of their historical habitat in the Pacific Northwest. Nearly half the remaining populations are at risk of extinction. The threats to salmon span almost every major freshwater ecosystem from the Los Angeles River to Canada. The crisis has made the unthinkable seem all too possible: a Pacific Northwest without salmon.

Unlike any other endangered species, salmon affect nearly everyone in the Pacific Northwest, either directly or indirectly. Indeed, this fish has long been the quintessential symbol of life there. Salmon provided the basis for the earliest cultures, economies and religions of indigenous peoples, and even after settlers arrived, treaties assured Native Americans access to salmon on their historic fishing grounds. In the past, activities related to fishing generated about one billion dollars in personal income annunally and provided as many as 60,000 jobs. The situation is now much more bleak, because millions of people—nearly every resident of the Pacific Northwest—compete with salmon for the waters in which the fish spawn and migrate: Dams, irrigation, mining, logging and cattle grazing all act to destroy salmon habitat.

With so many people affected, proposals for protecting salmon are highly contentious. All parties agree that conservation measures should be driven by science, but scientific opinion about how to help salmon recover is itself splintered. Although the root causes of the problem have long been well documented, investigators are just now beginning to understand that human activities have selectively eliminated some populations of salmon while favoring others, resulting in the loss of much of the genetic heritage in these amazing animals.

Curiously, such changes have received far less attention than has the dwindling number of fish. For example, in 1996 the National Research Council published a definitive 450-page treatise on the salmon crisis, and that tome discussed the genetic effects of human activities in fewer than five pages. So here we try to show the risks confronting salmon populations and salmon biodiversity with a focus on the potential for genetic loss. To appreciate the issues requires first a basic understanding of salmon biology.

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