FEATURE ARTICLE
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
Unique Responses
Because salmon typically return to reproduce in the river where they were spawned, individual streams are home to local breeding populations that can have a unique genetic signature. Chinook in the coastal streams of Washington, for example, are genetically distinct from those of Oregon. And even within the Columbia River basin, clear genetic differences are evident. Stream- and ocean-type chinook within the Columbia watershed can be differentiated, as can stream-type populations from the upper and lower reaches of the river. Yet it would be a mistake to believe that genes control all the differences between these fish.
Take, for example, the age of maturation, which the two of us have studied recently. The chinook that reproduce at high elevations tend to spawn at around five years of age, when their bodies are large; the chinook that use lower spawning grounds reproduce earlier in life, at around age four. Although this difference might well be genetically programmed, the state of the ocean also influences it: If food is plentiful when these fish first enter the sea, they tend to return sooner than they would otherwise. Such findings clearly demonstrate that salmon react in complex ways to natural variations in the environment. It should come as no surprise, then, that salmon populations respond differently to human-
induced changes to their environment.
First, consider fishing. Simply put, fishing has harmed some salmon stocks more than others. In the Columbia River Basin, for example, harvest rates for different populations of chinook varied by as much as 10-fold through the 1980s. The annual harvest rate for ocean-type chinook from the lower Columbia River, from the Snake River (fall run) and from the upper Willamette River was often more than 50 percent. By comparison, stream-type chinook from the upper Columbia (spring run) and from the Snake River (both spring and summer runs) experienced annual harvest rates below 10 percent.
Why do these disparities exist? For one, ESUs migrate at different times, so depending on the timing of the river harvest, populations may be exploited to different degrees. Additionally, ocean-type chinook tend to remain near the coast, where they are more accessible to fishers, while stream-type chinook migrate far offshore, essentially out of reach. Consequently, reduction of fishing will help some ESUs but have little impact on others. According to a study by Michelle McClure and colleagues of the National Marine Fisheries Cumulative Risk Initiative, if fishing stopped completely, the annual population growth of some ocean-type stocks of chinook in the Columbia River would increase by at least 20 percent—enough to reverse their current decline toward extinction. But such a harvest moratorium would have virtually no impact on stream-type chinook.
The effects of fishing on salmon, however, go far beyond simply catching more fish of one type than another. A 1981 examination of weight changes in pink salmon is particularly dramatic. William Ricker, of Canada's Department of Fisheries and Oceans, documented a decline of more than 30 percent in the average body weight of spawning pink salmon from the 1950s onward. This long slide appears to have originated in the late 1940s, when fishers shifted from selling their goods by the piece to selling them by weight, a change that prompted the use of nets that capture only the largest fish.
As it turns out, the new fishing practices exerted a selective pressure on these creatures and altered their genetic makeup. Because all pink salmon return to spawn at the same age (after their second year), the largest are those that have the fastest growth rate—a trait that is at least partially determined by genetics. Size selection by fishers gave the smaller ones a higher probability of survival, thereby favoring genes for slower growth.
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