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
Loss of Habitat
Another large-scale threat to salmon has been the destruction of their freshwater environment. While there is a voluminous literature on the specific habitat requirements of salmon, the general needs of these fish are really quite simple: They must have plenty of gravel in which to spawn; they need enough clean, cool water to swim, escape predators and find food; and they require sufficient vegetation along the river banks to protect the stream bed from excessive erosion or sedimentation, to add nutrients to the water and to provide woody debris as shelter from strong currents.
Over the past 150 years, mining, livestock grazing, timber harvesting, agriculture as well as recreational and urban development have eliminated or substantially disturbed salmon habitat. The numerical effect is obvious—there are fewer salmon in degraded regions than in pristine ones. As with fishing, however, habitat loss and destruction have the potential to reduce genetic diversity. The most obvious mechanism is the extinction of entire salmon populations. Indeed, most analysts believe that environmental degradation underlies the demise of many of the 106 salmon populations now considered extinct. Although some rivers have been subsequently recolonized, the unique genes of the original populations have been lost.
Large-scale disturbances in one locale also have the potential to alter the genetic structure of populations in neighboring areas, even if their habitats are pristine. Why? Although the homing instinct of salmon to their natal stream is strong, a fraction of the fish returning from the sea (rarely more than 15 percent) stray and spawn in nearby streams. Low levels of straying are crucial: The process provides a source of novel genes and ensures that a location can be repopulated should the fish there disappear. Yet high rates of straying can be problematic, because the misdirected fish may interbreed with the existing stock, diluting any local adaptations that may be present.
When environmental conditions are stable, straying rates are relatively low. But they can increase dramatically when streams suffer a severe disturbance. The 1980 eruption of Mount St. Helens, for example, damaged more than 500 square kilometers of forest and sent mud and other debris into several major tributaries of the Columbia River. For the next couple of years, steelhead returning from the sea to spawn were forced to find alternative streams. As a consequence, their rates of straying rose from 16 percent to more than 40 percent.
Although no one has attempted to quantify changes in the rate of straying as a result of the disturbances people have caused, there is no reason to suspect that the effect would be qualitatively different than what was seen in the aftermath of the Mount St. Helens eruption. Such a dramatic increase in straying from damaged areas to more pristine streams results in substantial gene flow, which can in turn lower the overall fitness of subsequent generations.
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