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
Damage from Dams
The extensive development of hydropower on the major rivers of the western U.S. has clearly disrupted populations of salmon. The Columbia River basin alone now contains thousands of dams (some only a meter or two in height and others more than 100 meters tall), which block off over a third of the original salmon habitat.
The damaging effects of these dams vary widely: In the Columbia basin, for instance, 10 percent of the dams over 15 meters tall have fish ladders, which assist adult salmon migrating upstream to pass up and over them. Young salmon traveling downstream have also been helped across dams with bypass systems that divert the small fish away from potentially deadly turbines. In some places, barges transport juvenile salmon downriver, keeping them out of harm's way. Yet the threat from dams is not limited to reducing the numbers of migrating fish. Again, a significant threat is to salmon biodiversity.
Consider, for example, what could happen to a few salmonid species, such as sockeye salmon and steelhead trout, that have populations that never move out to sea. Dams might exert a large selective force in favor of those fish that do not migrate. For both sockeye and steelhead in the Columbia Basin, this potential may become reality. Above several dams there, the status of anadromous steelhead and sockeye is dire. In contrast, the resident steelhead and kokanee (a form of sockeye) remain relatively healthy. Should this trend persist, sedentary stocks may well come to dominate streams where anadromous forms once were prevalent.
Other genetic modifications might come from the very engineering fixes made to protect these fish from harm. Dams on the Columbia and Snake Rivers are equipped with submersible screens designed to divert migrating juveniles away from turbines. Unfortunately, these measures do not benefit all fish. These screens steer as many as 95 percent of the stream-type chinook around turbines, but because of idiosyncrasies in behavior, these measures redirect as few as 15 percent of ocean-type chinook. One thus expects to see genetic shifts in favor of the stream types.
Fish ladders, too, have drawbacks. Although these devices have helped to bring survival rates for mature fish closer to historic levels, dams have certainly altered their upstream journey. Rather than swimming against a flowing river, adults now pass through a series of reservoirs punctuated by dams, where discharge from the turbine can disorient the fish and make it hard for them to find the ladders. Such impediments do not kill the fish, but they affect migration rates and, potentially, spawning. Salmon do not feed as they swim upstream to spawn but instead rely on stored energy. The changes to their migration route clearly have the potential to impose an artificial selective force.
Dams may also modify salmon habitat in more subtle ways. An indirect effect of the 92-meter Brownlee Dam on the Snake River provides a dramatic example. Historically, the upper Snake River produced some 25,000 to 30,000 chinook salmon that spawned during the early fall. The completion of the dam in the late 1950s not only rendered the vast majority of their habitat inaccessible, but also led to more extreme water temperatures downstream from the dam. These changes, in turn, altered the life cycle of the small population of Snake River chinook that remained. Today, young chinook emerge from the gravel later than they did before the dam was built, and thus they migrate downstream later, when temperatures are higher and water levels are lower.
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