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"In streams with high densities of salmon, the disturbance from spawning impacts virtually all aspects of stream ecology," he says. The female salmon wants to lay her eggs in a nice, gravel-bottom bowl that's free of fine sediments that can smother them. The largest female chinook (or king) salmon are more than a yard long and can weigh 45 pounds or more. The biggest nests are nearly a foot and a half deep and extend up to 17 square yards. Smaller species of salmon that spawn at higher densities are capable of even more widespread tilling, according to Moore. Using counts of spawning sockeye salmon for the last 50 years and previously measured nest sizes, Moore calculated that every summer the sockeye disturb at least 30 percent of the stream beds of two Alaskan streams he studied. And in years when salmon populations are high, sockeye dig up entire stream beds more than once, being forced to superimpose new nests on top of old nests when the females run out of room. The "rototilling" effect most happens wherever salmon are found in high densities, such as British Columbia, Alaska and some individual streams in the Pacific Northwest, according to Moore. Kennedy Creek, for example, is a small stream that flows into south Puget Sound and since 1968 has had chum salmon in high-enough densities that they have caused the amount of algae and stream insects to decline.

Scientists have long known of the habitat-changing activities such as dam building by beavers, but much of ecology has assumed that most other organisms simply react to the physical and chemical conditions around them. A handful of papers since 1999, including one by Moore in 2004, has focused on the disturbance created by spawning salmon and the ecosystem at large. In this most recent paper, Moore also outlines a conceptual framework that gives ecologists a way to formulate when the abundance of animals and their activities — be they spawning salmon or some other kind of animal — begin to significantly influence ecosystems. "One specific application to stream restoration in the Pacific Northwest, for example, deals with the practice of fertilizing salmon streams with the carcasses of dead salmon obtained from hatcheries," said Daniel Schindler, UW associate professor of aquatic and fishery sciences. "Although this does replace some of the nutrients that salmon returning from the sea would normally provide, it entirely ignores the fact that live salmon play a diverse suite of roles in streams, including helping to disperse those nutrients."

Source: University of Washington.

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