This, too, covers the redds. Pollution of the rivers also occurs because of the agricultural use of chemicallybased fertilizer, herbicides and pesticides, which leach into the aquifer and thence into the rivers. These pollutants kill the salmon. Hatchery fish management has introduced diseases to wild fish and often successfully competes with them. In 1986 some 2500 salmon leaped from the waters and died on the banks of the Duwamish River.
Biologists believe toxic chemicals in the water killed the fish.
Over fishing also plays a very important part in salmon depletion. In the Atlantic waters whole species have already been wiped out. Dredging and hydraulic mining blasts enormous quantities of mud and gravel into the rivers. This is fatal to the redds. Many dams should be removed and a few have been, but political reasons have frequently been a barrier.