Environmental Problems, Their Causes, and Sustainability 1

water withdrawal for irrigation. An example of suchdisruption is the Columbia River—North America’sfourth largest river (Figure 13-13).This basin has the world’s largest hydroelectricpower system. It has 119 dams, 19 of which are majorgenerators of inexpensive hydroelectric power. It alsosupplies municipal and industrial water for severalmajor urban areas and is a source of water for irrigatinglarge areas of agricultural land.Salmon are migratory fish that spawn in the upperreaches of streams and rivers. Their offspring migratedownstream to the ocean, where they spend most oftheir adult lives. The adults complete their life cycle byreturning to their place of birth to spawn and die (Figure13-14, left). A series of dams and extensive forestclearing of land adjacent to stream banks can severelydisrupt the salmon life cycle because salmon need freeflowing rivers to return, spawn, and lay eggs at thesites where they were hatched. This requires not cuttingnearby forests that can cloud salmon spawningsites with silt and cover spawned eggs. In other words,salmon need nearby intact forests.In 2001 research by ecologist Thomas Reimichenindicated that salmon can improve the health of thenearby forests. This occurs because bears feeding onlarge amounts of the salmon strew half-eaten salmonFish change formHuman captureFish enter riversand head forspawning areasSalmonprocessingplantTo hatcheryIn the fall spawning salmondeposit eggs in gravel nests and dieGrow to maturityin Pacific Oceanin 1–2 yearsNormalLifeCycleFry hatch in the spring . . .ModifiedLifeCycleEggs are taken from adultfemales and fertilized withsperm “milked” from malesGrow to smoltand enter the ocean . . .And grow in the streamfor 1–2 yearsEggs and young arecared for in the hatcheryFingerlings migrate downstream . . .Fingerlings arereleased into riverFigure 13-14 Normal life cycle of wild salmon (left) and human-modified life cycle of hatchery-raised salmon(right). Salmon spend part of their lives in fresh water and part in salt water.http://biology.brookscole.com/miller14269