Some salmon species swim hundreds or even thousands of miles to return home. Only a tiny fraction of adult fish ever reach their spawning grounds. Since salmon don't feed once they leave the ocean, many die from lack of stored body fat. Many others are caught in fishing nets or by a variety of predators. Others choke in the polluted waters near cities.
Some salmon eventually do make it back to their home rivers, only to be blocked by huge dams.
Consider that each female adult lays between 1,500 and 10,000 eggs. The current battle in the Pacific Northwest to save salmon in the Columbia River is looming as one of the most important in the nation. The river was once teeming with salmon. But in the nineteenth and early twentieth centuries, overfishing and habitat destruction from farming, grazing, mining, logging, and construction took their toll. Then, the Bonneville Dam was built in 1938, and the Grand Coulee Dam followed in 1941.