Salmon Fish

Many species of salmon are anadromous—they spawn, or lay their eggs, in fresh water; the young migrate to salt water and grow up there; and the fish return to fresh water to breed after they reach maturity. Other populations or species of salmon are landlocked, spending their entire life cycle in fresh water. The migratory instinct of members of the salmon family is remarkably specific, each generation returning to spawn in exactly the same breeding places as the generation before it. Some salmon migrate hundreds or even thousands of miles to reach their spawning grounds. Even those species that do not migrate from fresh water to salt water spawn in the same freshwater streams as did their ancestors.

Although usually drab in color before the breeding season, which varies with the species, members of the salmon family develop bright hues at spawning time. During this season the male usually develops a hooked snout and a humped back. Salmon typically spawn in rapidly flowing, clear streams with gravel and rocks in the bottom. Before mating, one parent excavates a nest, or redd, for the eggs. The female deposits eggs in the nest and the male releases sperm, or milt, over the eggs to fertilize them. The female then stirs up the stream bottom so that earth and stones cover the eggs and protect them. During the migrations and nest-building activity that precede mating, neither the females nor the males consume food.

The eggs hatch in two weeks to six months, depending on the species and the water temperature. The newly hatched young, called alevins, remain buried in the nest, living on nutrients absorbed from a yolk sac attached to the abdomen. When all the yolk has been absorbed, the young salmon, then called fry or fingerlings, emerge from the gravel to seek food. Their diet consists of microscopic plants and small animals, such as insects.