Fish are separated into males and females. A number of fishes, however, are born males, but change into females; or are born females and change into males.
Many fish, including trout and salmon, are oviparous. This means they lay eggs. Sharks and rays have internal fertilization and lay fertilized eggs. Most fish, including trout and salmon, lay unfertilized eggs. The eggs are fertilized externally by the male. Fish typically release hundred or even millions of eggs, which increases the chance that a few offspring will survive into adulthood.
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. The male, during this season, usually develops a hooked snout and a humped back. Before mating, one parent excavates a nest for the eggs; after the eggs are deposited and fertilized, the female stirs up the stream bottom so that earth and stones cover the eggs and protect them. The eggs hatch in two weeks to six months, depending on the species and the water temperature. During the migrations and nest-building activity proceeding mating, neither the females nor the males consume food.
The Atlantic salmon migrates to cold, fresh water in late spring or early summer, swimming upstream at an average rate of up to 6.4 km (4 mi.) per day. Because salmon can jump as much as 3.7 m (12 ft) out of water, they clear most obstacles in their path. The female lays as many as 20,000 eggs in October or November, after which time the adult salmon float downstream and return to the sea. Unlike the various species of Pacific salmon, the Atlantic salmon does not die after its first spawning but returns year after year to its breeding place. The newly hatched young, known as parrs or brandlings because of the dark transverse markings on their sides, remain in fresh water for about two years. At this time, the young, known as smolts, become silvery in color and descend to the sea. Upon the first return of the Atlantic salmon to its spawning ground, the fish is known as a grilse. After spawning, it is known as a kelt.
Salmon found in the North Pacific Ocean spawn only once, dying after depositing and fertilizing their eggs. The Chinook salmon migrates farther than any other salmon, often traveling 1600 to 3200 km (1000 to 2000 mi.) inland to its spawning ground. Its eggs usually hatch within two months, and the young descend to the sea when 5 to 7.5 cm (2 to 3 in) long.