In the wild, rainbow trout live between four and six years, and can live up to 11 years. They usually grow between 20 to 30 inches in length (51 to 76 cm), and weigh up to eight pounds; however, some can grow up to four feet in length (1.2 m) and weigh up to 53 pounds. Lake and ocean-dwelling rainbow trout, known as steelhead trout, grow larger than freshwater stream stocks. Their coloring depends on habitat, age, and spawning condition; they are usually blue-green to yellow-green, with pink stripes along their side, and small black spots on their back and fins, while steelheads are named for their steel-grey coloring on their head. Also called redband trout, rainbow trout are a member of the salmon family.
Spawning season is usually between January and June in the north, and September to November in the south, happening between three and fours years old. Steelhead are anadromous and return to their original hatching grounds in freshwater streams to spawn. Females build nests, called redds, in the gravel river bottom. They produce between 2000 and 3000 eggs per kilogram of weight. Eggs fall into the redds, where the males fertilize them. They hatch within seven weeks, and in two weeks the yolk sac is completely consumed and the fry begin feeding on zooplankton. As fry grow, they develop parr marks – dark vertical bars - on their sides. These juveniles are called fingerlings because they are roughly the size of a human finger. Juveniles remain in the river for up to three years before undergoing physiological changes, known as smoltification, which allow them to survive in seawater. Offspring of steelhead are not always anadromous, just as offspring of rainbow trout do not always remain in freshwater rivers. Populations can spawn multiple times in a lifetime.
Rainbow trout feed on aquatic and terrestrial insects, as well as fish and fish eggs and small crustaceans. Spawning salmon attract rainbow trout, which eat their eggs. Steelheads feed primarily on other fish, squid, and amphipods. Predators include lampreys, fish, birds, bears, river otters, seals, sea lions, and orcas.