Trout's Life Cycle


North America has a number of trout species, all of the family Salmonidae, including such exotic lines as the Apache trout of Arizona (Oncorhynchus apache) and the golden trout of California (Oncorhynchus aguabonita). The most common trout in the U.S. are rainbow trout (Oncorhychus mykiss), brook trout (Salvelinus fontinalis) and brown trout (Salmo trutta), a native of Europe that was introduced in the late 19th century. Though they differ in a few specifics -- brook trout spawn in the fall, rainbows in the spring, for example -- the life cycle of all the species is similar.

Born to Fin

  • The trout’s life begins as a fertilized egg. At spawning time, the female trout digs out a nest, or “redd,” in the bottom gravel of a stream; she lays her eggs, which are then fertilized by the male. The eggs hatch from February to March and emerge as alevin, each with a large yolk sac on its belly. The alevin remain near the nest until the yolk sacs are used up, when, as fry, they swim to shallow water, seeking protection from predators and feeding on zooplankton. After several weeks, dark markings appear along the sides of the fish, now known as parr. As they grow, the trout feed in the stronger currents. Trout from 3 to 9 inches long are known as fingerlings. When they reach sexual maturity -- the age varies by species and environment -- they will return to their natal stream, spawning a new generation.

Run to the Sea

  • Some trout, including rainbow, cutthroat and brown trout, will take to the sea as yearling fish. They spend their adult lives in the saltwater, and return like salmon after two or three years to their birth streams to spawn. Unlike salmon, however, some sea-run trout may live to return to the ocean and spawn another day.

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