Rainbow trout reproduction begins with spawning. Usually this begins at 3-4 years of age and occurs in the spring. However, through selective breeding, some trout spawn in fall and winter.

Prior to spawning, the males will undergo radical anatomical changes, including the lengthening of the jaw and the devolpment of a kype, ( a hook), at the tip of the lower jaw. Also, their spots and stripe intensify in color.

Stream dwelling trout spawn upstream, while those in lakes migrate to tributary streams or spawn in shallow areas where gravel bars have plentiful oxygen and little silt. The female will dig several redds (nests) for depositing her eggs. She will turn on her side and using her tail, pound against the bottom moving away gravel to create a depression half as deep and a bit longer than her body. Often, more than one male will join here, with the largest male defending his territory by attaching smaller fish with his kype.

Once the redd is dug, the female is flanked by the males, where they simultaneously release milt and eggs. Occassionaly, other males will later deposit milt in the redd as well. The female trout will then cover the eggs with the gravel removed for building the redd. She will continously spawn until all the eggs are released, usually 400-3,000 eggs depending on the size of the female. Once all eggs are released, the male and female trout abandon the redd.

Eggs will incubate from 1 to 5 months depending on species of trout and temperature of the water. During this rather long incubation period, the eggs are subject to a variety of dangers like disease, drought, flooding, or predators such as crayfish, insect and other fish - including trout. Eventually the eggs hatch into Alevins (free-swimming embryos with an attached yolk sac). The Alevins will feed on the yolk sac until it is absorb, and until their fins develop, then they will swim to the surface of the stream or lake and feed.

A trout's early life is perilous. Predators such as Kingfishers, herons and other fish do the greatest damage. Fewer than one percent of newly hatched trout survive.