Trout Life Cycle
Two to four weeks before spawning, male trout undergo physical changes that include developing a kype, a hooked lower jaw with strong teeth, and exhibiting vibrant colors. As mating time approaches, the females will begin searching for a site in the shade with clear water, gravel and a good water flow to build her redd, or nest. When a female finds a suitable site, she scoops out a little depression using her fins. The female works hard to create her nest while males attempt to impress her with mating dances. When the female is ready, the male swims up alongside her. The female releases the eggs and the male releases his milt and the eggs will be fertilized. The female covers the eggs with gravel and moves on to another site to lay more eggs with two or three different males at different sites. A 13-inch female trout may produce about 1,000 eggs over the course of a couple of days.
The fertilized eggs remain in darkness and undergo a process in the first hour of being laid called water hardening, where the pores in the eggs seal. The sticky eggs are now in the green egg stage and will remain so for approximately 20 days after spawning. Green eggs are very fragile and can be easily damaged during this stage. The eggs begin to change and become clear and pinkish in color and a set of eyes becomes recognizable. They look like two dark dots. This is called the eyed egg stage. The trout remain in the eyed egg stage for approximately 2-3 weeks. Alevins then appear! This form of the trout is not yet free-living, but feeds from a yolk sac attached to their bellies. As the alevins grow and develop, they remain in the redd until all the yolk sac is absorbed. Eggs and alevins are both very vulnerable to environmental disturbances at this stage. When the alevins no longer have yolk sacs, they become fry and float up to the surface actively looking for food. It may take up to 2-3 weeks for alevin to develop into fry.
Trout broodstock or parent fish live at the hatchery. Female trout produce eggs that are manually stripped from the fish in October or November to produce the fry.
The eggs are fertilized with milt (sperm) from male trout.
The eggs are then placed in special incubators where they receive a constant supply of oxygen rich water. At this early stage, the eggs are very tender and cannot be handled. Total incubation time from spawning to hatch is normally 4 to 6 weeks.
Halfway through incubation, eyes of the developing embryo can be seen through the transparent egg shell. This is called the “eyed” stage. At this time, eggs can be handled and transported without harm to the fish.