Brook Trout Life Cycle

Breeding success of male brook trout (Salvelinus fontinalis)in the wild

During the summer months adult Brook Trout are usually found in larger streams and lakes. In late summer and early fall as the length of the day begins to shorten the Brook Trout begin to migrate out of the lakes and up the larger streams into the smaller headwater steams. Here they seek out a spawning location in gravel beds in streams with cold, well-oxygenated water. Brook Trout seem to seek out areas where upwelling springs occur. The streams are usually fairly shallow, but not so shallow that the bed will be damaged by ice during the winter. Some Brook Trout may remain in lakes and spawn in gravel beds over upwelling springs. Females are capable of detecting upwelling springs or other gravel areas with ground-water flow and often deposit their eggs in these habitats.

Spawning usually begins in October and continues into the fall.  In spawning the female Brook Trout prepares a depression in the gravel of a stream using her tail fin.   This depression is called the redd. Hovering over the redd the female releases 15-60 pea sized eggs into the redd. As the eggs settle into the redd the male, who hovers nearby, releases a cloud of sperm, or milt, and the eggs are fertilized.  Individual males may display some territoriality with aggression increasing when joined by a female. The female then uses her tail fin to sweep the gravel back over the eggs and then moves on to construct another redd. The total number of eggs a female lays depends on her size, larger females laying more eggs.  The number of eggs produced by a female varies with her size.  A 4-inch fish produces less than 100 eggs; a 14 or 15-inch fish produces over 2,000 eggs, while a 22-inch fish will produce over 5,000 eggs.  One male and female perform actual spawning; however, each may spawn with different mates during the reproductive period. 

Several different accounts of the spawning act have been recorded for brook trout.  One account stated that the female turned on its side, flipped its tail several times and the eggs shot out.  Another reported that the female took a position on the bottom of the nest, with pectoral and pelvic fins spread against the stones, and at her side the male arched his body to hold the female against the bottom and both vibrating intensely as eggs and milt were discharged.  There are several extrusions followed by a resting period.  The eggs are adhesive for a short period after extrusion which allows those not lodged in gravel from being washed away.   After spawning is completed the female covers the eggs with gravel in a similar manner as when she constructed the redd. 

The length of the incubation period varies according to the temperature - at lower temperatures the incubation period is longer. For example, in 35 Fº water, the incubation period is about 144 days; in 50 Fº water, the incubation is only 44 days. The effect of temperature on the length of the incubation period is adaptive. No matter when during the fall/winter the eggs are laid, it allows the eggs to hatch in March and April when food for the larvae becomes abundant.

Newly hatched trout are referred to as alevin. They somewhat resemble tadpoles because of the pendulous yolk sacs protruding from their undersides. Alevin remain in the protective gravel of the redd as they use up the stored nutrients in the yolk sac.

When the nutrients are gone the young salmon emerge ("swim-up") from the gravel. At this stage they are referred to as fry. The fry leave the redd area and move to shallow water where they can find some protection from predators. After several weeks a series of dark vertical bands called parr marks appear along the sides of the young trout and they are now referred to as parr. The Brook Trout lose these marks as they grow, eventually becoming juveniles, then adults.

The life cycle of the Brook Trout

Reproduction