Spawning occurs in smaller tributaries from late March through early June. Eggs incubate in the gravel for one to two months depending on water temperature. Fry emerge in June and early July. Bull Trout Bull Trout generally mature after three years. Spawning occurs in the upper reaches of mainstem and tributary streams from late August through October.
Eggs incubate in the gravel through the fall and winter and emerge during the spring.
Two types of Bull Trout populations exist. One completes its life cycle within the upper reaches of its natal stream system. In the other population type juveniles again rear in the upper reaches of stream systems but migrate after two to three years to reside in lower areas of mainstem Grande Ronde, Snake and Imnaha Rivers where larger prey is more abundant. Mature fish move back upstream in late spring and early summer to spawn the following fall. Birkmaier has ranched all his life, and follows in the footsteps of family before him.