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.
To understand what the Salmonid and Resident fish require from their habitat in the Basin we first must understand the life cycles of these fish, when they are in the various reaches of the rivers, and what part of the life cycle they are in at a given time. The following are the life cycles of the major fish in the Basin. After spending one to four years in the ocean, adults migrate through the Columbia and Snake rivers into county streams from late April through June. May. Some juvenile chinook redistribute down stream from August to November as water temperatures decrease. Other juveniles continue to rear in the upper reaches of their natal streams through the winter.