Hybrids of bull trout and dolly varden can occur in watersheds where the two species co-exist. Lake and large river forms are often late maturing. These forms usually survive at least five summers before beginning their first spawning run. All adults do not spawn every year. Spawning migrations occur during the summer.
Migrant forms move downstream to over winter in rivers or lakes after spawning.
After emerging, fry disappear into the gravel along stream edges or in side channels. As they grow larger, they move to the deeper water of pools, riffles and runs. Juveniles remain near the bottom and are strongly associated with cobbles and boulders or woody debris. The diet of small fish and juveniles is mostly made up of aquatic insects taken either from the bottom or from drift. Bull trout will become piscivorous at 100-200 mm in length where other fish species are present.