Stream-resident bull trout are often separated from other populations by some barrier (like a waterfall or irrigation structure). Consequently, stream-resident populations are associated with headwater streams in mountainous regions where cold water and barriers are common. Typically, these streams are smaller and have higher gradients than those occupied by adfluvial and fluvial populations. In these headwater streams, resident bull trout are associated with deep pools and in-stream cover. Suitable overwintering sites are critical to maintaining viable populations. So in winter, resident bull trout may move to areas of upwelling ground water or deep pools.
Fluvial bull trout spend their adult lives (except when spawning) in large rivers and major tributaries. Typically, juvenile bull trout rear in small streams for at least two years (and often longer) before migrating to mainstem rivers and their larger tributaries. During the hot days of summer, mainstem bull trout concentrate in cooler areas and are often associated with the mouths of spawning streams like the Jocko. They also like areas with instream cover (lots of large woody debris (logs)).
Depending on conditions in the lake or reservoir, adfluvial populations of bull trout can use all parts of the lake: they might forage near the shore in fall and spring, and move to deep water in summer, most likely due to temperature constraints. Apparently, they also display daily and moon-phase patterns of habitat use in lakes. Some studies of adfluvial bull trout show little activity during the day and peak activity on dark, moonless nights. Other studies (such as one in Lake Koocanusa) suggest that even in the spring, bull trout are more abundant in deep water than near the surface.
Resident Adult Habitat