Degree of Threat: B : Moderately threatened throughout its range, communities provide natural resources that when exploited alter the composition and structure of the community over the long-term, but are apparently recoverable
Comments: Hybridization appears to be a common problem where isolated or remnant resident populations overlap with introduced brook trout (spawning times and conditions are similar). Brook trout have been widely introduced and now occupy most basins inhabited by bull trout, though they often occupy different streams or stream reaches. Hybrids are likely to be sterile and experience developmental problems, and sometimes sharp declines in bull trout populations have occurred (Leary et al. 1993). In Montana, introduced brook trout progressively depressed a bull trout population. See Rieman and McIntyre (1993). In western Montana, Kanda et al. (2002) found that F1 hybrids can reproduce but they found no evidence of hybrid swarms in which all individuals are of hybrid origin. Because hybridization generally involved female bull trout and male brook trout, they concluded that hybridization represents greater wasted reproductive effort for bull trout than for brook trout.
Introduced brown trout and rainbow trout have been associated with bull trout declines, apparently due to competitive interactions; lake trout may have a negative impact on bull trout, due to predation by lake trout on juvenile bull trout, probable competitive interactions, and increased harvest associated with increased fishing pressure for lake trout (see Rieman and McIntyre 1993). Lake trout can displace bull trout and may prevent bull trout from becoming established in certain low elevation lakes (Donald and Alger 1993).
Stocked, hatchery-reared steelhead that do not migrate to the ocean (residual steelhead) sometimes migrate over 12 km upstream from their release point and may move into areas occupied by threatened stocks of bull trout (McMichael and Pearsons 2001). Residual steelhead could pose a threat through ecological interactions.
Bull trout are threatened by activities that damage riparian areas and cause stream siltation; logging, road construction, mining, and overgrazing may be harmful to spawning habitat. This species is very sensitive and severely impacted by siltation of spawning streams. Timber harvest and associated activities may have negative impacts on stream channels through sedimentation and/or increasing flooding or scour events (Rieman and McIntyre 1993).
Habitat fragmentation may be a problem, but it is unclear whether the fragmented distribution is natural due to specific habitat requirements or caused by human impacts (Rieman and McIntyre 1993). Some migratory populations have been virtually eliminated by water diversions or habitat disruption (e.g., in the Bitterroot basin) (Rieman and McIntyre 1993).
Passage through screens of water diversion structures has been a problem in some areas, but currently specified screen regulations for Pacific Northwest salmonid fry apparently do not need to be modified for bull trout fry (Zydlewski and Johnson 2002).
Climate change (warming) is a potential threat because it would decrease the amount of suitable habitat (see Rieman and McIntyre 1993).
Extirpation in California probably resulted from two factors: 1) interaction with the introduced brown trout, and 2) indirect effects resulting from the loss of the McCloud River spawning population of chinook salmon; loss of the massive influx of nutrients provided by dying salmon altered the character of the stream (Minckley and Deacon 1991).
Overharvest and illegal harvest are past and present threats, contributing to population reductions and threatening existing small populations. In response, Idaho, Montana, Oregon, and Washington have instituted strict harvest guidelines.
Electrofishing can be harmful to individual survival and reproduction (see management section).
In the Klamath River basin, bull trout are threatened by habitat degradation caused by livestock grazing, timber harvest, and water withdrawals; drought has aggravated the situation and existing land and water management and regulatory mechanisms have failed to protect populations and habitat; non-native brook trout also are a serious threat because of possible hybridization and competition; probably fewer than 5000 individuals remain in 7 fragmented populations (Fisheries Action News, Winter 1994). Each of the seven populations is at a moderate to high risk of extirpation (USFWS 1997).
Columbia River population segment is threatened by habitat degradation, passage restrictions at dams, and competition from non-native lake and brook trout.
For further information, see the threats section for individual population segments of bull trout.