The native Bull Trout has been determined to be a separate species from the coastal Dolly Varden. Bull Trout are found in the Clark Fork and Flathead drainages of western Montana, and their slowly declining trend has led to their designation as a threatened species. Bull Trout are a sensitive species that do not tolerate high sediment levels in their spawning streams. Sediment can suffocate the developing embryos before they hatch. In Flathead Lake, where they achieve trophy sizes of up to 25 pounds, the Bull Trout life cycle has been studied extensively. Adult Bull Trout ascend the North and Middle forks of the Flathead River to spawn in small tributary streams; in some cases traveling well over 100 miles in a few months. They spawn in the fall and the adults return to the lake. Young fish may spend up to three years in the tributaries before returning to mature in Flathead Lake. In other river systems, Bull Trout may be a resident stream fish.
Often, native Bull Trout have been displaced through competitive interaction with introduced Brook Trout. Bull Trout and Brook Trout will interbreed, resulting in sterile hybrids, which leads to a further decrease in Bull Trout populations. The Bull Trout may be considered the Grizzly Bear of the fish world in relationship to its need for unaltered habitat. Young Bull Trout feed primarily on aquatic invertebrates but adults eat mostly other fish (Montana Fish, Wildlife, and Parks). Resident adults are 15 to 30 centimeters in length whereas migratory adults commonly exceed 60 centimeters (Rieman and McIntyre 1993).
Western Hemisphere Range
Both migratory and stream-resident Bull Trout move in response to developmental and seasonal habitat requirements. Migratory individuals can move great distances (up to 156 miles [250 kilometers]) among lakes, rivers, and tributary streams in response to spawning, rearing, and adult habitat needs (Swanberg 1996). Stream-resident Bull Trout migrate within tributary stream networks for spawning purposes, as well as in response to changes in seasonal habitat requirements and conditions. Open migratory corridors, both within and among tributary streams, and larger rivers and lake systems are critical for maintaining Bull Trout populations (Montana AFS Species Status Account
Sub-adult and adult fluvial Bull Trout reside in larger streams and rivers and spawn in smaller tributary streams, whereas adfluvial Bull Trout reside in lakes and spawn in tributaries (Montana AFS Species Status Account
). They spawn in headwater streams with clear gravel or rubble bottom (Brown 1971, Holton 1981).
The young feed on aquatic insects. The adults are piscivorous. A Flathead Lake study found fish comprising more than 99% of food biomass (Shepard et al. 1984, Leathe and Graham 1982).
The Flathead Basin population is largely adfluvial, living out most of their adult lives in a lake environment. Young are reared in spawning tributary streams (Shepard et al. 1984). Hybrids are sterile when crossed with Brook Trout. Bull Trout grow to lengths of 37 inches and weights as heavy as 20+ pounds (Montana AFS Species Status Account
Bull Trout reach sexually maturity in 4 to 5 years. Spawning takes place between late August and early November, principally in third and fourth order streams. Existing studies suggest that successful incubation of Bull Trout embryos requires cold water temperatures, a gravel/cobble substrate with high permeability to allow water to flow over incubating eggs, and low levels of fine sediment (particles smaller than 6.35 millimeters (0.25 inches) in diameter) that smother eggs and fry. Eggs are deposited as deep as 25 centimeters (10 inches) below the streambed surface, and fry do not emerge until 7 to 8 months later, depending on water temperature (Montana AFS Species Status Account
Because of their opportunistic feeding habits and late maturity, Bull Trout are vulnerable to over-harvest and poaching/accidental harvest, especially during migrations in tributaries (Leathe and Enk 1985, Long 1997, Schmetterling and Long 1999, Carnefix 2002). Some Montana Bull Trout populations (e.g. Swan, South Fork Flathead, Kootenai, Flathead and Blackfoot) have responded well to more restrictive angling regulations or closures (Tom Weaver, personal communication), and initial conservation efforts in Montana focused on such measures. Harvest is currently permitted only in Swan Lake. Some level of poaching (Swanberg 1996, Long 1997) and accidental harvest due to misidentification (Schmetterling and Long 1999) probably continues to impact Bull Trout populations, but is difficult to detect, quantify, prosecute or prevent (Montana AFS Species Status Account
). Recent efforts to reduce misidentification include a Bull Trout Identification and Education webpage at the Montana Fish Wildlife and Parks website.
The State of Montana began development of a Bull Trout restoration plan in 1993. The final plan, published in June 2000, identifies 115 Bull Trout core areas and connecting nodal habitat within twelve Restoration/Conservation Areas (RCAs); sets goals, objectives and criteria for restoration; outlines actions to meet those criteria; and establishes a structure to monitor implementation and evaluate effectiveness of the plan. The stated goal of the plan is to ensure the long-term persistence of complex (all life histories represented), interacting groups of Bull Trout distributed across the species' range and manage for sufficient abundance within restored RCAs to allow for recreational utilization (Montana Bull Trout Restoration Team 2000, Montana AFS Species Status Account