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To help assess the influence of whiriing disease (WD) on wild rainbow trout (i.e. RBT-hybrids see results), we used radio telemetry to identify spawning areas and movement patterns by RBT in the lower Blackfoot Watershed of Montana. Telemetry confirmed a majority of Blackfoot River RBT rely on tributaries for spawning; however movement patterns also suggest mainstem spawning. Spawning migrations to tributaries began in early March as maximum daily water temperatures approached 43 °F. From presumed wintering areas in the Blackfoot River, pre-spawning movements averaged 10.1 miles (range: 0.5-47.4) to tributary spawning sites. Migratory RBT spawned in the lower reaches of most spawning streams with peak spawning occurring in late April, which translates to predicted fry emergence by late June. Fish captured downstream of the Clearwater River spawned primarily in the lower reaches of smaller, higher gradient tributaries that support low-level whirling disease infections during the post-emergence period. RBT implanted with transmitters upstream of the Clearwater confluence spawned primarily in Monture Creek, the lower portion of which supports a high level of WD during the infectious post-emergence period. Spawning in Monture Creek extended further upstream than observed in the lower river tributaries. Initial findings suggest "lower" River RBT are currently at a reduced risk of contacting WD; whereas RBT in the "middle" River are at a higher, but variable risk of contracting WD depending upon where early rearing occurs. Prior to WD, the middle Blackfoot River was identified with high juvenile (age and I) RBT (winter) mortality and recruitment problems. Introduction Whirling disease (WD), caused by the myxosporean parasite Myxobolus cerebralis, has been associated with significant declines in some wild RBT populations in the western United States (Nehring and Walker 1996; Vincent 1996). First detected in Montana in 1 994, this disease has been described as one of the single greatest threats to wild trout (MWDTF 1996). Clinical signs of the disease include the characteristic "whirling" behavior, black tail, skeletal and cranial deformities (MacConnell and Vincent 2002). Myxobolus cerebralis has a complex, two-host life cycle involving the aquatic oligochaete worm Tubifex tubifex, and a salmonid (member of the trout family). The development and severity of the disease is dependent on species, fish age and size and parasite dose at time of exposure (Vincent 2002). However young trout, particularly RBT have been shown to be the most vulnerable when infected at less than nine weeks of age (Ryce 2003). High mortality and recruitment collapse can occur in infected populations. This type of collapse is now rapidly occurring in the fluvial RBT of the Rock Creek drainage near Missoula, Montana (FWP unpublished data). Recent population surveys in the Blackfoot River indicate early stages of similar RBT declines in the middle Blackfoot River, an area of highly infected spawning streams (Results Part II, Results Part IV). WD was first detected in the Blackfoot Watershed in 1 995 in Cottonwood Creek, near Ovando. Since then, the disease has increased in distribution and intensity in both

The Big Blackfoot River Fisheries restoration report for 2004 and 2005