Preliminary summary of spawning migrations and tributary use of telemetered 

 rainbow trout in the lower Blackfoot River basin: An initial evaluation of WD risk 



Abstract 



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 



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