Movements of and Habitat Use by Bull Trout in Arrowrock Reservoir, Idaho

Publication Date


Type of Culminating Activity


Degree Title

Master of Science in Biology



Major Advisor

Matthew R. Dare


Despite the growing body of literature on bull trout Salvelinus confluentus life history and ecology, there is limited information on the habits of bull trout in lakes and reservoirs. I used telemetry to examine the movements and habitat use of bull trout overwintering in and around Arrowrock Reservoir from 2004 through 2005. During winter 2004, I focused on movements and habitat use of 12 bull trout implanted with acoustic transmitters that resided in Arrowrock Reservoir. Bull trout displayed a three- stage pattern of residency in Arrowrock Reservoir: 1) arrival with a brief residence near the transition of the river and reservoir, 2) dispersal throughout the reservoir, and 3) return to original arrival location with a prolonged residence prior to emigration. Separate dispersal patterns were observed for sub adult and adult fish, subadult dispersal was shorter in duration and less extensive than that of adults. During all three stages I observed movements in open water areas of the reservoir characteristic of a foraging piscivorous salmonid. The majority of bull trout were observed in offshore areas, which was disproportionate to the availability of these areas. To determine if bull trout were influenced by contrasting biotic conditions in Arrowrock Reservoir from 2002 through 2005, I examined the proportion of bull trout implanted with radio and acoustic transmitters using the reservoir and the destination of emigrants. During winter 2003 Arrowrock Reservoir was reduced to less than 1% of its full capacity to replace the darn's original valves. Following the repair, over 400,000 rainbow trout Oncorhynchus mykiss and kokanee Oncorhynchus nerka fingerlings were stocked to restore the sport fishery. I was able to compare the use of the reservoir by bull trout implanted with transmitters when conditions differed dramatically: 2002, represented baseline conditions, the reservoir drawdown in 2003 was a major anthropogenically created disturbance, and in 2004 the reservoir returned to baseline conditions with the addition of a dramatically increased food base. There was no significant difference in the proportion of bull trout overwintering in Arrowrock Reservoir among years. However, a greater proportion of fish made upstream migrations into the Boise River (suggesting spawning behavior) in 2004 compared to 2002 and 2003. These observations suggest conditions in the reservoir improved following the draw down. Bull trout would likely benefit by maintaining water surface elevations of at least 947 msl in Arrowrock Reservoir in order to maintain water depth in the most frequently used areas.

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