Influences of Body Size and Environmental Factors on Autumn Downstream Migration of Bull Trout in the Boise River Basin

Publication Date


Type of Culminating Activity


Degree Title

Master of Science in Biology



Major Advisor

Peter Koetsier


Jason B. Dunham


Mark Fuller


Little is known about the relative importance of migratory corridors and seasonally important feeding habitats in relation to bull trout size. An understanding of size-related habitat use and migratory behavior is particularly important for managing bull trout in highly manipulated reservoir systems. I used radio telemetry to determine size related patterns of autumn migration during 2001-2003 in the Boise River and Arrowrock Reservoir in southwestern Idaho. Downstream migrating bull trout (n = 158, 21-73 cm TL) captured in the autumn at a picket weir on the North Fork Boise River were implanted with radio transmitters. Tagged fish were located weekly throughout their autumn migration. Rate of migration was compared to fish size, stream temperature and discharge for 93 bull trout that traveled to the reservoir. Probability of migration to the reservoir increased with fish body length. Bull trout captured later in the season had a lower probability of migrating to Arrowrock reservoir. Migration rate (km/day) increased with fish length. Stream temperature was not significantly related to migration rate. Higher stream discharge during the first week after tagging resulted in slower migration rates. The results of this study indicate that fish size and stream discharge play an important role in determining the probability and length of time that bull trout use reservoir habitats for overwintering.

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