Lateral and Longitudinal Displacement of Stream-Rearing Juvenile Bull Trout in Response to Upstream Migration of Spawning Adults

Ontogenetic shifts in fish habitat use often occur to reduce the risk of predation or cannibalism. We examined the large- and small-scale spatial behavior of juvenile Bull Trout Salvelinus confluentus during the spawning migration of adfluvial adult Bull Trout into a tributary (Smith-Dorrien Creek, Alberta) where juveniles rear. We evaluated whether juvenile spatial behavior suggested a direct response to adults that may be interpreted as an attempt to reduce predation risk. Our results showed that the juvenile response to the influx of adults was size dependent, with larger juveniles migrating downstream into Lower Kananaskis Lake and smaller juveniles moving into shallower reaches and margins of the creek. From 1992 to 2002, the downstream migration of juveniles occurred after the upstream migration of spawning adults. Juveniles migrating downstream into the lake were predominantly age 3, with some age-2 individuals. The lateral movement of juvenile Bull Trout in the creek in response to the presence of adult Bull Trout spawners was also assessed by comparing water depths occupied before, during, and after the spawning period. Median depths of water used by juveniles decreased when adults were present and then shifted to prespawning water depths once spawners had left the creek to return to the lake. These results suggest that behaviorally driven habitat segregation of juvenile and adult Bull Trout occurs at both a small scale (within the creek) and a large scale (within the watershed), likely to reduce cannibalism and to maximize juvenile survival.