Temporal and Spatial Variability in the Migration Patterns of Juvenile and Subadult Bull Trout in Northeastern Oregon

Conservation planning for threatened bull trout Salvelinus confluentus requires the identification of temporal and spatial movement patterns to better understand the patch size and connectivity requirements of different behavioral strategies (e.g., resident and migratory) and life stages (e.g., juvenile and adult). Although these patterns have been identified for adults, less is known about the movement patterns of juvenile and subadult bull trout. Thus, we evaluated the movement patterns and abiotic and biotic cues associated with migration timing of fluvial juvenile and subadult (150‐300 mm) bull trout in the South Fork Walla Walla River, northeastern Oregon. From 2002 to 2005, we tagged 1,636 individuals with passive integrated transponder (PIT) tags and monitored subsequent movements (within the study area) and migrations (exiting of the study area) by PIT tag detection at antennae and active detection during the mark‐recapture sampling season. Juveniles and subadults exhibited downstream movements and migrations throughout the year; movement and migration activity peaked in August, and migrations occurred predominantly at night (94%). We modeled migration timing in response to abiotic and biotic cues and observed distinct seasonal patterns in migration timing that were associated with changes in minimum temperature. However, the seasonal models based on temperature explained only 23‐35% of the variation in migration timing, suggesting the influence of additional variables. Based on the temporal and spatial continuum of movement observed here, we believe that management must address the occupancy of multiple habitat types and migration corridors throughout the year.