Hydrologic Correlates of Activity in Bull Trout Holding Station in a Large Dynamic River

Recent meta-analyses have determined that changes in river flow regimes negatively impact fish communities.  Mitigating these effects is difficult due to the lack of empirical knowledge on the relationship between water flow and ecological response.  Quantifying fish behaviour and energy use across a range of river flows can provide a mechanistic dimension to observed population changes.  We used a longitudinal mixed-effects model to assess the effects of hydrologic parameters on axial swimming muscle electromyograms (EMGs) of bull trout (Salvelinus Confluentus) in a large hydropeaking river (river discharge ranging from 0-1790 m3/s).  Two hypotheses were tested: 1) bull trout must counteract the drag force of flowing water and therefore, swimming muscle activity is positively correlated to river discharge, 2) rapid changes in river discharge alter the spatial arrangement of flows and result in peaks in activity as fish attempt to re-align themselves with optimal focal point velocities.  Within-hour changes in river flow from 0-1045 m3/s did not elicit a hyperactive response in bull trout.  We found medium-large positive effects (r = .36-.56) of hourly average discharge on swimming muscle activity.  However, when a subset of EMG transmitters was calibrated to swimming speed, we found that most often these bull trout were not actively beating their tails when at-large in the river.  Not including these periods of rest, bull trout swam at preferred hourly speeds of 0.53 BL/s which is similar to swimming speed estimates of salmonids from lentic environments.  These findings are consistent with the growing body of literature that suggests that trout use strategies for energy economy such as shedding vortices, using pectoral fins for substrate contact and/or seeking out micro-flow habitats to reduce the drag forces of flowing water.