Improving Wild Sockeye Salmon Migration Success through a Large-Scale Experiment

Traversing high flows downstream of dams is a physiologically and energetically challenging activity that could influence survival and spawning success following passage. ‘Delayed mortality’, however, is an under-appreciated consequence of dam passage that has been over-looked by managers and researchers. We conducted a large-scale experiment to refine the operations of a dam in British Columbia to mitigate high post-dam passage mortality of wild sockeye salmon. We tracked 637 fish (acoustic accelerometer and PIT telemetry) under two flow conditions: (1) a baseline condition – currently used by managers – which is intended to promote high attraction efficiency by releasing water adjacent to the entrance of a vertical-slot fishway, and (2) an experimental condition that released water away from the fishway entrance in an attempt to reduce the physiological stress associated with burst swimming through high flows. Survival to spawning grounds was ca. 20% higher under the experimental flow condition, a difference that equates to ca. 2000 more sockeye salmon (15% of total spawners) reaching spawning grounds. We found no evidence that spawning success was related to flow condition. Our research highlights the importance of considering delayed mortality and carryover effects when monitoring dam passage.