Effects of Swim Speed and Activity Pattern on Success of Adult Sockeye Salmon Migration through an Area of Difficult Passage

We used electromyogram telemetry to examine swim speeds and passage success for 12 adult sockeye salmon Oncorhynchus nerka migrating through Hell\u27s Gate, a reach in the Fraser River Canyon that is notorious for creating conditions that impede salmon migrations. Fishways exist in one segment; however, the approach immediately downstream of the fishways is also very challenging to migration. Average swim speeds (in body lengths per second; BL/s) were slower (P-0.017) and residency times shorter (P=0.058) in the approach for fish that successfully entered the fishways (mean=1.85 BL/s, SE=0.71; mean=34.57 min, SE=17.39; n=7) compared with those of fish that were unsuccessful (mean=4.23 BL/s, SE=0.85; mean=1,742 min, SE=1,367; n=5). Migrant-specific swim-speed patterns revealed that fish alternated at different time scales between relatively fast and slow speeds. Continuous swimming at greater than maximum sustained speeds (Ucrit) never exceeded 3 min in the case of any of the successful migrants, but unsuccessful migrants exhibited one or more periods (each of \u3e10 min duration) of continuous swimming at speeds greater than Ucrit. Unsuccessful fish disappeared downstream and died without passing Hell\u27s Gate. Hyperactivity seems to have played a significant role in lack of passage success. It is unclear why some fish swam at relatively fast speeds for extended periods of time while others did not. It is possible that unsuccessful migrants swam fast because they used relatively turbulent and fast-flowing migration paths. Some individuals may also be prone to swim at relatively fast speeds regardless of migration path