Coastal marine and in-river migration behaviour of adult sockeye salmon en route to spawning grounds

Although behaviour and physiology of the reproductive migration of Pacific salmon Oncorhynchus spp. have been studied for the upriver migration, equivalent information for the coastal marine migratory phase has been difficult to obtain. Acoustic acceleration transmitters equipped with pressure sensors provide a tool to study swimming activity and migration depth of salmon in both marine and fresh water. Ocean migrating sockeye salmon O. nerka bound for the Fraser River, British Columbia (Canada), were intercepted approximately 200 km from the river entrance, tagged and tracked as they crossed several acoustic receiver lines in coastal waters and the lower Fraser River. Acceleration data were converted to swim speed using an existing statistical model. Data from 55 tagged fish revealed that swim speed and depth varied among migratory locales. Migration difficulty (freshwater migration distance and elevation to natal stream) was related to swim speed in the marine environment. Some of the variability in swim speed and depth was explained by diel and tidal cycles. In the marine environment, average (+/- SEM) swim speed estimate was similar to 1.23 body lengths (BL) s(-1) at an average depth of 13 +/- 0.058 m (range = 0-171 m), whereas the average swim speed in the river was significantly higher at similar to 1.57 BL s(-1) at an average (+/- SEM) depth of similar to 9 +/- 0.018 m (range = 0-21 m). Consistent with the physiological literature, coastal migrating fish were swimming near metabolically optimal speeds (0.9-1.2 BL s(-1)). Overall this study demonstrates that using acoustic accelerometer transmitters can provide valuable insights into behaviour of homing sockeye salmon in both marine and freshwater environments.