Influence of river speed on path selection by migrating adult sockeye salmon (Oncorhynchus nerka)

We applied stereovideographic techniques to investigate path selection, ground speed, and swimming speed in adult sockeye salmon (Oncorhynchus nerka) ascending the Seton River, British Columbia. We tracked three-dimensional trajectories of salmon through 10 reaches and characterized the current profile at each site. At sites with relatively slow currents, the encountered river current speeds along the fish's trajectory were significantly lower than the mean current speed of the site. However, at higher current speed sites, fish experienced current speeds at or above the average current speed of the site and increased their ground speed through these sites. Observed in situ swimming speeds were 1.4-76.0 times greater than swimming speeds expected based on tailbeat frequency - swimming speed predictive relationships established in flume studies. We conclude that (i) at sites with relatively slow or moderate current speeds, fish minimize exposure to high-speed currents to minimize energy expenditure, (ii) at sites with high-speed currents, fish may change their migration strategy, minimizing time spent searching for low current speed pathways and increasing their ground speed to expedite passage, and (iii) laboratory-derived predictive equations may only be appropriate for predicting in situ swimming costs at sites with moderate and linearly flowing currents.