Analysis of movements and behavior of smolts swimming in hydropower reservoirs

Hydroelectric dams can be a barrier to migrating fish. An understanding of smolt swimming behavior in reservoirs is of importance to effectively design bypass systems and protect downstream migration. In this paper, probability distributions that describe smolt swimming behavior are derived based on the combined analysis of measured fish locations and Computational Fluid Dynamics (CFD) simulations. Acoustic tags were used to record three-dimensional positions of Chinook salmon (Oncorhynchus tshawytscha) in the forebay of Rocky Reach Dam and sockeye salmon (Oncorhynchus nerka) and steelhead (Oncorhynchus mykiss) in the forebay of Priest Rapids Dam. CFD simulations were used to determine flow velocity and acceleration at the measured fish locations. Fish swimming behavior in relation to flow field variables was assessed by solving Newton's Second Law at each individual measured fish location. Probability distributions of swimming orientation and thrust for selected values of flow acceleration were obtained. Both fish drifting with the flow and actively swimming were considered. According to this study, as flow acceleration increases, smolts tend to orient themselves against the flow and are less likely to drift. Out of the three species, steelhead show the strongest tendency to avoid flows with significant vertical accelerations. The percent of fish drifting in low flow acceleration zones (<2.0 × 10−4 m/s2) was between 16 and 24%. However, for flow accelerations in the order of 2.0 × 10−3 m/s2, less than 8% of fish were drifting. For fish actively swimming, increasing flow acceleration results in larger fish thrust.