A Biological Turbulence Intensity Index for Pacific Lamprey Passage of Artificial Fishways

Pacific Lampreys (Entosphenus tridentatus) is a culturally and ecologically important anadromous parasitic lamprey that spawns in Columbia River tributaries. Lamprey have a unique burst-and-attach swimming mode (saltation) used to pass challenging hydraulic sections or to climb, but passage efficiency is low at fishways of four lower Columbia Dams (~50% per dam). Past observations suggested that serpentine weir sections in the upper fishways with high turbulence are a bottleneck for this species. We designed and built an experimental vertical slot within an experimental flume to understand how velocity, slot length and turbulence conditions of water entering the slot affect lamprey passage success. Behavioral observations were coupled with measurements of the flow field. The flow field was measured with an ADV for 18 different scenarios from a combination of 3 slot lengths, 3 mean flow velocities, and 2 turbulence treatments, without and with a turbulence-inducing wall. Results show that turbulence intensity increases with slot length and mean flow velocity regardless of turbulence treatment. High turbulence zones consistently formed toward the downstream entrance of the slot for the control treatment whereas similar zones were distributed through the entire slot length for the turbulence treatment. These changes alter hydraulics and sensory cues encountered by passing fishes and may limit fish passage in longitudinal sections with sustained high turbulence intensity by affecting swimming capacity and/or behavioral responses. To account for these processes, we propose a new biologically meaningful turbulent intensity, I_bio, quantified as the ratio between the root of the averaged TKE along the path which minimize it through the slot and the average velocity for which lamprey initiate saltation. Our results indicate that I_bio may be a good predictor of passage probability through a fishway and a potentially a better metric of fish passage rate than velocity and depth alone.