The effect of water depth on passage success of arctic grayling through two Denil fishways

Arctic grayling (<i>Thymallus arcticus)</i> are found throughout parts of North America, Europe and Asia. Denil fishways are being used in efforts to recover populations and provide passage for grayling through water diversions in Montana and elsewhere. This study evaluated how water depths in 1.83 m and 3.66 m Denil fishways affect upstream passage for grayling. The study was performed in a laboratory at the Bozeman Fish Technology Center (BFTC) in Bozeman, MT. Fifteen passage trials of different treatment combinations of approach depth and sill depth were performed in each ladder. A total of 310 grayling, in groups of 10 per trial, were allowed to volitionally attempt to swim upstream through each ladder. For the 1.83 m Denil, 81 of 137 participants successfully ascended resulting in an overall passage of 59.1% among all trials compared to 82 of 147 participants successfully ascending the longer structure for an overall passage of 55.8%. A quasi-binomial regression model was used to predict passage success for different combinations of approach and sill depths. Our study showed upstream passage success increased with deeper water depths at the sill for both ladders with the exception of the configuration with the shallowest approach depth and deepest water depth at the sill.