Westslope cutthroat trout passage in a scaled Denil fishway

Abstract

Westslope Cutthroat Trout (Oncorhynchus clarkii lewisi) are a species of concern in the state of Montana and has become the focus of conservation efforts and research. Habitat fragmentation, caused by structures such as dams, culverts, and weirs, is one of the largest threats to Westslope Cutthroat Trout. Denil fishways have been installed at low-head diversions to facilitate their movement past these structures and maintain habitat connectivity. Recent research has focused on scaled Denil fishways, which require less water for operation than standard sized Denil fishways and leave more water for competing uses such as agricultural irrigation. The purpose of this study was to examine the passage of Westslope Cutthroat Trout in a 0.6-scale Denil fishway to determine the hydraulic conditions that best allow for passage. To do this, we prescribed twelve treatments of headwater and downstream approach depth combinations. Each treatment was replicated three times for a total of 36 trials with 10 fish in each trial. Fish movements and passage efficiencies were tracked using PIT tag telemetry. Overall, 68% (256/379) of the fish successfully passed through the fishway. Mixed effects statistical modeling was used to relate passage success to hydraulic variables and fish length. Results from this analysis indicate headwater to tailwater depth ratio and bulk tailwater velocity (as measured at the downstream end of the fishway) are the best metrics to predict the passage efficiency of Westslope Cutthroat Trout in a scaled Denil fishway. In general, passage success increased with lower headwater to tail water depth ratios (i.e., depths at the up and downstream ends of the fishway are similar) and lower tailwater velocities.