| Summary: | The Big Hole River in southwest Montana supports the only indigenous, self-sustaining fluvial population of Arctic Grayling (Thymallus arcticus) in the conterminous United States. Denil fishways have been installed at 63 low-head irrigation diversion dams throughout the basin to provide grayling and other fishes year-round access to critical habitats; however, their efficiency has not been evaluated comprehensively. We quantified attraction, entrance, and passage efficiency of grayling, trout (Brook Trout Salvelinus fontinalis and Brown Trout Salmo trutta), and suckers (White Sucker Catostomus commersonii and Longnose Sucker C. catostomus) at six Denil fishways during 14 trials conducted over a range of fishway slopes and hydraulic conditions. Tagged hatchery-reared grayling and wild fish of several taxa were released downstream of diversions in each trial, and upstream passage was monitored for 72 hours using passive integrated transponder telemetry. Attraction (60.4-84.3%) and entrance (44.3-78.6%) efficiencies limited overall fishway efficiencies (19.1-55.8%), but passage efficiencies (96.2-97.0%) were high for all taxa over all conditions tested. Attraction of hatchery-reared grayling was limited at fishways with shallow upstream depths (low fishway discharges) and low attraction flows, but wild trout and suckers were less limited by these effects. Entrance of grayling and trout was limited at high fishway discharges and fishways with steep slopes, but both species groups were more likely to enter fishways with deep downstream depths than those with shallow downstream depths. Entrance appeared to be limited by turbulent plunging conditions associated with shallow downstream depths. Denil fishways demonstrated great promise for improving habitat connectivity for grayling and other fishes in the Big Hole River basin; however, attraction and entrance were key bottlenecks that limited overall fishway efficiency. Maintaining high attraction flows and deep downstream depths should be the focus of adaptive ...
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