Triano, B., K. M. Kappenman, T. E. McMahon, M. Blank, K. C. Heim, A. E. Parker, A. V. Zale, and K. Plymesser. 2022. Attraction, entrance, and passage efficiency of Arctic Grayling, trout, and suckers at Denil fishways in the Big Hole River Basin, Montana. Transactions of the American Fisheries Society 151:453-473. https://doi.org/10.1002/tafs.10362
The Big Hole River basin in southwest Montana supports the only indigenous, self-sustaining fluvial population of Arctic Grayling Thymallus arcticus in the conterminous United States. Denil fishways were installed at 63 low-head irrigation diversion dams in the basin to provide grayling and other fishes year-round access to critical habitats; however, their efficiency has not been evaluated comprehensively. We quantified all components of fishway efficiency (approach, attraction, entrance, and passage) for hatchery-reared grayling, wild trout (Brook Trout Salvelinus fontinalis and Brown Trout Salmo trutta), and wild suckers (White Sucker Catostomus commersonii and Longnose Sucker C. catostomus) during 14 field trials conducted at six Denil fishways over a representative range of fishway slopes and hydraulic conditions using passive integrated transponder telemetry. Attraction (60.4-84.3%) and entrance (44.3-78.6%) efficiencies were variable across test conditions and reduced overall fishway efficiencies (19.1-55.8%). In contrast, upon entry, passage efficiencies were high (96.2-97.0%) for all taxa across all test conditions. Attraction of hatchery-reared grayling was limited at fishways with shallow upstream depths (low fishway discharges) and low attraction flows, but wild fish were less limited by these conditions. Entrance of grayling and trout was limited at deep upstream depths (high fishway discharges) and fishways with steep slopes, especially when plunging entrance conditions associated with shallow downstream depths were present. However, both grayling and trout were more likely to enter fishways with deep downstream depths than those with shallow downstream depths, and deep downstream depths demonstrated promise for increasing entrance at fishways with high discharges and steep slopes. Our results indicate that Denil fishways have enhanced aquatic connectivity for fishes in the Big Hole River basin, and maintaining high attraction flows and deep downstream depths could increase attraction and entrance, thereby improving overall fishway efficiency.