Efficacy of three Denil fish ladders for low-flow fish passage in two tributaries to the Blackfoot River, Montana

Publication Date



Brown trout, denil, diversion, fish ladder, fish length, fish passage, irrigation, juvenile, marking, salmo trutta, slope, structures, trout, upstream

Journal or Book Title

North American Journal of Fisheries Management


We evaluated the efficacy of three Denil fish ladders retrofitted to three irrigation diversions in two tributaries to the Blackfoot River, Montana. Fish ladders were 2.4-6.1 m long, with slopes of 9.6-15.8% and mean velocities of 18-140 cm/s. We captured west-slope cutthroat trout Oncorhynchus clarki lewisi, brown trout Salmo trutta, bull trout Salvelinus confluentus, and slimy sculpins Cottus cognatus upstream of the fish ladders (treatment groups), and in three nearby stream reaches without a ladder (control groups). Fish were marked and then released downstream of the fish ladders (treatment) or immediately downstream of their capture reaches (control). Fish lengths were similar between treatment and control sites (t-tests P > 0.05). We resampled the reaches 5-7 d after marking and recaptured similar numbers of marked westslope cutthroat trout and brown trout in both treatment and control sections (chi(2) = 0.13; df = 2; P = 0.94). No slimy sculpins were recaptured in treatment or control sections. Lengths of recaptured westslope cutthroat trout ranged from 92 to 305 mm after the trout ascended the ladders and were similar to lengths of marked fish in those sections (t-tests; P > 0.05). Although it was the longest, the fish ladder with the lowest slope and velocities appeared to limit small (<300 mm) brown trout movement. Recaptured brown trout in the treatment reach were significantly larger (P = 0.05; range = 215-352 mm) than marked brown trout. Fish ladders on irrigation diversions require frequent maintenance. When we evaluated the three structures, only one was operating correctly. Nevertheless, the ladders demonstrate promise for passing small stream-resident or juvenile fishes over irrigation diversions







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