Assessment of Trout Passage through Culverts in a Large Montana Drainage during Summer Low Flow

Abstract

AbstractWe used a combination of methods to assess the degree of fish passage restriction from road culverts during summer low flow for westslope cutthroat trout Oncorhynchus clarkii lewisi and brook trout Salvelinus fontinalis across a large drainage basin. The FishXing fish passage model classified 41 of 45 (91%) culverts as barriers to upstream passage for 152‐mm westslope cutthroat trout. Population sampling upstream and downstream of 23 culverts revealed little differences in westslope cutthroat trout or brook trout above and below culverts, although density declined upstream when culvert slopes exceeded 4.5% and outlet drops exceeded 20 cm. A passage experiment with marked trout at 12 culverts showed that the proportion of upstream movement averaged 2.45 times lower through culverts (mean, 0.37) than through natural stream reaches (mean, 0.63; χ2 = 26.2, P < 0.001). Outlet drop was the most important factor affecting passage success; probability of passage was low for small trout (<100 mm fork length) at outlet drops greater than 15 cm and for large trout (>100 mm) at outlet drops greater than 21 cm. Agreement between FishXing model predictions and observed upstream passage through test culverts was low overall (17%, n = 12); the model tended to overestimate the number of impassable culverts, underscoring a need for further field testing to refine the model. Overall, the high degree of upstream movement observed in our study for juvenile and adult westslope cutthroat trout and brook trout during the summer indicates that culvert passage is an important management consideration for stream salmonids during this period.