Assessment of trout passage through culverts in a large Montana drainage during summer low flow
culverts, trout, fish passage, barriers, upstream, upstream passage, slope, juvenile, adult, salmonids
We used a combination of methods to assess the degree of fish passage restrictionfrom road culverts during summer low flow for westslope cutthroat trout (Oncorhynchus clarkiilewisi) and brook trout (Salvelinus fontinalis) across a large drainage basin. The FishXing fishpassage model classified 41 of 45 (91%) culverts as barriers to upstream passage for 152-mmwestslope cutthroat trout. Population sampling upstream and downstream of 23 culverts revealedlittle differences in westslope cutthroat trout or brook trout above and below culverts, althoughdensity 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 upstreammovement averaged 2.45 times lower through culverts (mean, 0.37) than through natural streamreaches (mean, 0.63; v2 ¼ 26.2, P , 0.001). Outlet drop was the most important factor affectingpassage success; probability of passage was low for small trout (,100 mm fork length) at outletdrops 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 testculverts was low overall (17%, n ¼ 12); the model tended to overestimate the number ofimpassable culverts, underscoring a need for further field testing to refine the model. Overall, thehigh degree of upstream movement observed in our study for juvenile and adult westslopecutthroat trout and brook trout during the summer indicates that culvert passage is an importantmanagement consideration for stream salmonids during this period.
North American Journal of Fisheries Management
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