An assessment of culverts of fish passage barriers in a Montana drainage using a multi-tiered approach
Burford, Duncan Drake, Jr.
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Restriction or blockage of fish movement due to culverts may have important consequences to fish populations. Problems include the direct loss of critical upstream habitats, and the loss of connectivity with neighboring populations; however, culvert barriers may also serve to protect native species from nonnative species encroachment. Culverts can impede fish movement due to high water velocities, inadequate water depths, and excessive outfall heights among other factors. Most previous studies have focused on fish passage at only a few culverts and thus the extent to which culverts impede fish passage across large drainage basins is largely unknown. In this study, a multi-tiered approach was used to assess fish passage through culverts throughout the upper Clearwater River drainage, Montana. The FishXing software program was used to assess juvenile and adult fish passage at 46 culverts across a wide range of stream discharge. At a subset of 20 culverts, population sampling upstream and downstream of each culvert at low flow was used to determine the degree to which culverts are influencing relative abundance, size structure, and species presence. At a further subset of 10 sites, passage was measured directly at low flow by monitoring the movement of marked fish through culverts with differing physical characteristics. FishXing results indicated that 76-85% of the culverts were barriers, depending on the streamflow and lifestage assessed. The population sampling results indicated that there were a only few differences in relative abundance or size structure at the culverts sampled at low flow, and there was little evidence to suggest that species presence was different below versus above the culverts. The direct passage study results showed that some passage occurred at 90% of the culverts studied at low flow. However, 7 of the 10 culverts showed some degree of passage impedance. There was a moderately significant positive relationship between passage impedance and outlet height (r2 = 0.41, P = 0.047). In summary, the culverts were restricting passage to upstream habitats that may be important for spawning, growth, and survival, but they were generally not isolating populations nor serving as barriers to protect native species from non-native species encroachment.