Spatial variation in spawning habitat of cutthroat trout in a sediment-rich stream basin

We examined distribution and habitat characteristics of spawning sites of cutthroat trout Oncorhynchus clarki at various spatial scales to assess effects of sedimentation within a large basin in Montana. Redd density varied widely across the basin; nearly all (99%) of the 362 redds observed occurred in two high-elevation headwater tributaries. Redd density at the reach scale was positively correlated (r(2) = 0.72, P = 0.001) with abundance of spawning gravels. Other habitat variables, such as gradient, width, depth, embeddedness, bank stability, and percent riffle, were not significantly correlated to redd density. Taylor Fork redds contained some of the highest proportions of fine sediments (<6.35 mm, mean = 41.6%; <0.85 mm, 17.9%) observed in egg pockets of salmonid redds in the Rocky Mountain region. Cache Creek, a highly disturbed subbasin, had significantly greater proportions of fine sediments smaller than 0.85 mm in redds than the undisturbed Wapiti Creek subbasin. High fine-sediment levels in redds led to very low estimated embryo survival (mean, 8.5%, but sedimentation did not appear to limit recruitment. Our data suggest that compensatory juvenile survival and high embryo survival in the small proportion of redds with good substrate quality may buffer the effects of the high sediment levels in the basin.