Adult Coho Salmon and Steelhead Use of Boulder Weirs in Southwest Oregon Streams

The placement of log and boulder structures in streams is a common and often effective technique for improving juvenile salmonid rearing habitat and increasing fish densities. Less frequently examined has been the use of these structures by adult salmonids. In 2004, spawner densities and redd counts of coho salmon Oncorhynchus kisutch in seven Oregon streams were compared between 10 reach pairs: reaches with artificially placed boulder weir structures (treatment) and reaches without weirs (control). In addition, based on annual spawner survey data collected from 2001 to 2005, redd density of steelhead O. mykiss and spawner and redd densities of coho salmon were examined to assess differences among main‐stem reaches with boulder weirs, main‐stem reaches without weirs, and tributary reaches without weirs throughout one basin (West Fork of the Smith River (WFS)). Numbers of coho salmon spawners and peak redd counts were significantly higher (P ≤ 0.05) in treatment reaches than in control reaches in the first study. In contrast, no differences existed in coho salmon spawner counts or steelhead redd counts among reaches within WFS. Coho salmon redd densities differed significantly among the three reach types in WFS; redd densities in tributary reaches were higher than those in main‐stem reaches either with or without boulder weirs. Both spawner density and redd density were positively correlated with percent gravel. Results from these two related studies suggest that the placement of boulder weirs in bedrock channels leads to localized increases in spawner abundance, although other factors (e.g., amount of spawning area or gravel) appear to influence coho salmon and steelhead spawner abundance and redd construction at a watershed scale. This also suggests that gravel sources are an important factor to consider when placing boulder weirs or other instream structures designed to improve spawning habitat.