Life history responses to salmon habitat restoration

Three similarly sized, yet naturally varying, watersheds in the Lower Columbia River Basin were the focus of a Before-After-Control-Impact (BACI) study from 2001-2024. The study was designed to evaluate the effectiveness of habitat restoration on salmon and steelhead ( Oncorhynchus spp.) recovery. The intensity of restoration was planned to increase smolt abundance, but the consequences for life history diversity were not initially considered. However, the streams support diverse juvenile life histories of coho salmon ( O. kisutch ) and steelhead ( O. mykiss ), which may benefit resilience in ways that are underappreciated and potentially comparable to enhancing productivity. To further evaluate life history responses to restoration, we compared density, fork length (FL), age at outmigration, biomass, and growth (for coho only). We tested for effects of basin, restoration, and their interaction, while controlling for confounding factors such as yearly variation and density dependence. The best-supported models all included a main effect of basin, indicating that natural watershed processes produced juvenile fish populations that differed significantly from one another. Prior to any restoration, Mill Creek, the control (reference) stream, showed the highest coho density and the lowest steelhead density and biomass compared to the impact (treatment) streams. Mill Creek also produced the oldest but smallest steelhead, and smallest, slowest growing coho smolts. When the analysis identified effects of restoration (for coho density, growth, and biomass) or a restoration-by-basin interaction (for coho and steelhead FL), both treatment streams showed positive responses. While life history diversity can enhance the resilience of salmon populations, restoration projects rarely account for it. This study suggests that effective restoration can preserve life history diversity while targeting specific habitat factors and life stages that limit population recovery.