Risky main stems are a poorly understood constraint on salmon recovery in coastal California rivers

ABSTRACT Objective Investment in salmon habitat restoration often focuses on natal streams, with a goal of improving access for spawning fish and increasing early life survival of juveniles. This work is justified by a large body of research; however, return on investment in natal streams (at least in terms of increased adult abundance) has, so far, been marginally successful at best. The factors that constrain salmon populations may shift in space and time, and accurately tracking these constraints requires monitoring the full spatial and temporal scope of the salmon life cycle. In particular, juvenile salmon emigration through main-stem rivers to the ocean is not well studied and is poorly understood. Low emigration survival can prevent successful upstream habitat restoration from being realized as increased adult recruitment. Methods We present acoustic telemetry studies from two coastal California rivers to estimate seaward emigration survival of Coho Salmon Oncorhynchus kisutch from their natal streams. We contrast emigration survival with other life stages and with relative restoration investment in main-stem rivers. Results In both watersheds, survival was low and highly variable across years. Main-stem river segments had the lowest survival, and survival was lower for migrants that traveled greater lengths in risky main stems. Main-stem emigration occurred over a very short time span, yet survival was similar to or lower than that of other juvenile freshwater life stages (e.g., oversummer and overwinter survival), which occur over much longer time spans. We also found that later emigrants had lower survival than earlier emigrants in both rivers. A summary of restoration investment in both watersheds indicates that much less planning and restoration implementation is occurring in main-stem rivers than in small natal streams. Conclusions Taken together, our results suggest that juvenile emigration success is likely an important constraint on salmon population recovery. Greater understanding of and greater investment in main-stem rivers are sorely needed.