Density-Dependent and Density-Independent Processes Interact to Govern Coho Salmon Productivity

A quantitative assessment of the processes constraining vital rates such as productivity and survival is an essential component of understanding how, why, when, and if habitat restoration increases salmon productivity.  Here we present results of ten years of coho salmon and habitat monitoring from the Hood Canal Intensively Monitored Watershed encompassing four streams in western Washington State.  Using a life-cycle monitoring approach, we measured the abundance of spawning adults in the fall, stream-rearing parr in the summer, and seaward migrating smolts in the spring.  We evaluated the influence of density-dependent processes and environmental predictors on survival.  Interestingly, at both sites where density-dependence was detected, effects were observed at younger (egg to parr) but not older (parr to smolt) life stages.  Parr to smolt survival (median = 15.6%, range = 0.9 - 32.5 %) showed a strong year effect, as trends within each of the four sites tended to track each other through time, suggesting a regional climate signal that was shared among the study populations.  A small number of habitat treatments have been implemented in two of the watersheds but the limited spatial and temporal distribution of these treatments has made responses in fish productivity difficult to detect.