Hierarchical Modeling of Late‐Summer Weight and Summer Abundance of Juvenile Coho Salmon across a Stream Network

Spatial variation in stream habitat quality can lead to network‐level patterns in the survival and growth of juvenile coho salmon Oncorhynchus kisutch that provide important insights into the factors influencing the freshwater population dynamics of this species. Our objectives were to quantify the relationships among summer habitat conditions, coho salmon density, and coho salmon parr abundance and weight across an extensive stream network over 3 years. We used hierarchical linear models to assess the factors influencing coho salmon weight and abundance at the levels of individual fish (fork length and parasite burden), habitat unit (surface area, cover, and density), reach (temperature and density) and stream (total nitrogen, soluble reactive phosphorus, and discharge). Habitat‐unit‐level surface area and stream‐level minimum discharge were important predictors of both the abundance and weight of coho salmon parr. An area × discharge interaction term was also important in models of weight and reflected network‐level patterns of the highest abundances and highest parr condition in the middle portions of the basin. In these locations, streamflow, habitat area, and temperature were moderate compared with the warmer, higher‐discharge downstream reaches and cooler, lower‐discharge (or even intermittent) upstream reaches. We conclude that in the study basin coho salmon parr weight was limited by habitat unit density and minimum summer discharge in the headwaters and abundance was limited primarily by space (pool area) in the headwaters and warm summer temperatures in the lower portions of the basin.