Linking riparian shade and the legacies of forest management to fish and vertebrate biomass in forested streams

Determining the factors that limit abundance and biomass of fish is fundamental to effective fisheries management. In streams, pool availability, cover, and habitat complexity often limit fish—particularly salmonids—and many restoration efforts are directed toward addressing physical habitat factors. However, the availability of prey, and the factors that influence prey abundance, can also influence the abundance, biomass, and growth of fish and other consumers. Both habitat and prey availability can be influenced by characteristics of the riparian forest in headwaters. In this study, we evaluate how variables associated with stream habitat, primary production, and macroinvertebrate biomass account for variability in the biomass of cutthroat trout and total vertebrates (fish and salamanders) across a series of paired stream reaches with contrasting forest structure. Each of nine stream pairs consisted of an old-growth reach and a reach bordered by 40- to 60-yr-old second-growth riparian forest. We evaluated relationships between response and explanatory variables for each forest structure type using correlation analysis, regression analysis, and AICc model comparison analyses. We also conducted correlation and regression analyses on within-stream reach pair differences across the nine study streams. Canopy openness, nitrate concentration, periphyton chlorophyll a accrual, total invertebrate biomass, predatory invertebrate biomass, cutthroat trout biomass, and total vertebrate biomass were all positively correlated with each other, while temperature was negatively correlated with biotic variables. Within reach pairs, canopy openness emerged as the strongest correlate with top predators, with differences in canopy openness explaining 84% of the variation in vertebrate biomass differences in the paired analysis. Other habitat metrics were poorly correlated with invertebrate, fish, and salamander biomass for all analyses. Overall, these results suggest that for the stream reaches surveyed here, resource availability—as regulated through bottom-up, autotrophic pathways—is a dominant control on fish and other consumers. This highlights the importance of food resource limitation for fish and top predators in headwater streams, and illustrates how differences in canopy structure can affect bottom-up drivers of stream food webs.
Keywords: aquatic–terrestrial linkages, bottom-up controls, Oncorhynchus clarkii clarkii, riparian forests, trophic linkages