Thresholds of juvenile steelhead, Oncorhynchus mykiss, resource use and stable isotope composition in a river network

Background/Question/Methods

River networks are characterized by predictable downstream changes in environmental conditions such as habitat structure, temperature, disturbance frequency, solar radiation, and the degree of terrestrial influence. These gradients present species with physiological and behavioral opportunities and challenges, and can influence their resource use, movement, survival, population dynamics, and ultimately the ecological role species play in river food webs. Despite nearly two decades of research demonstrating the primacy of predation by stream-dwelling fishes, we have little ability to predict how the role of such top predators should change across these gradients within a river network. We conducted a watershed-scale survey of the density, diet, and stable isotope composition of juvenile steelhead (Oncorhynchus mykiss) and their terrestrial and aquatic prey to evaluate how resource use, trophic interactions, and the importance of terrestrial energy shift down river networks. We sampled juvenile steelhead diets and tissue stable isotope (d¹³C, d¹⁵N, dD) composition, and characterized the macroinvertebrate assemblage at 14 locations in the South Fork Eel watershed (spanning 1.9 to 346 km² drainage area). 

Results/Conclusions

We identified a high degree of direct consumption of terrestrial prey sources in small tributary sites, on average accounting for 50% of diet biomass. Steelhead tissue stable isotopes had high correspondence to average diet composition by site, with increases in terrestrial diet items resulting in more depleted d¹³C and more enriched dD signatures. Across the river network, direct consumption of terrestrial prey declined to very low levels (<15%) at locations with drainage areas in excess of 20km², a position previously predicted to be a key threshold in algal productivity. Steelhead density across sites was relatively constant, suggesting that juvenile steelhead are heavily subsidized by terrestrial energy in tributary locations, and that stable isotope signatures integrating consumption history over 2-3 months indicates only limited movement of individuals from locations where they were caught during our sampling. We conclude that steelhead populations in this watershed are maintained by the mosaic of habitats and prey resources that vary across river networks.