Engineered Foodwebs: Management Alters Trophic Relationships in a Reservoir System

Deep drawdown of reservoirs (defined as the drawdown below conservation pool) may induce highly variable and complex effects in fish communities and ecosystems. Here, we examined trophic relationships among fishes in two high-head reservoirs in the Willamette Basin under different drawdown regimes including Fall Creek (after a deep drawdown) and Hills Creek (reference site without deep drawdown) during late summer and early fall. To identify trophic relationships among consumers (e.g., dietary overlap and piscivory), we used stable isotopes ratios of nitrogen, carbon, and sulfur from primary producers, macroinvertebrates, zooplankton, and fishes. Our findings are consistent with expected overall reductions in fish densities after deep drawdown. Under such conditions, generalist predators may switch their diets from piscivory to the remaining and more abundant prey (e.g., zooplankton). This provides the foundation for food web manipulation through management alternatives which alter predicted cost-benefit ratios and influence the dynamics around species of concern. In this case, such alterations may have the potential to alleviate predation on ESA listed juvenile salmonids which have been found to regularly rear in such reservoir systems.