Effects of Extreme Hydrologic Regimes on Juvenile Chinook Salmon Prey Resources and Diet Composition in a Large River Floodplain

Climate and hydrologic variability are defining characteristics of California rivers. Recently, the region experienced an unprecedented drought, and the probability of similarly warm, dry conditions is predicted to increase. In addition to warming air and water temperatures, climate change projections predict increased flooding and sea level rise, likely aggravating the water resource issues that already challenge the western United States. Water managers balance many public interests, including the conservation of native fishes, such as the Chinook Salmon Oncorhynchus tshawytscha. Given projected changes in climate and hydrology, there is an urgent need to understand how salmon respond to these conditions. In this study, we examined how young salmon responded to extreme drought (2012-2015) versus flood (1998-1999) conditions in the Yolo Bypass, a floodplain-tidal slough complex of the Sacramento River, California. We found that the diets of juvenile Chinook Salmon were dominated by aquatic-riparian insects during flooding and were dominated by zooplankton during the drought. Although juvenile salmon that were caught during the drought seemed to have eaten a higher number of prey items on average, they also had higher metabolic costs. Therefore, it is likely that juvenile salmon must augment their foraging behavior to offset higher temperatures and prey shifts. Finally, preferentially consumed, calorically valuable prey (i.e., larger zooplankton and aquatic-riparian insects) have become rare due to habitat degradation and biological invasions, and resource managers must consider re-establishing productive off-channel habitats, such as riparian corridors, floodplains, and wetlands.