An Examination of the Processes That Regulate Fish Size Downriver of Dams in the Western United States

Results from a recent meta-analysis of fishery data collected downriver of Western U.S. dams indicates that rapid increases or decreases in dam discharge resulting from alterations in electricity demand (termed “hydropeaking”) can have significant negative effects on the size and catch of a recreationally important introduced salmonid (rainbow trout) in tailwater ecosystems. In addition, low discharge from reservoirs, particularly in the summer months, correlates with a shift in size structure towards smaller rainbow trout. However, it is unclear whether changes in dam operations have a direct effect on fish size (e.g., via energetic costs), or if another variable influenced by dam operations is indirectly responsible for the change in size structure (e.g., temperature, invertebrate foodbase, nutrient supply). We built upon our previous analysis by using structural equation and temporal trend analyses to identify the processes that regulate the size of introduced salmonids and other native fishes downriver of dams across the West. Results will be used to inform and improve the management of fishery resources in the Colorado River downriver of Glen Canyon Dam.