Decreased streamflow impacts fish movement and energetics through reductions to invertebrate drift body size and abundance: Reduced streamflow changes drift, fish behavior and fish energetics

Streamflow drives ecological processes across multiple trophic levels making it a master variable in lotic systems. In mountain systems, especially those that are regulated, increased frequency of droughts and reductions in snowpack may alter future streamflow regimes and impact ecological processes. We monitored invertebrate drift abundance, size, and diversity as a function of streamflow. We then related these variables to fish movement and energetic efficiencies in the Upper Shasta River in California, above and below a large streamflow diversion. Invertebrate drift biomass was significantly less at impaired flows compared with unimpaired flows, and average body size of invertebrates decreased with decreasing streamflow. Generally, fish movement was greater at the impaired flow site (>50% of the time fish were tracked). Fish movement at the upstream site was negatively related to the size of individual prey items and amount of prey available, and significant drivers were not detected in the flow-impaired site. Energetic efficiency was reduced by over 70% when search foraging took place, and the net rate of energetic intake was below 0J/s for low-flow periods. Our results suggest that fish foraging behaviour may be influenced indirectly by altered streamflow through changes to amount and size of invertebrate drift. A shift to foraging behaviour, coupled with low food availability, results in decreased energetic efficiency. Future prescriptions of flow rates to regulated rivers should account for changes to invertebrate drift, fish behaviour, and fish energetics on seasonal time scales.