Decreased streamflow impacts fish movement and energetics through reductions to invertebrate drift body size and abundance

Abstract

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 0 J/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.

Do precipitation extremes drive growth and migration timing of a Pacific salmonid fish in Mediterranean‐climate streams?

“…), which may influence their motivation to feed during late summer despite the increased effort required to acquire food in low‐drift and low‐flow conditions (Caldwell et al. ).…”

“…While growth was negligible overall, there was a slight tendency for the smallest fish to show positive growth, suggesting that smaller individuals may be better able to withstand the harsh late summer conditions. For young fish, summer growth and body condition are linked to overwinter survival (Ebersole et al 2006, Evans et al 2014 and downstream life history decisions (Satterthwaite et al 2012), which may influence their motivation to feed during late summer despite the increased effort required to acquire food in low-drift and low-flow conditions (Caldwell et al 2018).…”

Drift and emergence patterns of nonbiting midges (Chironomidae, Diptera, Insecta) in a subtropical river

“…Consequently, emergence patterns in the subtropical Sinos River, appear to be related to the regularly occurring spring floods. In a Californian stream Caldwell, Rossi, Henery, and Chandra () observed higher drift frequencies of a macroinvertebrate community dominated by nonbiting mitches and mayflies at high stream flow conditions. The authors do not interpret this behavior as catastrophical drift, but as an adaption of the life history timing to disperse effectively.…”

Food web perspectives and methods for riverine fish conservation

“…Bioenergetics models have also been linked with foraging theory to describe the energetic profitability of habitat conditions for drift‐feeding fishes (Hughes & Dill, 1990). These drift‐foraging bioenergetics models are increasingly being applied to inform metrics of stream habitat quality (Jenkins & Keeley, 2010; Rosenfeld et al, 2014; Urabe et al, 2010), carrying capacity (Wall et al, 2016), and ecological responses to flow alteration (Caldwell et al, 2018; Rosenfeld & Ptolemy, 2012).…”