How tough a passage? Nonnative fish entrainment survivability at a major dam in the western USA

The Colorado River downstream of Glen Canyon Dam (GCD) in the southwest USA demonstrates anthropogenic modification including altered flow and temperature regimes and established nonnative species in upstream reservoirs. Water overallocation and megadrought are contributing to new potential invasion pathways for nonnative fishes through dam releases. In this study, we quantified factors that influence survival of fish entrained at GCD, a pathway for nonnative fishes to leave Lake Powell and enter the Grand Canyon as reservoir levels remain near historic lows since the reservoir initially filled. We measured conditions of entrainment through GCD using a mechanical Sensor Fish device and evaluated potential barotrauma by comparing our results to previous studies. Under measured conditions, a fish's gas bladder would expand by a factor of 2.7 due to entrainment, potentially resulting in injury from barotrauma. We estimated members of Centrarchidae may survive entrainment at problematically high rates across all water levels where entrainment is expected. Minimizing nonnative fish entrainment through GCD is best achieved through maintenance of water levels higher than those in recent years, potentially decreasing both entrainment likelihood and survival.