Passage survival of European and American eels at Francis and propeller turbines.

We present the magnitude of losses of European eel Anguilla anguilla and American eel A. rostrata in passage through propeller and Francis turbines at hydroelectric projects. Survival and injury rates and types were turbine type related. Overall, eel survival was higher (mean ± 90% CI = 95.1 ± 5.3%,) and injury rate lower (12.5 ± 10.5) at Francis than propeller turbines (survival = 80.7 ± 6.4%; injury rate = 25.7 ± 7.9%). The common injury type at Francis turbines was bruises and at propeller turbines was severance. Blade shape and thickness of the leading edge of the blades (rounded, thick buckets of Francis turbines v. flatter, sharper edged blades in propeller turbines); eel entry routes into the turbines; their flexible, cylindrical body shape and orientation probably contributed to these differences. Relationship between survival and injury and turbine characteristics was turbine specific. For Francis turbines, one negative correlation (r = -0.986, P < 0.01) between survival and runner speed was found and two positive correlations between injury rates and fish length (r = 0.740, P < 0.10) and number of blades (r = 0.835, P < 0.05) were noted for propeller turbines. Several severely injured eels remained active 48 h after turbine passage suggesting caution is warranted when using telemetric movement for estimating eel survival. We conclude there is a need to (a) better understand travel paths and approach orientation of eels through turbines; (b) determine where only eel passage is of concern at hydropower plants that have both turbine types and therefore preferential operation of Francis turbines may be considered; (c) inform hydropower plant operators where turbine replacement is being considered and downstream eel passage is of concern that replacement by Francis or bulb turbines may prove beneficial for eel passage.