Estimating and Comparison of Turbine Passage Survival of European Eels at Two Hydro-Power Stations on the Rhine River

The European eel stock collapsed in the 1980’s and continues to decline gradually. Europeon Union (EU) council established a management plan in 2004 and in September 2007 regulations were initiated to aid in the recovery of the European eel stock. Each European States (EEC) has to develop a management plan. France chose to focus on the three main causes attributed the eel stock decline: hydro station dams, fisheries, and water quality. An R&D program was launched in 2008 to study and understand the eel’s behaviour and impact of dams, including hydropower stations. This study is part of one of the actions of this R&D program. The aim of this study action is to estimate injury and survival rates of eels passing through the turbines. EDF R&D and Normandeau Associates Inc. collaborated in studies at two hydro power stations on the Rhine River in France. Both stations had Kaplan turbines, one with 4 bladed units and the other with 5 bladed unit. The HI-Z tag recapture technique was chosen to estimate 1 and 48 hour survival probabilities and the rate and type of injuries. Both turbines were evaluated with 350 eels ranging in size from 600 to 1000 mm, mean approximately 700 mm. Recapture rates were very high (more than 96%), which provided high precision within 4%, 90% of the time on the survival and injury estimates. Direct survival rate 1 hour after recapture was estimated at 93.2% (4 blades) and 82.6% (5 blades) and survival rate at 48 hours was estimated at 92.4% (4 blades) and 78.6% (5 blades). The injury rate was 7.6% for the 4 bladed unit and 27.5% for the 5 bladed unit.The main causes of injury and death were attributed to being struck by a turbine blade or other structural components of the turbine. Half of the eels that passed through the 5 bladed turbines were also X-rayed and autopsied to check for any injuries not apparent by visual examination. The presence of an additional blade (5 versus 4) appears to be one of the main factors for the considerably lower eel survival and higher injury rates; the radius and thickness of the leading edge of the blades are also being examined.  Determining the portion of eels that emigrate through turbines, especially those with five or more blades is necessary to develop a viable recovery plan for eels on the Rhine River.