A Long Way Down – Escapement Success of European Silver Eels from a German River System

The direct monitoring of silver eel escapement from rivers is challenging and managing authorities are therefore usually reliant on estimates and model outputs. In order to validate the reliability of escapement numbers that were used for the German Eel Management Plan (EMP) we assessed the total silver eel downstream migration in a German drainage system (Schwentine River) during a three-year monitoring. The observed numbers were compared to estimated escapement values from the EMP of the respective river basin district and to the output of the widely used German Eel Model II (GEM II). In addition, we analyzed potential environmental triggers for the onset of downstream migration and assessed the benefit of fish protecting devices for the passage and survival of eels at a hydropower station. Results indicate that the annual silver eel escapement from the Schwentine River is far below estimated values that underlie the EMP of the respective river basin district. This highlights the need for a careful consideration of indirect estimations which include multiple assumptions and uncertainties. In contrast, GEM II predictions were close to monitored escapement numbers, indicating the potential of the model to accurately calculate silver eel escapement when system-specific input data are carefully assessed.