Assessing downstream passage of European silver eel at a tidal pumping station: acceleration as a proxy for eel activity.

Background: To complete their lifecycle, diadromous fish often need to pass anthropogenic barriers in regulated rivers and estuaries (e.g., pumping stations, weirs, hydropower facilities). The safe and timely passage of the endangered catadromous European eel (Anguilla anguilla), through pumping stations is a major concern and European legislation stipulates that safe downstream passage must be provided at hazardous intakes. To implement effective mitigation measures, specific knowledge on eel passage behaviour at barriers is needed.

Methods: We used acoustic telemetry with acceleration sensors tags, to understand eel movement and activity, migration routes, escapement success, and delay at a tidal pumping station. Tri-axes accelerometers measured eel acceleration in three directions and provide a root-mean-square (RMS) value over the measurement period, providing a proxy for eel activity. A network of 10 receivers was placed along the migration route to track 40 tagged individuals. Telemetry data were analysed using visual investigation of eel detections and Generalised Additive Mixed Models (GAMMs) for analysing acceleration data.

Results: We found that 75% of the tagged eels migrated to the estuary via the pumping station (PS), 5% used other routes, and 20% did not migrate seaward that season. Acceleration data showed that eels significantly increased their activity up until the moment of PS passage, from an overall mean RMS acceleration of 1.04 m/s² (95% CI = 0.93-1.18) when the pumping station started pumping (between 1 up to 4 h before eel passage), to 1.14 m/s² (95% CI = 1.04-1.26) at 10 min before the eels passed through the pumps, and 1.66 m/s² (95% CI = 1.32-2.08) 1 min before passage. Most eels passed the pumping station at night, and we found that eels had the highest movement activity between 15:00 and 00:00 with a peak around 19:00, which coincided with the moment of PS passage.

Conclusions: Acceleration provides a proxy for fish movement activity and our study demonstrates how including accelerometer sensors with telemetry can help understand movement of endangered species at migration barriers. This information is vital for implementing strategies to improve outward migration success, towards the spawning grounds in the Sargasso Sea, and thus the conservation and restoration of eel populations.