Seaward migration of European Eel through consecutive migration barriers: passage at a pumping station and tidal sluice

Seaward migration of diadromous fish is frequently impeded by anthropogenic barriers, yet the effects of consecutive obstacles on migration success remain poorly understood. We used acoustic telemetry to quantify passage success, barrier delay, and determinants of barrier passage for European Eel ( Anguilla anguilla) at a pumping station and subsequent tidal sluice which are separated by a lake. Of 40 tagged individuals, 35 passed the pumping station and 27 successfully migrated seaward via the tidal sluice, with a mean cumulative barrier delay of 34 days. At the pumping station, eels in better body condition accumulated more passage opportunities before passing, while those with higher migration speeds required fewer. Eels that encountered more passage opportunities at the pumping station missed fewer at the tidal sluice. Passage at the pumping station increased with longer discharge events and higher wind speeds; lower moon illumination increased passage at the tidal sluice. Management must account for barrier-specific passage drivers and cumulative delays along source-to-sea migration routes.