Migration behaviour of silver eels (Anguilla anguilla) in a large estuary of Western Europe inferred from acoustic telemetry

• Silver eels' migration was followed in a macrotidal estuary using acoustic telemetry.
• Residence times varied significantly with estuary compartments.
• Most individuals left the estuary together with the flood.
• Total length and body weight were correlated with directional migration speed.

Despite intensive research on eels, the behaviour of silver eels in estuaries during their migration remains poorly documented which creates serious gaps in planning the restoration of the European eel population. Estuaries are complex environments that can be exposed to large human pressures which could impede, delay migration or impact fish reproductive potential. This study investigated the estuarine migration of female silver eels in the Loire River using an acoustic telemetry system. An array of 31 hydrophones was deployed in the Loire estuary and 51 female seaward migrants were tagged with acoustic transmitters and released 20 km upstream of the estuary, at 100 km from the river mouth. 94% of the silver eels could be followed down to the river mouth. Mean global estuarine speed was 4.5 km days−1, i.e., 0.05 m s−1 and residence times varied significantly between upstream and lower compartments. Mean directional migration speed was found to be 48.6 km days−1, i.e., 0.56 m s−1, and appeared correlated with total length and body weight. Also, daily escapement rate was highly influenced by river flow.