Early or late? Just go with the flow: Silver eel escapement from a Mediterranean lagoon

Migration is an important yet dangerous phase in the life of numerous animals, including the European eel (Anguilla anguilla), which migrates thousands of kilometers between its growing habitats and spawning area. The behaviors and mechanisms involved in the migration of silver eels to their spawning area have been extensively documented in rivers, estuaries and marine environments but rarely in Mediterranean lagoons, despite the high abundance of eels in lagoons. To fill this gap, we used an ARIS 1800 acoustic camera to monitor silver eel migration (escapement) in a channel that connects the Bages-Sigean lagoon (South of France) to the Mediterranean Sea. The camera was deployed over 905 days from October 2018 to April 2022 and coupled with the monitoring of 10 environmental parameters. Silver eel escapement mainly occurred in the first hours of the night, between October and December, and to a lesser extent in March and April during a few mass migration episodes. Interestingly, silver eels that escaped from October to December were mostly males, shorter than 45 cm, whereas silver eels that escaped in March and April were mostly females (>45 cm). The night migration of silver eels and their tendency to migrate in large groups is likely a strategy to limit their risk of predation. They escaped principally when the water flowed seaward, which limits their energy demand for migration. Finally, we emphasized that knowledge gaps on the behavior of males silver eels at sea need to be addressed as a priority to improve the understanding of eel ecology.