Initial marine movements of silver European eels Anguilla anguilla following their emigration from an English chalk stream

The conservation management of the facultative catadromous and critically endangered European eel Anguilla anguilla is inhibited by substantial knowledge gaps on their behaviours and migrations. Integral to understanding the silver eel migration back to the Sargasso Sea for spawning is generating knowledge on their initial marine movements. Here, the downstream migration of 25 silver eels from the River Test, southern England, and their initial movements in the sea were measured using acoustic telemetry. Of 24 eels detected migrating downstream, 19 were successfully tracked into the tidal area (taking 1–34 days), with in-river barriers not delaying their emigration. Thirteen eels were then detected in the marine environment, with 11 immediately moving westwards towards the Atlantic Ocean. These 11 eels took up to 25 days to move to areas outside of the marine receiver array, a linear distance of approximately 80 km from the river mouth. While eels were generally detected moving downstream in the river during darkness, marine detections occurred across the 24-h cycle, with detection depths up to 50 m. These movement data indicated eels generally migrated relatively quickly out of the river, with most then moving through inshore areas in the direction of their spawning area.