Assessing migratory bottlenecks and escapement of silver eel (Anguilla anguilla) in the highly urbanized North Sea Canal basin, the Netherlands

Habitat fragmentation and migration barriers have attributed to the decline of
European eel (Anguilla anguilla). The migration of silver eels through pumping stations, ship locks, and migration facilities within the North Sea Canal basin using acoustic telemetry and overall escapement by mark–recapture was studied. A network of 61 acoustic receivers and three PIT-tag stations was built, and 305 silver eels were tagged with acoustic transmitters and 3923 with PIT-tags. Of all the silver eels that were detected, 55% passed barriers between the polders and the canal and 46% also passed the barrier complex at IJmuiden to sea, mainly via the ship locks. Overall, silver eel escapement to the North Sea per year averaged 81,629 silver eels and 14.3% suffered mortality in the pumping station at IJmuiden. Migration speed was lower for silver eels that initiated their migration upstream compared with silver eels further downstream, higher for silver eels passing barriers later in the migration period, and highest at sea. This study of silver eel movement and escapement in the North Sea canal basin indicated several bottlenecks. Passage success along barriers varied strongly between sites and types of barriers. While at two smaller locations, silver eels migrated through the pedal valves in the ship lock gates, a small passage facility and the pedal valves in a ship lock at complex IJmuiden did not enhance silver eel
migration. The barriers and unnatural canal system caused additional delay during silver eel migrations. Mitigation measures for management could include installing fishfriendly pumps, using pumping stations only during the day, and in addition opening ship locks and pedal valves at the beginning of the night