Passage efficiency and behaviour of sea lampreys (Petromyzon marinus, Linnaeus 1758) at a large marine–freshwater barrier

Passage success of sea lampreysPetromyzon marinusand behaviour in the vicinity ofman-made structures such as marine–freshwater barriers are poorly understood. To mit-igate these migratory problems, a better understanding of passage behaviour is needed.To investigate this, 25 sea lampreys were tagged with V7 VEMCO acoustic transmittersat a large marine-freshwater barrier, consisting of ten sluice gates and a ship lock com-plex in the Netherlands. Overall passage success was assessed to be 16%, with four sealampreys passing the sluice gates, while none passed the ship locks. Successful passagethrough the barrier occurred either at the beginning or at the end of a discharge period,when water level differences between sea and lake and thus water velocities were rela-tively low. Most sea lamprey showed exploratory searching behaviour but only for ashort duration, before leaving the area again when unsuccessful. Low passage successand search duration were likely related to the unnatural infrequent occurrence of flowand only short lasting windows with suitable conditions for passage at the sluice gates.To mitigate the poor migration success of sea lampreys at the marine-freshwater barrierwith the current adapted sluice management, a fish passage facility with lower watervelocities and longer or continuouspassage opportunities is neede