• acoustic telemetry;
  • Anguillicoloides crassus;
  • diurnal migration;
  • diving behaviour;
  • migratory speed;
  • sea entry

To study the migratory behaviour in wild northern European silver eel Anguilla anguilla during sea entry and early marine migration, 32 individuals were tagged with acoustic transmitters and registered at four automatic listening station arrays from the mouth of the north Norwegian River Alta and throughout the Alta Fjord. The A. anguilla entered the fjord during all parts of the tidal cycle and did not seem to utilize the outgoing tidal currents. They migrated mainly during the night, in both the river mouth and the fjord. On average, they spent 2·7 days travelling from the river mouth to the outermost array, 31 km from the river mouth, corresponding to an average migratory speed of 0·5 km h−1. The A. anguilla generally migrated in the central part of the fjord and in the uppermost 10–25% of the water column, but with frequent dives to greater depths. Already 4 km after sea entry, A. anguilla were observed diving deeper than 130 m within 20–30 min periods. Hence, this study demonstrated that A. anguilla may perform an active diving behaviour during the early marine migration. The study took place in a pristine area with a minimum of anthropogenic interventions and by individuals from a population still uninfected by the introduced parasite Anguillicoloides crassus. The results may therefore be used as a baseline for future studies of the A. anguilla early marine migration.