Vertical movements of Atlantic salmon post-smolts relative to measures of salinity and water temperature during the first phase of the marine migration

The migratory behaviour of hatchery-reared Atlantic salmon, Salmo salar L., post-smolts during the first phase of the marine migration was examined to assess their susceptibility to salmon lice, Lepeophtheirus salmonis (Kroyer), infestations. Swimming depths of eight post-smolts relative to the measured salinity and temperature were monitored for an average of 11.4 h following release outside the mouth of the River Eio using depth sensitive acoustic transmitters. Vertical salinity and temperature distributions were simultaneously recorded along the migratory route. Mean swimming depth was 1.7 m (individual mean 0.5-2.1 m). There was no overall preference among all the post-smolts for specific salinity concentrations. Typically post-smolts migrated the majority (68%) of their time at salinities < 20 psu (brackish water), and as a result outside the reported salinity tolerances of sea lice. Furthermore, post-smolts chose the warmest water layer during their coastal migration.