Counterintuitive active directional swimming behaviour by Atlantic salmon during seaward migration in the coastal zone

Abstract

Acoustic telemetry was used to track salmon smolts during river migration and into the open marine coastal zone. We compared migration direction and speed with particle tracking simulations to test the hypothesis that marine migration pathways are defined by active swimming current following behaviour. Habitat-specific survival rates, movement speeds, depths and directions in riverine, estuarine, and coastal habitats were also quantified. Salmon post-smolts did not disperse at random as they entered the unrestricted, coastal zone of the North sea; rather they chose a common migration pathway. This was not the most direct route to marine feeding grounds (ca. 44° N); north in the direction of the prevailing currents. Particle modelling showed that the actual post-smolt migration route was best predicted by active swimming at 1.2 body length.sec.-1 at a bearing of 70° from north but not by current following behaviour. Fish migrating in larger groups and earlier in the migration period had increased migration success. We conclude that: post-smolts have preferred migration routes that are not predicted by the shortest direction to their ultimate destination; they do not simply use the current advantage to migrate; and that they actively swim, occasionally directly against the current prevailing at the time.