Diving and feeding of adult Atlantic salmon when migrating through the coastal zone in Norway

Abstract

Atlantic salmon post-spawners from a population in northern Norway were tagged with data storage tags (N = 773), and the depth use and diving behaviour of recaptured individuals within the coastal zone were examined, both on their outward migration to sea (N = 44) and their return to the natal river after overwintering at sea (N = 34). In addition, the stomach contents of 909 returning adults caught in the fjord were examined to determine the extent to which, how recently and on what species they had fed. The tagged individuals migrated through surface waters and performed aperiodic dives, regardless of whether they were leaving the fjord as post-spawners (kelts) or returning after a winter or more at sea. However, diving behaviour differed between the fjord and outer coast. During both outward and return migration, dives when fish were likely in the fjord were shallower than on the outer coast. Deep dives of longer duration were more frequent on the outer coast than in the fjord. The stomach content analysis of salmon captured in the fjord did not show strong evidence of recent feeding: 58% of the salmon had empty stomachs, and most stomach contents were highly digested fish (mainly herring, but some capelin and unidentifiable species). We conclude that the inferred diving behaviour in the coastal zone, both on outward migration to sea and on return to the natal river, did not provide sufficient evidence of foraging within the water column, and hypothesize that diving in search for navigation cues is a more likely explanation.