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They dive but they do not eat

Publisert 08.06.2022

New study of adult Atlantic salmon in near-coastal areas shows that they migrate through surface waters and perform aperiodic dives - both on outward migration and return.

Photo: Eva B. Thorstad
Photo: Eva B. Thorstad

 

Human activities in the coastal zone may impact Atlantic salmon during their migration to and from the high seas. Information on the depth use of Atlantic salmon in these areas both inform on their biology, and on the effects of human activities and mitigation measures.

 

Data storage tags require that the tagged fish is recaptured to retrieve data

Atlantic salmon post-spawners from the River Alta in northern Norway were tagged with data storage tags. The depth use and diving behaviour of recaptured individuals within the coastal zone were examined, both on their outward migration to sea and their return to the natal river after overwintering at sea. A total of 773 salmon were tagged, and we obtained data from 44 salmon on outward migration and 34 salmon on return migration.

In addition, the stomach contents of 909 returning adults caught in the commercial fisheries in the fjord were examined to determine the extent to which, how recently and on what species they had fed.

Surface migration with dives

The tagged Atlantic salmon had 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.

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 salmon had stopped feeding before they reached the near-coastal areas

The stomach content analysis of salmon captured in the fjord during the return migration from the ocean to the river did not show strong evidence of recent feeding: 58% of the salmon had empty stomachs, and most stomach contents were highly digested fish.

For the salmon with remains of digested fish in their stomachs, they had mainly eaten herring, but also some capelin and unidentifiable species.

Diving in coastal areas near the rivers is likely for navigation purposes

We conclude that the 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.

We suggest that diving in search for navigation cues is a likely explanation. Diving for the purpose of navigation may involve individuals exploring different layers within the water column to find cues for navigating out of the fjord and back to the natal river. Atlantic salmon return to their natal river with a high precision, and seemingly use cues sequentially learned during outward migration to orient through near-coastal and fjord areas to locate the natal river.

 

Further reading

Hedger, R.D., Kjellman, E.B., Thorstad, E.B., Strøm, J.F. & Rikardsen, A.H. 2022. Diving and feeding of adult Atlantic salmon when migrating through the coastal zone in Norway. Environmental Biology of Fishes 105: 589-604.