Depth use of adult Atlantic salmon during the first and last phase of the marine migration

Abstract

The Atlantic salmon has declined in numbers over the last decades. Given the species’ anadromous nature, management is a challenge, and information is lacking, especially from the marine migratory phase. Information about repeat spawners and their migratory behaviour may be of major importance in management questions, as these individuals have been found to contribute significantly to spawning populations. The aims of this study were to analyse depth use during the first and last phase of the marine migration and to investigate if the Atlantic salmon forage during the return migration. During the years 2008-2014, 630 post-spawned Atlantic salmon were tagged with depth-recording data storage tags in the river Alta. Of these, 41 salmon were recaptured. Usable data were retrieved from 29 salmon during outward migration and from 20 salmon during return migration. In addition, stomachs were collected from 939 returning salmon in the Alta fjord. The salmon spent most of the time in the upper five meters of the water column in the fjord and outer coast both during outward and inward migration. Number of depth recordings at 0-5 m decreased with time from seawater entry during outward migration from 96% during the first 24h after seawater entry, to 75% at the fourteenth day. During the inward migration, the proportion of recordings at this depth interval was smaller in the outer coast areas (50%) than in the inner and outer fjord (88 and 91% respectively). Diving intensity increased with time and distance from the estuary, both during outward and inward migration, and so did the proportion of diving salmon and the overall maximum diving depths. Large individual variations in depth use was observed, and the deepest recorded dives were 337m during outward migration and 97m deep during inward migration. The stomach analyses showed that a large proportion (59%) of the returning salmon had empty stomachs. A proportion of the salmon (14%) also had only small fractions of stomach contents. All feeding salmon had fed exclusively on fish, including herring, capelin and sand eel, but the contents were often highly digested, suggesting that the food intake had occurred some time ago. This suggests that other explanations than feeding have to be considered for diving behaviour during inward migration. The proximity to surface and diving behaviour observed in both outward migrating and inward migrating salmon is likely a combination of factors like orientation, predator avoidance, control of body temperature, or other unknown factors.