High Level of Predation of Atlantic Salmon Smolt During Marine Migration

Summary

The early marine migration of Atlantic salmon through coastal areas and fjords is a potential bottleneck in the migration where high losses can occur. Here we use a displacement experiment to attempt to identify mortality hotspots or bottlenecks within a fjord in Western Norway, where earlier studies have indicated exceptionally high mortality and poor recruitment over multiple years. Atlantic salmon (Salmo salar) smolts from the Dale River were tagged with prediction sensor acoustic transmitters and released at one of six sites within the fjord, which was covered by several receivers up to a large bridge across the fjord. There was a consistently high level of mortality throughout the migration, where none of the smolts from the first three release groups made it past the bridge to the outer fjord (mean survival = 17%). There was a strong effect of release distance on survival, suggesting that releasing the fish further out in the fjord and closer to the bridge improved survival. Mark-recapture analysis revealed the importance of the predation sensors for calculating survival; without using the information from predation sensors, survival through the system was high and the confidence bands were narrow; however, knowing which individuals had been eaten reduced estimated survival substantially. Once smolts arrived at the end of the tracking array at the bridge, passage of the bridge was slow, and we found that salmon spent time moving laterally along the structure rather than directly through it and appeared to be delayed.