The Atlantic salmon (Salmo salar) is one of the most recognized fish species that we eat and is one of the largest salmon species. Like most salmon, they start their lives in freshwater streams before migrating to the ocean as adults and then returning to rivers to lay their eggs to start the cycle again. During their journey to the ocean, the young salmon face several challenges navigating through narrow bodies of water such as fjords to reach the sea. Fjords may be daunting obstacles, as they can stretch for tens or hundreds of kilometres and leave the young fish vulnerable to dangers such as marine predators, changes in the saltiness and temperature of the water, and artificial structures including nets and bridges. A previous study investigating the migration of these salmon found that their return rate to these rivers was very low when the salmon had to cross a fjord compared with when they were manually transported across on their way to the ocean. Knut Vollset and colleagues from the Laboratory for Freshwater Ecology and Inland Fisheries, Norway, set out to discover where and why salmon numbers were dropping during their outward migration through the fjords in western Norway in 2022, by placing salmon at specific starting points while tracking their movements and survival.
The research team caught 70 young wild Atlantic salmon from the River Dale in western Norway, before the youngsters set off to the ocean through the fjords around Osterøy. They tagged each fish with a radio tracker and predation sensor to determine which fish survived their arduous migration and which did not. The researchers then released the fish carefully at seven different sites along their outbound migration route – the first in freshwater as they left the river before proceeding through the inner fjord, with the final site near the Nordhordland bridge, which crosses the fjord on the fish's way to the sea. They then tracked the fish's locations with radio receivers located along the fjords.
Reviewing the tracking observations, the scientists found that only 17 of the released fish made it as far as the bridge, with 51 being eaten by predators as they proceeded through the inner fjord; none of the fish that were released from the three farthest sites at the beginning of the youngsters' migrations survived the journey. The distance that the young fish must travel during their migration was crucial in determining their survival. The bridge also presented a significant obstacle to these fish, as they did not immediately pass beneath it. Although it is still unclear whether the structure significantly reduced successful migration, the recordings from the trackers suggest that the bridge does change their movements. When the researchers tested the water for aluminium – which is released by artificial structures such as bridges and is toxic for fish – they found that although the aluminium concentrations were higher than average, and could have impaired the fish, they were not high enough to be lethal.
Vollset and colleagues suspect that the fjords around Osterøy may form a ‘survival bottleneck’ that young salmon have to negotiate and the main cause of lower salmon survival is predation by larger fish – such as saith and cod, which are attracted to the fjords by fish farms. And the farther the youngsters have to travel, the higher the chance they will be snapped up by a predator, although the researchers point out that toxic aluminium and other factors such as parasites may also make the young salmon more vulnerable to predators.
