MILJØFORSK-Miljøforskning for en grønn samfunnsomstilling

Even though Atlantic salmon live a large part of their life in freshwater, the bulk growth to attain large body sizes occurs at sea. Mortality during the ocean migration is high and variable, and at the same time, this is the least known life-stage of Atlantic salmon. Variable conditions in different areas of the Atlantic Ocean may have large consequences for variation in size of the spawning populations among years and rivers. Reduced salmon survival during the ocean phase has been documented during recent decades, but without knowing the reason for this. The purpose of the Salmotrack project was to use new electronic fish tagging technology to reveal secrets about their ocean migration stage - and thereby be able to better understand and manage the species. This has been the most comprehensive project to date on mapping the salmon migration in the Atlantic Ocean. The electronic tags used were small data loggers and pop-up satellite transmitters. The data logger was inserted into the body cavity of the fish and, for example, depth and temperature where the fish were located were recorded. These data were downloaded from the tag when tagged salmon were recaptured. The satellite tag was attached to the base of the dorsal fin, and used the same technology as the data logger tags, but also detected light levels during the fish migration. Data on light levels were used to increase the precision of the estimated migration routes of tagged fish. The advantage of these tags is that they detach from the fish at a pre-programmed date, or when the fish dies, float to the surface and send the data to us via satellites. By using advanced models developed during the project, combined with data from these tags, we were for the first time able to follow the salmon continuously throughout its entire ocean journey. The project used data from both previous projects and new data collected during the project period. The data were also used to map the ocean migration of salmon originating from different rivers and countries (Norway, Denmark, Ireland, Iceland and Canada) and which factors that may affect their migration and survival in these areas - including where, when and why they die in the sea. The results showed that the Atlantic salmon migrate considerably further north and dive more frequently and deeper (to almost 1000 m depth) than previously known. The northernmost record of all tracked individuals was at 80 ° N, which was a Danish fish. This shows that also southern populations use areas where the cold polar and warm Atlantic water meets at the polar front. During summer and autumn, the salmon were feeding and frequently dived to the thermal stratified layers in the upper part of the water column where prey items may aggregate. In mid and late winter, the salmon generally moved more southward, the stratified temperature layer was reduced, and the salmon performed less diving but were generally diving deeper. The results also showed that the migration of adult salmon from the river and into the open ocean for some populations largely followed the ocean currents, but where others seemed often to crossed the main ocean currents. The results also showed that most salmon migrated in a single loop pattern in a counter-clockwise direction. Many salmon therefore entered the coast south of their home river and followed the coast in a northward direction back to the river. Salmon from different rivers had different degrees of geographically overlapping feeding areas, which explain why salmon from different areas can vary in annual growth and survival. Large salmon also seemed to have more predators than previously thought, including whales and sharks. An increasingly warmer ocean results in the polar front being pushed northward, which may particularly affect the southernmost salmon populations negatively. These southern populations are exposed to higher temperatures and may have to migrate longer distances to reach the feeding grounds. Overall, the project has contributed to a new and important knowledge of the open ocean migration of salmon and how this varies between different populations and environmental conditions. This knowledge can in the future be incorporated into revised management models, where both natural and anthropogenic factors are taken into account in order to better predict changes in stocks abundances and make mitigation strategies in this regard.

The open ocean migration is the main production phase of Atlantic salmon, but also the least understood of the species life cycle. Only small ecosystem changes affecting growth and survival can have dramatic effects, as seen for this species in the last y ears. Mapping the population specific marine feeding areas and understanding the temporal and spatial factors affecting salmon abundance are therefore probably the biggest challenges in salmon ecology today. New development within satellite tracking techn ology has now for the first time made it possible to track ocean migrating salmon with miniaturized pop-up satellite tags (PSATs) over long periods. By utilizing a combination of new data and data collected from a very recent large-scale project, combined with extensive national and international cooperation and one postdoc position, the present project aim to 1) model the individual complete migration pattern of salmon from different locations around and in the North Atlantic Ocean. We will further 2) an alyse the vertical diving patterns and model marine growth, as well as 3) estimate the main area and time of mortality and identify their possible main predators. Finally we will 4) test if nesting success of sea bird colonies can assist in making salmon survival prognosis. In sum the suggested project will add unique and new information about salmon migration in the open ocean, which will be important as basic knowledge to understand the present situation, temporal trends and assist in risk assessments f or the future of salmon populations. It will also assist in the management and conservation of this economically important species.