Ocean-cruise with salmon from southern Norway - we follow salmon from River Nidelva on a dangerous journey in the ocean

Publisert 17.07.2020

This is the preliminary story of the travels of salmon tagged with satellite tags in River Nidelva at Arendal. The story includes a new record, with bluefin tuna or porbeagle involved.

Migration routes of individual salmon from the River Nidelva at Arendal, tagged with satellite tags. Analyses and map by John F. Strøm
Migration routes of individual salmon from the River Nidelva at Arendal, tagged with satellite tags. Analyses and map by John F. Strøm



We tagged 16 kelts in May last year, which were adult salmon that had stayed in the river since spawning the previous autumn, and released them at the river mouth to follow them on a new ocean journey.


The pop up satellite tags were programmed to detach from the fish and surface during the autumn and winter. As long as the tags are attached to the fish under water, they do not have contact with the satellites. When the tag reaches the surface, it tells the satellites where it is, and transmits the data that are stored during the journey.

The tags recorded depth, temperature and light during the migration. Light recordings provide information on when the sun is rising and setting every day during the journey. Based on such data, the migration route between tagging and the position where the tag surfaced can be estimated.


The salmon from River Nidelva migrated northwest, north of Shetland, and towards Iceland. The salmon that retained the tag the longest, until 23 August, continued towards Greenland, between Iceland and Jan Mayen (see figure).

Nine of the tags surfaced 30-111 days after tagging. The positions where they surfaced are shown by yellow squares in the map. The estimated migration routes between the river and surface position are shown by dots.

We had planned to follow the fish longer, but there were technical problems with several of the tags, so we plan to tag more salmon next year.

The migration routes of salmon from River Nidelva resemble the route of salmon tagged in River Etne the same year, see cool animation of the migration tracks from the River Etne here.


Two of the tagged salmon were apparently eaten by larger fish after one month’s ocean migration. This is evident from the temperature recordings, because they were eaten by fish that were more warm-blooded than salmon. Similar to most fishes, the temperature of the salmon body resemble the temperature of the surrounding water. Some fishes have a higher body temperature than the water they swim in, but not as high as mammals, such as whales and seals.

Before they were eaten, these two salmon were swimming in water that was slightly below ten degrees. Suddenly, the temperature increased to between 20 and 25 degrees in a couple days, before in declined to around ten degrees again. During these two days, the salmon and their tag was likely in the stomach and digestive system of a more warm-blooded fish.

These two salmon were likely eaten by bluefin tuna or porbeagle. This has earlier been documented to happen with salmon tagged further south in Europe and in Canada, but never with salmon tagged in Norway, so this is a new record.  


People in the Nidelva area provided invaluable help with catching fish and assistance during tagging. Such collaboration is deemed necessary, but also very pleasant, when carrying out such projects. Thanks to all those helping us and working with us.


The study was funded by the Norwegian Research Council, Environment Agency and County Governor in Agder.