Jumping bluefin tuna from DTU Aquas 2018 tagging project. Photo: Brian MacKenzie, DTU Aqua

This year’s tuna tagging project under way in the Skagerrak

onsdag 28 aug 19
af Helle Falborg


Brian MacKenzie
DTU Aqua
35 88 34 45


Kim Aarestrup
DTU Aqua
35 88 31 42


Lars Skou Olsen
Chef for Forskning og Naturbevarelse
Den Blå Planet, Danmarks Akvarium
44 22 22 60

Help DTU Aqua collect information about tuna in Denmark

DTU Aqua is urging everyone to report sightings of tuna in Danish waters, as this information is important for documenting and describing the whereabouts of bluefin tuna.
If you see a tuna, please contact Brian MacKenzie at DTU Aqua by email, text message or phone (brm@aqua.dtu.dk/ +45 2131 5814), ideally with photos and/or video, information on time and place of the observation, the number of tuna and their approximate size, as well as whether you have seen any possible prey, for example herring, mackerel or garfish, leap out of the water in an attempt to escape.
Once again, DTU researchers are tagging bluefin tuna to find out why the huge fish have returned to Denmark as summer visitors. Thirty-two tuna have been tagged in four days.

On Saturday morning, about 70 boats carrying recreational anglers left Skagen at the northern tip of Jutland to help DTU researchers tag bluefin tuna on day one of this year’s tagging project. And the tuna were out there. On Saturday, Sunday, Monday and Tuesday, the researchers tagged and released 32 bluefin tuna, and the work will continue over the next almost two weeks. Unfortunately, one tuna did not survive. This has now been reported to the International Commission for the Conservation of Atlantic Tunas (ICCAT), and falls under the commission’s scientific quota.

In contrast to the two previous years where DTU Aqua has tagged tuna, the researchers had not previously received reports of tuna leaping out of the water in the Skagerrak before the tagging started. But the bluefin tuna has definitely returned to Danish waters.

“Some of the questions that we’re trying to answer with our research include: Why do bluefin tuna come to Denmark in the summer? Why did the bluefin tuna disappear several decades ago, and what can we do to ensure that it returns in future?” says project manager Professor Brian MacKenzie, DTU Aqua.

Invaluable help from experienced anglers
The DTU researchers are receiving invaluable help from experienced anglers who make their boats, equipment, time and, not least, their know-how available in order to catch the big fish and transfer them to the researchers, who can then place one or more tags on each fish.

“The tuna are handled as gently as possible, and the tagging involves transferring the fish to the tagging boat for a short period of time where a hose is immediately placed in the fish’s mouth to provide lots of sea water and sufficient oxygen. Then the tag is attached, and a small sample of the fish is taken for the purposes of genetic research. The entire procedure only takes a few minutes, before the tuna is returned directly to the water,” says Professor Kim Aarestrup, DTU Aqua, who is responsible for tagging the fish.

There are three different types of tags:

  • All the fish have a small plastic tag attached to them with a unique number. If the tag is submitted to the ICCAT when the fish is caught by a fisherman, it provides information on where the fish was caught and how much it has grown since it was tagged.
  • A number of tuna are marked with sophisticated tags, which store data about the fish’s surroundings (depth, temperature and light), before transmitting it via satellites after the tag is automatically released from the fish following a predetermined time interval. This data can be used to study where the fish has been, and it also reveals where the fish spawns. Researchers can also see its preferred feeding areas, and how long it stays in these ‘fish restaurants’.
  • Some tuna will also be fitted with acoustic tags, which can be detected by advanced monitoring stations which are part of a global network.
  • A tissue sample is also taken from each of the fish. This is used for genetic analyses to reveal which of the two Atlantic subsets of bluefin tuna the fish belong to.

“The findings will help us understand how factors such as fishing, climate and the availability of food can affect where the tuna go to as well as their migratory behaviour. We will also learn more about whether the two subsets of bluefin tuna spend time together. Hopefully, this will help us to better understand population dynamics, and thus enable us to provide advice on sustainable management in future, ensuring that tuna continue to be part of the Danish fish fauna,” says Brian MacKenzie, DTU Aqua.

 DTU forskere i gang med at mærke en tun (2018) Foto: Kim Birnie-Gauvin, DTU Aqua.
DTU researchers measuring and tagging a bluefin tuna during the 2018 tuna tagging. Photo: Kim Birnie-Gauvin, DTU Aqua.

Fascinating giant fish
Up until 1962, the bluefin tuna visited Danish waters every summer, when there was widespread fishing. But then they basically disappeared, and it is only in recent years that the tuna, which can sometimes grow to up to three metres in length, can once again be seen leaping out of the Skagerrak. Today, there is a ban on tuna fishing in Denmark, as Denmark does not have a quota, but the tagging project has been granted an exemption. 

“The tuna is an iconic fish, which has become a powerful symbol of the benefits of regulating and protecting fish stocks. The return of the bluefin tuna to Danish waters represents a fantastic opportunity for Danish researchers to contribute facts about the tuna’s migratory patterns, and thus improve the management of the world’s tuna population,” says Lars Skou Olsen, Head of Animals and Aquariums at the National Aquarium Denmark (Den Blå Planet), which is contributing to the research project.

This year’s tuna tagging project is sponsored by National Aquarium Denmark, STARK, the Nordic Council of Ministers and the International Commission for the Conservation of Atlantic Tunas (ICCAT) in collaboration with DTU.

Results from previous years
This is the third year that researchers from DTU Aqua have tagged bluefin tuna. In 2017, only four tuna were tagged, while in 2018 an impressive 91 fish were tagged with one or more markers. So far, this has provided the following information:

  • We know that two of the fish we tagged in 2017 swam to the Bay of Biscay. This is because the tags transmitted signals about their respective positions two and four months after the tagging.
  • We also know that at least one of the tuna that we tagged in 2017 returned to the Skagerrak in 2018. The tag fell off after 359 days—in the Skagerrak.

Follow us on this page at fiskepleje.dk, where regular updates are posted with findings from this year’s tagging project in the Skagerrak.

Watch the video about the 2018 tuna tagging project

16 MAJ 2021