The Atlantic bluefin tuna has returned to Danish waters and this year the tagging project has tagged the largest tuna in the project with an estimated weight of approx. 380 kg.
DTU Aqua and SLU Aqua have been tagging Giant Atlantic Bluefin Tuna again this year (ABFT). In total, 134 ABFT were caught and tagged by DTU Aqua during this year's tuna tagging. This is slightly below the number tagged in 2022. Unlike last year, weather conditions were generally difficult in Skagerrak, which affected the number of days we could go out to fish, and thus the number of tuna caught and tagged. Despite this challenge however, we are satisfied with the season
The tagged tuna ranged from 217-290 cm. This means that most of the tuna will reach well up on the first floor of your house if you stand them up. All the biggest fish were caught in Øresund, and the largest caught on a Danish boat was 290 cm.
Although it is not possible to weigh the fish onboard the tagging boat as they are released as quickly as possible (they usually spend less than 2 minutes on the tagging boat!), we can estimate – using the most accurate length-weight relationship table for ABFT – that this 290cm fish weighed about 380kg. In addition, the tuna caught in Øresund are generally very fat, with this individual having a girth of 202cm. So, although it may not be recognized officially, this tuna probably represents the Danish angling record for ABFT, which was previously 372kg, set all the way back in 1950 at the exact same location (Øresund).
Photo 1. Measuring the circumference of the tuna.
So far, the mean length appears to be greater in Øresund than in Skagerrak, an observation also made last year (Figure 1). This shows that, at least in part, there are different fish in the two locations. It therefore makes sense to continue to tag both in locations to assess and understand both groups of ABFT in our waters.
Figure 1. Length of tagged ABFT in 2022 in Skagerrak (blue) og Øresund (red). Mean length was 249 cm in Skagerrak og 261 cm in Øresund.
All ABFT were tagged with a standard floy tag (from ICCAT) and an acoustic tag from Thelma Biotel. The tags make it possible to obtain information from the fish over the next several years. For example, the acoustic tags work for 7-10 years and can thus be recorded at listening stations around the Atlantic and Mediterranean and, for that matter, down in the Sound for many years to come. The listening stations are all part of an international collaboration called the European Tracking Network (ETN). The collaboration ensures that our tagged fish are detected all over the world, including the North Sea, the Mediterranean and the Atlantic Ocean.
Tunas tagged by DTU have already been detected in several places in the world.
Previous results suggest that ABFT follow a similar migration route year after year, so it will be interesting to see if this is the case here. We have already recorded ABFT from several locations around Europe, including off the coast of Belgium, in the Eastern and western part of the English Channel, and in the Mediterranean Sea.
The good old days have returned
An old tuna angler, many years ago, wrote, “tuna numbers would never return to the "good old days. But you can always hope”. The last few years of fishing and tagging have shown that the wish has come true - the good old days have returned, at least in terms of the ABFT being here again. Now it is just a matter of making sure it stays that way, and here our research has an important role to play.
Af Kim Aarestrup, Henrik Baktoft, Kim Birnie-Gauvin og Lene Klubben Sortland, DTU Aqua, Institut for Akvatiske Ressourcer. Foto: Bettina Pedersen