Follow the journey of the world’s largest tuna

The Atlantic bluefin tuna is iconic —a gigantic fish, the world’s largest tuna, and one of the ocean’s fastest swimmers.  

Finally, the species is fully back along the Norwegian coast, after a solid comeback over the last decade.  

But how long do the tunas stay up here in the north? Where do they go next? And will they return next year?

Satellite tagging provides researchers with new answers.

Tagging bluefin tunas to follow their spectacular journeys

"Since 2018, we have been satellite-tagging Atlantic bluefin tuna here in Norway, which is as far north as this species travels. The tunas weighed between 200 and 400 kilograms, so we are talking about large animals" says researcher Keno Ferter at the Institute of Marine Research (IMR).  

"Never before have bluefin tunas been tagged with electronic tags this far north, so this is completely new data we have gathered."  

The tags stay on the fish for an entire year before they "detach" and send the information "home" to the marine researchers. Then the researchers can calculate their migration route.  

This has provided new insights into the behaviour of the bluefin tuna.

Watch their journey here: 

Likes Norway in late summer – leaves before winter

As seen in the animation: The Atlantic bluefin tuna does not mind taking a little swim.  

"We’re seeing spectacular migrations," smiles the researcher. "For example, one of them crossed the Atlantic Ocean twice before swimming all the way to the eastern Mediterranean. We're talking about around 45,000 kilometres."  

Additionally, the researchers could see how long the tunas stayed in Norwegian waters.  

"We found that the tunas arrived here in mid-August and stayed until mid-October. On average, their stay in Norway lasted 68 days" Ferter explains.

Has favourite areas – swims quickly between them

The marine researchers also observed that the tunas had defined hotspots where they stayed for some time.  

"After their stay in Norway, most individuals swam to the middle of the Atlantic, where they overwintered. But some of them travelled directly to warmer waters and overwintered around the Canary Islands instead. In May, most of the fish gathered in the Mediterranean to spawn, before swimming north again." 

Between these areas, they swam very quickly.  

"What was also surprising, was that the tunas had different diving patterns, depending on where they were. This could be because the tunas adapt to the prey available in different areas and how those prey species behave," suggests Ferter.  

All the fish from which the researchers gathered full-year data, and that were not caught along the way, returned to Norway the following year.

Important knowledge for management

The new knowledge about the bluefin tuna’s migration route is crucial for developing a sustainable, international management of both the tuna and the species it preys on.  

"The fact that the bluefin tuna has specific favourite areas is an important finding, as it shows that these large bluefin tunas may be more accessible for targeted fishing when they follow a consistent migration pattern," explains IMR stock manager and researcher Leif Nøttestad.  

Additionally, the bluefin tuna is not particularly modest at the dinner table.  

"The tuna’s formidable appetite and high food intake have consequences for the fish stocks they prey on, such as mackerel. This is something that must be considered in stock advice and the management of these species", says Nøttestad.

Reference

Ferter K., Pagniello C.M.L.S. m.fl. "Atlantic bluefin tuna tagged off Norway show extensive annual migrations, high site-fidelity and dynamic behaviour in the Atlantic Ocean and Mediterranean Sea". Proc. R. Soc. B 291: 20241501.