Tagging programmes to better understand tuna migration patterns

The challenge

The Bay of Biscay is one of the most important feeding areas in the Atlantic for juvenile tunas of several species, such as albacore and bluefin tuna, which has led to the development of traditional summer fisheries. Recently, it has been observed that this feeding area is also used by adult bluefin tuna, even during winter.

Although there was a long-standing hypothesis that juveniles of several tuna species return every summer to the Bay of Biscay, attracted by the abundance of prey such as anchovy, there was no scientific evidence to confirm it.

This information is essential to improve the management and conservation of tuna stocks, supporting the sustainability of fisheries.

The solution

AZTI staff deploy tags to study tuna growth and migrations over periods of several months or years. Different types of tags are used:

• Pop-up tags: devices that record data from multiple sensors (pressure, temperature, light and time). They are attached to the dorsal area of the fish and programmed to detach after one year. Once they reach the surface, they transmit all the recorded data via satellite.

• Archival tags: unlike pop-up tags, these are implanted through minor surgery and store the information internally. Recovering the data requires the cooperation of fishers to retrieve the tag.

• Spaghetti tags: these do not record data. They are plastic tags with a serial number and contact address, used to track growth and movements when tagged fish are recaptured.

Through the “tag and release” programme, AZTI experts tag tunas that are then released back into the sea. After some time, professional and recreational fishers who encounter tagged individuals report the recapture, thereby contributing to scientific research.

Tagging campaigns have delivered major successes, significantly improving knowledge of marine species, including their migratory movements, stock structure and growth patterns.

The results

• First scientific evidence of bigeye tuna site fidelity to the Bay of Biscay:
  In June 2024, a specimen tagged 11 months earlier in waters off Gipuzkoa was recaptured near Suances (Cantabria), confirming that this species returns to—or remains in—the area year after year.
• Pioneering record of the annual migration of albacore:
  In 2022, an albacore tagged by AZTI with electronic technology was recaptured south of Ireland two years later.
  The tag recorded, for the first time, its winter migration route, depths, temperatures and light levels, revealing repeated summer visits to the Bay of Biscay.

These findings confirm the value of tagging programmes in uncovering complex migration patterns and provide essential knowledge for more sustainable, biologically realistic management of tuna fisheries.