Tagging programmes to better understand tuna migration patterns

The challenge

Bigeye tuna (Thunnus obesus) is a tropical species that inhabits warm waters worldwide. Although individuals can reach up to 150 kg, catches in the Bay of Biscay mainly consist of juveniles weighing between 10 and 30 kg. While the stock is considered slightly overexploited, current scientific assessments do not classify it as being at risk of overfishing.

Although it was hypothesised that juveniles of several tuna species return each summer to the Bay of Biscay—following patterns similar to those of Atlantic bluefin tuna and albacore, attracted by the abundance of prey such as anchovy—there was no direct evidence to confirm this behaviour.

Such information is essential to improve tuna management and conservation, supporting the long-term sustainability of these fisheries.

The solution

Through a tag-and-release programme, AZTI experts attach spaghetti tags—a simple technology that enables monitoring of tuna growth and movements—to tuna that are subsequently released back into the sea.

Recreational fishers who encounter tagged individuals report their findings, thereby actively contributing to scientific research.

These tagging campaigns have delivered significant successes, improving knowledge of marine species, including their migratory movements, stock structure and growth.

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.