First scientific evidence of the loyalty of bigeye tuna to the Bay of Biscay

• The recapture of a specimen monitored with an AZTI tag provides unprecedented data on the distribution of this little studied species in this oceanic region.
• The discovery highlights the importance of collaboration between sport fishermen and research centres to promote the conservation and sustainable management of fisheries.

Pasaia, 6 November 2024– Recent recapture of a bigeye tuna in the Bay of Biscay has provided key data on the behaviour of this species, little studied in the region and of great gastronomic value.

The specimen was tagged by sports fisherman Imanol Ansa on board the Hondarribia (Gipuzkoa)-based Julenimanol, during a recreational fishing day organised by the Arraindenok association in collaboration with AZTI, on 30 July 2023. Almost a year later, on 30 June 2024, the same fish was recaptured by the fishing boat San Fermín Berria, some 26 miles off the coast of Suances (Cantabria) and 185 kilometres west of its first tagging point. During this period, it grew by 9 cm and its weight increased by 4 kg, reaching 10.5 kg.

This finding provides new evidence of the ‘fidelity’ of bigeye tuna to these waters, suggesting that bigeye tuna, like albacore and bluefin tuna, may return annually or be continuously resident in the area.

Collaboration between recreational fisheries and science

This discovery has been possible thanks to the ‘tag and release’ programme led by AZTI, with the support of the Basque Government. This initiative allows recreational fishermen to contribute to scientific research through the use of spaghetti tags, a simple technology that facilitates the monitoring of tuna growth and movements. This data is essential to better understand their migratory patterns.

‘Each recapture provides us with a unique window into the migratory patterns, growth and feeding habits of less common species such as bigeye tuna. These data allow us to improve the management and conservation of tuna, favouring the sustainability of fisheries,’ explains Iñigo Onandia, AZTI researcher and expert in tuna tagging.

About bigeye tuna

Bigeye tuna (Thunnus obesus) is a tropical species that inhabits warm waters all over the world. Although it can weigh up to 150 kg, juveniles weighing between 10 and 30 kg are mostly caught in the Bay of Biscay. Although its population is slightly overfished, current scientific assessments do not consider it to be at risk of overfishing.

The new data provided by AZTI on the migration and behaviour of bigeye tuna in the Bay of Biscay reinforces the theory that juveniles of several tuna species return to these waters every summer, following similar patterns to those of species such as bluefin tuna and albacore tuna, attracted by the abundance of prey such as anchovy. This underlines the importance of continuing with tagging days to improve knowledge about key species in the economy of the Basque Country.