GEAR

Context

Atlantic bluefin tuna (Thunnus thynnus, ABFT) is a high-value commercial species whose biology and ecology make it particularly sensitive to environmental changes. It has high metabolic demands, shows a preference for temperate waters in its adult phase and requires warmer waters during early life stages, as well as a strong dependence on the distribution and abundance of prey due to its position in the food web.

In this context, understanding how environmental changes affect the physical condition, physiology and population dynamics of bluefin tuna is essential to move towards an ecosystem-based fisheries management approach grounded in scientific evidence, ensuring the sustainability of both the species and the fishery, particularly under global warming and ongoing ocean changes.

The recent approval by ICCAT of Recommendation 24-06 (October 2024), amending Recommendation 23-08, opens the possibility of developing a pilot bluefin tuna fattening project in the Bay of Biscay. This activity had not previously been carried out in the region due to technical limitations and adverse oceanographic conditions, especially in autumn and winter.

Fattening facilities, conceived as marine “living labs”, offer a unique research opportunity by enabling the collection of biological, physiological and genetic data from specimens under known environmental conditions. The involvement of public research institutions, including AZTI, will enhance knowledge of the species and fattening processes, while enabling the calibration and validation of methodologies applicable to wild populations.

Objectives

The overall objective of the project is to establish the scientific, methodological and logistical foundations for implementing a biological and genetic sampling programme in submersible fattening cages, where bluefin tuna specimens captured in the Bay of Biscay will be fattened for approximately three months, in accordance with Recommendation 24-06.

Specifically, the project aims to:

• Reduce uncertainties related to the physical condition, growth, and physiological and metabolic changes of individuals during the fattening process.
• Improve knowledge of population structure, mixing patterns and connectivity between populations.
• Calibrate and validate alternative methodologies to improve biometric relationships, monitoring and biomass estimates.

To achieve these objectives, the following actions are planned:

• Assess growth and condition index using acoustic techniques and stereoscopic cameras, establishing a length–weight relationship specific to the Bay of Biscay, both at capture and after fattening.
• Improve size and biomass estimates using acoustics (multibeam sonar and echosounders).
• Analyse fat content and its variability as an indicator of condition and feeding processes.
• Continuously monitor stress and internal temperature using heart-rate bio-loggers.
• Determine the relationship between the chemical composition of carbonated structures (otoliths and spines) and environmental conditions during fattening.
• Evaluate and calibrate genetic markers to determine reproductive status, comparing them with traditional methods such as gonadal histology.
• Analyse how physical processes influence environmental DNA (eDNA) detection to reduce uncertainty in abundance estimates.
• Study relatedness among individuals using SNP-based genetic markers.

Expected impact

The project’s integrated approach will improve knowledge of the physiology, condition and development of bluefin tuna during fattening, optimising associated practices and contributing to the sustainability and efficiency of this activity in the Bay of Biscay.

Data obtained from fattening facilities, conceived as controlled experimental platforms, will be highly valuable for transfer to wild populations, supporting calibration and validation studies of methodologies applied in marine environments.

The implementation of a systematic biological and genetic sampling programme, together with the creation and maintenance of biobanks, will strengthen data availability for the scientific community, in line with European open data commitments. This information is particularly relevant for stock assessment models and for reducing current uncertainties in the management of Atlantic bluefin tuna.

Furthermore, the project will contribute to improving estimates of removed biomass, validating acoustic methodologies and supporting ongoing surveys and management models, strengthening the scientific basis required for more robust and sustainable fisheries management. Collaboration between science and industry, led by AZTI, will be key to maximising both the scientific and applied impact of the results.

Funding