The Atlantic bluefin tuna (ABFT), Thunnus thynnus, is an emblematic large and highly migratory pelagic fish with physiological specializations for its lifestyle, in-cluding partial endothermy and obligate ram ventilation. Even though general mi-gratory patterns are known, little information is available on the effects of body size and increased temperature on underlying biological migratory processes, which is of great interest in the context of ongoing climate change. We used high-resolution acoustic telemetry and heart rate biologgers to follow movements and cardiac activity for a year on twenty individuals (mass range 25 to 200 kg) held in a sea-pen (50m diameter and 30m depth) off the coast of Malta, Mediterranean Sea. During this period, in the summer 2023, a heat wave occurred with temperature increases of ~4˚C above the previous years’ summer average for about a week. Preliminary results indicate increased median daily depth use as temperature increased but only in smaller sized fish, larger fish seemed just to decrease their overall activity. The results will contribute to a better understanding of the behavioural and physiological responses of ABFT to environmental conditions, which is key to predicting their migratory behaviours in both current and future climates, and is essential for effective management of this highly exploited and socio-economically important species.