Insights into swimming energetics and spawning dynamics in Atlantic bluefin tuna

The Atlantic bluefin tuna (Thunnus thynnus) is a large pelagic predator specialized for long migrations between feeding and spawning grounds. As an obligate ram ventilator, tuna must swim continuously, making locomotion a potential energetic constraint. However, both the energetic costs of sustained swimming and the fine-scale dynamics of spawning behaviour remain poorly understood. Today I will present two complementary studies I contributed to during my postdoctoral work at Ifremer addressing these questions. In the first, conducted at the Infrastructure for Atlantic bluefin tuna aquaculture in Spain, we used a stereo-camera system to quantify spontaneous swimming speeds across body sizes, from juveniles to adults. Absolute swimming speed increased with size but relative speed declined. Literature estimates of metabolic rate of swimming bluefin indicate much lower costs of transport and clear energetic advantages of being large for this migratory species. In the second study, we monitored adults over one year in an offshore sea-pen off Malta using high-resolution acoustic telemetry and heart rate biologgers. We detected recurrent nocturnal episodes of elevated swimming and cardiac activity during summer. By combining behavioural and physiological data we identified distinct spawning-associated signatures, providing the first high-resolution description of the spawning behaviour in this species. Together, these studies offer novel mechanistic insight into bluefin tuna ecology and tools to better model migration energetics and identify spawning behaviours in wild populations.