Bluefin tuna swim in schools to dilute the risk of being attacked, confuse predators, save energy by taking advantage of the wake of its neighbors and locate food areas more effectively, without any individual leading the group.
Coordination without a leader, carried by the lateral line
Each tuna perceives the movements of its immediate neighbors thanks to its lateral line, a sensory organ which detects vibrations and pressure variations in the water, supplemented by highly developed peripheral vision. This local information is sufficient: each fish adjusts its speed and direction in relation to a few nearby conspecifics, which collectively generates coherent school shapes, without an individual taking the role of leader. This decentralized mode of coordination allows almost instantaneous reactions to danger, with the entire group able to change direction in a fraction of a second.
The school also offers a hydrodynamic advantage: by placing itself slightly behind a neighbor, a tuna can exploit the vortices created by the body in front of it, which reduces the muscular effort necessary to maintain its cruising speed. In a species like the bluefin tuna, capable of traveling several thousand kilometers during its transatlantic migrations, this cumulative energy saving over the duration of the journey represents an important physiological issue.
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Living in groups also increases the chances of detecting schools of prey such as sardines or anchovies: the more eyes there are, the greater the probability that an individual will spot a feeding opportunity, and the information is quickly propagated to the rest of the school by simple observation of changes in the trajectory of neighbors.
A behavior exploited by fishing in the Mediterranean
This gregariousness, valuable for the survival of the species in the natural environment, paradoxically makes it vulnerable to humans. Tuna purse seiners locate the surface schools by plane or by sonar then encircle them in one go with large nets, a technique that is extremely effective precisely because hundreds of individuals are grouped in the same place at the same time. This fishery has strongly contributed to the historic decline of the Mediterranean stock of bluefin tuna, today regulated by strict international quotas.
