This paper reviews results of some experiments conducted in French Polynesia on tuna behaviour. A method based on the simultaneous use of two techniques, acoustic tracking and acoustic surveys, was used. Experiments were conducted within the framework of the ECOTAP program, a joint program between two national research institutes (IFREMER and ORSTOM), and a territorial institute (EVAAM). Acoustic tags equipped with pressure sensors were used in order to record horizontal and vertical movements of one yellowfin tuna (Thunnus albacares) and two bigeye tuna (T. obesus). Trackings lasted between 13 to 24 h. In the same time, echogram data were recorded between the surface and a depth of 500 m on board the tracking vessel. As the maximum range of the acoustic tags is small (a few hundred meters), vessel and tagged fish horizontal movements are therefore treated as equivalent. Echogram data from the sounder and data on the swimming depth of the fish given by the acoustic tag are then considered as having been obtained at the same time at the same place. Comparison between the swimming depth of the tagged fish and the echogram data from the sounder clearly shows the important role of scattering layers, assimilated as food, on vertical and horizontal tuna movements, during daytime as well as during night-time. The method used during these experiments allows to observe a new explanatory factor of tuna behaviour: the biotic environment. At small temporal and spatial scales, structure of the biotic environment and its dynamic appear to be a key factor to understanding the vertical and horizontal tuna movements. The simultaneous technique presented here must now be improved by using behavioural activities sensors. By this way, it would be possible to elucidate different tuna foraging phases in relationship with the dynamic of scattering layers.
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