SIMULTANEOUS OBSERVATIONS OF TUNA MOVEMENTS AND THEIR PREY BY SONIC TRACKING AND ACOUSTIC SURVEYS

This paper reviews results of some experiments conducted in French Pol ynesia on tuna behaviour. A method based on the simultaneous use of tw o techniques, acoustic tracking and acoustic surveys, was used. Experi ments were conducted within the framework of the ECOTAP program, a joi nt program between two national research institutes (IFREMER and ORSTO M), and a territorial institute (EVAAM).Acoustic tags equipped with pr essure sensors were used in order to record horizontal and vertical mo vements of one yellowfin tuna (Thunnus albacares) and two bigeye tuna (T. obesus). Trackings lasted between 13 to 24 h. In the same time, ec hogram 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 i s small (a few hundred meters), vessel and tagged fish horizontal move ments are therefore treated as equivalent. Echogram data from the soun der and data on the swimming depth of the fish given by the acoustic t ag 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 rol e of scattering layers, assimilated as food, on vertical and horizonta l tuna movements, during daytime as well as during night-time. The met hod used during these experiments allows to observe a new explanatory factor of tuna behaviour: the biotic environment. At small temporal an d 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 wou ld be possible to elucidate different tuna foraging phases in relation ship with the dynamic of scattering layers.