The diurnal vertical behaviour of yellowfin tuna observed from sonic tagging experiments in the western Indian Ocean (Comoros archipelago) is compared with the vertical profiles of temperature and dissolved oxygen concentration. Two different behavioural situations: off-FAD and FAD-associated yellowfin tuna, are characterized by the relationships between swimming depth and vertical structure of temperature and dissolved oxygen concentration (i.e. depth and thickness of the mixed layer, of the thermocline layer and of the oxycline) observed in the tracking area. Gradients of both paramelers are shown to be the key factors which determine the vertical swimming behaviour. From an analysis carried out on the whole oceanographic data set of the Western Indian Ocean, oxyeline is found to match with the depth of the 4.2-4.3 ml.1−1 dissolved oxygen concentration.

Vertical distributions of yellowfin tuna, for both off-FAD and FAD-associated situations, are modelled by using normal-derived distributions. These models indicate the probability of the presence of yellowfin tuna, related to the vertical profiles of temperature and dissolved oxygen concentration. A single mode distribution is fitted to describe the FAD-associated situation, and a bi-modal one for the off-FAD situation. Positions of the modes are determined by the relationship between the vertical swimming behaviour and the position of either maximum gradient of temperature or dissolved oxygen concentration. An iterative calculation of normal distribution standard error is conducted to adjust the shape of the eurve to cover the entire layer in which yellowfin tuna is considered to be present. Both models were applied in a remote area, east of the Seychelles, where purse seine catches are important. The predicted vertical distribution seems to be realistic and matches the observations given by echosound pictures of tuna schools obtained in the same area by purse seiners.