Analysis of small-scale movements of yellowfin tuna around fish-aggregating devices (FADS) using sonic tags
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The ongoing ultrasonic experiments of phase 2 of the Regional Tuna Project (Commission de l'Ocean Indien) are reported and analysed in relation to the local environmental parameters. Although 7 yellowfin tuna (Thunnus albacares) were tagged during the first 3 cruises of the project, only 5 of them are considered in this paper, as they were tracked around FADs. Seasonal trends affect the horizontal movements associated with FADs: strong association and homing behaviour during the fishing season vs. weaker association when the fishing season comes to an end and the fish start migrating from the area. The vertical movements are analysed in order to provide a typology of the distribution of tuna in relation to different features (FAD association by day and at night, transits between FADs by day and by night) from which other variables are considered: life stage juvenile or adult), moon phase and thermal structure.