Colonization of drifting fish aggregating devices (DFADs) in the Western Indian Ocean, assessed by fishers’ echo sounder buoys

Floating objects drifting in the surface of tropical waters, also called drifting fish aggregating devices (DFADs), attract hundreds of marine species including tuna and other species. Taking advantage of this associative behavior, industrial tropical tuna purse seiners have been increasingly deploying artificial man-made DFADs to facilitate the catch of tuna. Yet, the reasons driving this associative behavior are not fully understood. Some hypotheses are under consideration for pelagic species that would partially explain and regulate the dynamics of the aggregations at different time scales, including species colonization and departure processes. Currently, most of the DFADs are equipped with satellite linked echo-sounder buoys, which provide information on the accurate geo- location and rough estimates of the aggregated fish biomass underneath along the trajectory of the DFAD. This study investigates the colonization process of DFADs in different periods in the Western Indian Ocean, using information from 962 echo sounder buoys of DFADs deployed between 2012 and 2015 by the Spanish fleet (67716 day observations). It was found that tuna species arrived at DFAD before non-tuna species (13.49±8.35 and 21.69±15.06 days, respectively). Results provided evidences on the relation between object depth and colonization process, finding that tunas arrive earlier to deeper objects. The analysis revealed period and species-specific colonization patterns, suggesting that both non-tuna species and tuna may have different behaviors depending on the period. This study will contribute to the understanding of the ecology and behavior of target and non-target species which are necessary to assure the sustainability of tuna resources.