Diel behaviour of tuna and non-tuna species at drifting fish aggregating devices (DFADs) in the Western Indian Ocean, determined by fishers’ echo-sounder buoys

As tropical pelagic species are attracted by floating objects in the surface of the ocean, industrial purse seiners deploy thousands of man-made drifting fish aggregating devices (DFADs) to facilitate their catch of tunas. However, the sharp increase in the use of DFADs leads to some ecological concerns, such as producing higher amount of by-catch or alteration of natural behaviour of fish. We used fishers’ satellite-linked GPS buoys equipped with echo-sounders to continuously collect acoustic samples under remote DFADs and investigate the diel behaviour patterns of the associated fish (i.e., non-tuna species and small and large tunas) and their potential biological interactions. Results showed a strong correlation between the presence of non-tuna species and small tunas, and between small and large tunas. Diel biomass dynamics were highly variable and seem to be both species and region-specific, which may suggest adaptive behaviour patterns. Tuna associated with DFADs in the Somalia area showed a clear night-time associative behaviour. In contrast, tuna in the NW Seychelles associated with DFADs to a greater degree during daytime. In the Mozambique Channel, large tuna showed daytime associative behaviour, while small tuna showed a maximum biomass at sunrise, decreasing over the day. The associative behaviour of non-tuna species was slightly variable, being uniform near the equator or showing two peaks when increasing latitude. This study shows the importance and effect of biological factors on the associative behaviour of the fish and serves as a first step towards improving pre-set selectivity of purse seine fisheries using DFADs. The fish presence and density may improve DFAD attraction and detectability and the observed periodicity by species and area shows both similarities and differences with published literature.