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We describe the capacity and composition of the large-scale purse seine fishery of the western Indian Ocean using drifting fish aggregating devices (DFADS) over the last two decades. In recent years, the fishery has been composed of 46 purse seiners of about 90 m length overall, representing a total fish hold volume of 97,000 m3. Purse seiners have shown a steady increase in length and capacity since the early 2000s and were assisted in 2022 by 13 support vessels of about 40 m length overall which are essentially devoted to the management of DFADs and the satellite-tracked buoys used for locating them and estimating the size of tuna aggregations. An average of about 13,600 DFADs have been reported to have been annually deployed in the fishery during 2019-2022, suggesting that more than 20,000 deployments occur every year in absence of data for Seychelles, and Oman and Tanzania in 2022. This figure is corroborated by the daily buoy position data available at the Secretariat since 2020. Data on the types of FOBs deployed and encountered at sea indicate that there has been a major decrease in DFADs using fishing nets over the last two decades, with about 5.6% of all DFADs occurring in the Indian Ocean still including some nets in their materials. Information on FOB types also shows that many DFADs found at sea are not equipped with a buoy, calling for further studies to estimate the importance of the component of derelict DFADs that cannot be tracked remotely. Daily buoy positions show the high dynamics of activations-deactivations of buoys over the years and a marked seasonality linked to the oceanography of the Indian Ocean and the cessation of the fishing activities of some purse seiners before the end of each year due to the limits of catch implemented through IOTC Res. 21/01. The total number of buoys monitored in the fishery was around 11,000 in late June 2023. More than 8,300 fishing sets were made on FOBs in 2022, resulting in a total catch of 310,000 t, mostly composed of skipjack tuna.
