Abstract
Marine pollution has increased over time, becoming a major source of concern. A non-negligible proportion of these waste and pollutants are from sea-based sources, especially fisheries, due to derelict fishing equipment. Tropical tuna purse seine fishing vessels contribute to this problem by deploying large numbers of drifting Fish Aggregating Devices (dFADs), as a significant portion of these floating objects eventually end up derelict, potentially contributing to marine pollution and threatening sensitive ecosystems such as coral reefs. The general objective of this thesis is to use scientific analyses of dFAD trajectory and fishing data to propose mitigation measures to reduce these problems in the Indian and Atlantic Oceans. First, it is demonstrated that prohibiting deployments in areas most likely to lead to beachings has the potential to be effective for reducing the beaching rate. Results indicate that 21% to 40% of beachings could be prevented if deployments were prohibited in high risk areas, roughly delimited by the areas south of 8°S latitude, the Somali zone in winter, and the western Maldives in summer for the Indian Ocean, and in an elongated strip of areas adjacent to the western African coast for the Atlantic Ocean. Next, the identification of areas within the fishing ground where most dFADs exit, as well as the passage of a large number of dFADs close to ports, provides support for the implementation of recovery programs to collect these dFADs at sea and reduce their loss. These two measures appear to be complementary since areas predicted to benefit less from closures are more likely to benefit from recovery programs, particularly in the northwestern Indian Ocean and the northern Gulf of Guinea. Finally, the evaluation of a Lagrangian transport tool to simulate the trajectories of dFADs shows that the efficiency of this tool at the basin scale is relatively good in the two oceans, that the accuracy to simulate the trajectories is better in the Indian Ocean than in the Atlantic Ocean, and that this accuracy depends on the depth and the spatial resolution of the forcing currents product used. This tool could be used in an operational mode in the future to anticipate the trajectories of dFADs that could lead to loss or beaching and therefore be used as a complementary mitigation program to the other two measures described above (prohibiting deployments and recovery at sea). The results obtained during these various works thus constitute a solid basis to define new recommendations to mitigate the risks of loss and beachings of dFADs and thus contribute to the preservation of our oceans and our coasts.