Abstract
Due to their highly migratory nature, the management of tuna stocks is done at the global ocean scale favouring local management specific to each country. Regional fisheries management organizations (RFMOs) such as the Commission for the Conservation of Atlantic Tunas (ICCAT) have been created in each ocean to coordinate fishing statistics among fishing countries and stock assessment work. As the first tuna port in West Africa, Côte d'Ivoire is witnessing intensive exploitation, even overexploitation, of the three main species of tropical tunas (yellowfin, skipjack and bigeye) present in the waters of its EEZ by industrial offshore fleets of different nationalities. By definition, stock assessments are carried out at the Atlantic Ocean scale by ICCAT and little information is produced to characterize the dynamics of local abundance over time and measure the consequences of the state of exploitation on this local resource. An approach based on the estimation and characterization of the fluctuations of the local tuna resource is necessary, and this cannot be done without a good understanding of the local and global factors that condition the presence of the species in the Ivorian EEZ and their migratory behaviour with the rest of the Eastern Atlantic. This thesis is part of a general context of the management relationship from global to local known as "glocalization".To achieve these objectives, we estimated the local tropical tuna resource of the Côte d'Ivoire EEZ using a seasonal spatio-temporal model (VAST) developed by Thorson et al. (2020a) applied to European (Spain and French) purse seiner CPUE. The indices relating to the abundance of this resource allowed the characterization of its seasonal and interannual dynamics over a 20-year study period. As tropical tunas are migratory species, in addition to the analysis of the seasonality of the local abundance, we analyzed their migration phenology dynamics through the Côte d'Ivoire EEZ. The other analyses allowed us to study the effects of internal (environmental conditions) and external (level of abundance and global exploitation) factors on the dynamics of this local resource. By highlighting the dependence of local resource fluctuations on both local and global factors, this study reinforces the idea of combining global management measures with local efforts ('glocalization') to optimize the management of Côte d'Ivoire's tuna resources, as well as those of coastal countries around the world.Furthermore, the ICCAT Atlantic Tropical Tuna Tagging Programme (AOTTP) provided an opportunity to actively participate, in parallel to the main objective of this thesis, in the estimation of some parameters characterizing the uncertainties associated with catch-recapture data. Therefore, this component should be seen as an overview of the implications and active local contributions to research programmes conducted at global scales and as a prerequisite for successfully managing "glocalized" tuna fisheries, i.e. combining this transfer of spatial scale (from global to local).