Abstract
Representing the majority of the world’s tuna catches, tropical tuna species are of critical importance due to their essential role as food and economic resource. The sustainable management of this valuable resource depends on an accurate estimate of the abundance of the exploited populations and the impact of fishing pressure on them. The present thesis provides a new direct abundance index for tropical tuna populations that account for their free-swimming and associated components. Indeed, tropical tuna species are characterized by a singular behavioral trait that causes them to associate with floating objects drifting at sea. This characteristic has led to the development of a specific fishing mode widely used in tuna purse seine fishery, consisting in the capture of schools associated to floating objects. Recent decades have thus seen the massive deployment of thousands of floating objects known as fish aggregating devices (FADs), specifically designed to attract and concentrate tuna schools. The drifting FADs are equipped with satellite-linked echosounder buoys, which ensure their continuous monitoring, providing fishers with near-real time information on their location and associated tuna biomasses. This thesis presents a standard methodological framework for processing the information from echosounder buoys for scientific use, including a new approach based on supervised learning for processing the acoustic data they provide. The analysis of these data has allowed improving the general knowledge on the associative dynamics of tuna aggregations. Ocean-specific differences were evidenced, with notably longer periods of absence of tuna under FADs in the Indian Ocean than in the Atlantic Ocean. The novel index for estimating tuna abundances proposed by this thesis also exploit this associative behavior. It relies on a modelling approach combining data on the dynamics of the occupancy of floating objects from echosounder buoys with data on the associative dynamics of tuna individuals from electronic tagging. An initial application to skipjack populations in the Western Indian Ocean has made it possible to provide time series of absolute and relative abundances, used for stock assessments of this species. This new index addresses the current critical need for complementary methods for estimating tropical tuna abundances, expressed by all regional fisheries management organizations.