1 SUMMARY In the lack of direct estimate of tuna abundance, the French tropical tuna fleet of purse-seiners provides a way to monitor the tuna stocks particularly through the calculation of Catch per Unit of Effort (CPUE). The objective of this study was to analyze the individual track of purse seiners with the aim to identify which part of the searching behaviour should be used to perform the fishing effort. In this context, Vessel Monitoring Systems (VMS) data are useful to analyse the different fishing activities of a purse seiner at a small scale (i.e.. stop, track and cruise), generally embedded into the concept of fishing effort. A state-space model (run in a Bayesian framework) was applied to speeds and turning angles from Vessel Monitoring Systems (VMS) data, to identify the different "states" of the fishing behaviour of a purse seiner over a fishing trip. Then a threshold was fitted to distinguish the activities "fishing" and "stop" without fishing (from the "still" state), "tracking" (from "meander"), and "cruising" (from "straight"). On a subset of VMS data corresponding to observers' data, some differences of fishing efforts between fishing on free-swimming schools and schools associated with FAD (Fish Aggregating Devices) are highlighted. RESUME Etant donnees les difficultes d'estimation directe de l'abondance de thon, la flottille francaise de thoniers senneurs tropicaux offre la possibilite de surveiller les stocks de thon, particulierement grâce au calcul de Capture par unite d'effort (CPUE). L'objectif de cette etude etait d'analyser la trajectoire individuelle des thoniers senneurs, dans le but d'identifier quelle parties du comportement de recherche doivent etre utilisees dans le calcul de l'effort de peche. Dans ce contexte, les donnees de Vessel Monitoring Systems (VMS) sont utiles pour analyser les differentes activites de peche d'un thonier senneur a fine echelle (c'est-a-dire arret, recherche active, route-recherche), generalement inclus dans le concept d'effort de peche. Un modele a espace d'etat (avec une approche bayesienne) a ete applique aux vitesses et angles de changement de direction issus des donnees de Vessel Monitoring Systems (VMS) pour identifier les differents etats du comportement de peche d'un thonier senneur pendant une maree. Ensuite un seuil a ete ajuste pour distinguer les activites de « peche » et d' « arret » sans peche (a partir de l'etat « immobile »), « recherche active » (a partir de « sinueux »), et « route-recherche » (a partir de « tout droit »). Sur un sous-jeu de donnees VMS correspondant aux donnees Observateurs, certaines differences d'effort de peche entre bancs peches sur Bancs Libres et ceux peches sur bancs associes aux DCP (Dispositifs de Concentration de Poisson) sont mises en evidence. RESUMEN
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