Abstract
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.