MULTI-FLEET NON-EQUILIBRIUM PRODUCTION MODELS INCLUDING SURFACE TO ASSESS TUNA STOCKS DYNAMICS.

Production models have proven for long to be very useful assessment tools. But two main problems make difficult their use for investigating the status of tropical tuna stocks. The first problem is the calculation of the effective fishing effort. Indeed, many different fleets with heterogeneous and changing catchabilities are often exploiting the same population. Consequently, scientists in charge of tropical tunas stock assessment are seldom in a position to estimate accurately a standardized effective fishing effort (e.g. an effort proportional to the fishing mortality) which is targeting the considered stock. The other important problem concerning the use of production models for tropical tunas stock assessment is the fluctuation of the overall size of the exploited area. Indeed, for tuna fisheries, the estimated production curve and its associated MSY are closely linked to the exploited surface and the stock biomass located in the fished area interacts more or less strongly with a “cryptic ” part of the population located outside the fishing area. In this paper, our goal is to address both categories of problems by formulating different multi-fleet non equilibrium production models incorporating or not the fished surface. A maximum likelihood approach is provided to estimate the models parameters in a bayesian context. Once parameters have been estimated, the model can be used to estimate the overall effective effort and the stock status. The comparison of different formulations of the model may help to better understand the fishery dynamics