Productions jointes et mesures de gestion des pêcheries mixtes : application à la pêcherie chalutière langoustinière du golfe de Gascogne

The use of poor selective gears in a multi-species and multi-size ecosystem induces desired joint productions corresponding to by-catches and unwanted joint productions that are discarded. These joint-productions can worsen external effects consequences and they complicate moreover classical right-based solutions to fisheries management. A bio-economic analysis of production process using low selective gear is presented in the first part of the thesis with an application to the Nephrops trawl fishery in the bay of Biscay. The production function is spilt into a catch function and a sorting function. The analysis of the catch function highlights that all productions are not joint productions. There is a kind of joint productions control by the fishermen that is useful to fisheries management. The analysis of microeconomic sorting behaviours, from a sorting function defined with analogy to the catch function, then enables to discuss incentives to discards. Discards are considered as an output of a sorting task. In the second part of this thesis, we present a bio-economic analysis of management measures to reduce negative consequences of joint-productions. A theoric analysis of selectivity shows that social benefits of selectivity improvement depend on the level of effort in the fishery and on the costs of effort. Besides, it is shown that when selectivity is not observable, fishermen are incited to adopt non selective technique. A cost-benefit analysis of several scenarios of improving selectivity is then realized from a bio-economic simulation model applied to the Nephrops fishery. Endogenous behaviours highlight the need to combine technical measures with access regulation measures: when the fishery is non selective, access regulation is complicated to implement and improving selectivity without access regulating do not resolve overcapacity problems. The example of the Nephrops pot fishery in Loch Torridon (Scotland) then enables to discuss profitability conditions of an alternative fishing gear to trawl, pot, whose selectivity is observable and that does not produce joint productions. Profitability of a pot metier in the bay of Biscay is discussed under several conditions. Transition phases are analyzed and highlight the question of transitory subsidies to technical change. The need for a spatial management of these activities is underlined in this case.