Defining viable recovery paths toward sustainable fisheries

This paper develops a formal analysis of the recovery process for a fishery, from crisis situations to desired levels of sustainable exploitation, using the theoretical framework of viable control. We define sustainability as a combination of biological, economic and social constraints which need to be met for a viable fishery to exist. Biological constraints are based on the definition of a minimum resource stock to be preserved. Economic constraints relate to the existence of a guaranteed profit per vessel. Social constraints refer to the maintenance of a minimum size of the fleet, and to the maximum speed at which fleet adjustment can take place. Using fleet size adjustment and fishing effort per vessel as control variables, we first identify the states of this bioeconomic system for which sustainable exploitation is possible, i.e. for which all constraints can be dynamically met. Such favorable states are called viable states. We then examine possible transition phases, from non-viable to viable states. We characterize recovery paths with respect to the time of crisis of the trajectory, which is the number of periods during which the constraints are not respected. The approach is applied to the single stock of the bay of Biscay Nephrops fishery. The transition path identified through the viability approach is compared to the historical recovery process, and to both open-access and optimal harvesting scenarios.