In the case of fisheries, the lack of an efficient property rights structure (uniform, exclusive, and transferable property rights) is widely recognized as the prime impediment to higher profitability of the fishing industry and to the prevention of over-exploitation. This study uses an empirical model for measuring the economic efficiency implications of the competitive exploitation of a commonly shared fishery. While lack of data prevents the empirical application of theoretical state-of-the-art fishery models, the specification of the model used here does not prevent an analysis of the central efficiency question. Within a 'surplus-production' model of the North Sea fishery, economically optimal levels of fishing effort, cost, revenue, and rent are estimated. These are compared with activity levels that are considered optimal in the biological sense. The paper begins with a review of the optimality conditions. Section II presents an empirical model, whose application to the multi-species North Sea fishery is discussed in Section III. Section IV presents empirical results.