Sustainable Exploitation of Fish Stocks by Long-Term Profit Maximization with Special Regards to Cod in the Baltic Sea

Abstract

An important aim of fishery management is the conservation and sustainable exploitation of fish stocks whereas maximization of profit is the aim of fishermen. At first view this seems to be contradictory. The main management tools for stock exploitation are fishing effort (summarizing all effort limitations) and the so called technical measures (mesh size). The presented new model, using these control variables, is intended to replace the Gordon Schaefer model and based on the Beverton and Holt yield-per-recruit model. The Beverton and Holt model, however, do not take into account the feedback of stock-size changes on recruitment and therefore also on yield. Overcoming this disadvantage is one finding in this paper. For the computation of optimal exploitation pattern the Shepherd approach for construction of sustainable yield curves was extended by incorporating the control of mesh size along with the current exclusive control of fishing effort. This extension turns all fishing mortality reference points into reference functions of the age at first capture. The sole economic important demersal stock in the western Baltic Sea is the cod stock of that area. This paper describes an equilibrium model for the computation of long-term harvesting strategies for that stock including the control of mesh size into the well-established exploitation models. It was shown that the stock is more reliably protected against collapsing by economic considerations rather than by measures chosen according to the precautionary approach. If the Baltic cod stock is exploited according to the suggested approach with an optimized, much larger mesh size in the steady-state, a larger profit is ensured simultaneously with improved stock conservation. This model can also be applied for the maximization of the overall profit within mixed-species management by overlaying the single-species results. Here, however, it is only possible to find an overall optimum which generally differs from the single species optimum. An economic reasonable limit of exploitation is given by the FMSY-curve – the fishing mortality resulting for a given age at first capture in the maximum yield for this age while the Fpa-curve according to precautionary approach has to be considered as the final line of defense against stock collapse. The precautionary approach only calls for changes in effort limitations or technical measures if a violation of its principles has occurred in a multi-species fishery due to ecological reasons. Apart from stock protection the main objective of this paper is to find at the same time appropriate values for the management control variables defining an equilibrium maximizing the profit (maximum economic yield – MEY) of the fishery.