Length-based empirical estimation of SPR for small-scale, data-poor fisheries

Despite their significant contribution to global yield, and considerable importance to the social and economic wellbeing of many millions of people, small-scale fisheries have largely been overlooked in the development of conventional fisheries science. Many of the modern quantitative methods and management approaches, including harvest policies and quota systems, require a level of information simply beyond the capability of most fisheries. It is becoming increasingly apparent that in addition to the vast array of complex stock assessment methods, there is a need for simple, cheap procedures based on local indicators collected directly from the catch to make assessment and management feasible in small, cash-poor fisheries. In this study, we investigate the use of a length based model, parameterised with empirical knowledge and life-history invariance theory, to estimate the Spawning Potential Ratio (SPR) of exploited fisheries directly from a population’s size composition. Key parameters derived empirically for the model are: the ratio of natural mortality to the von Bertalanffy growth parameter (M/k), L&, and an estimate of size at maturity. Simulation modelling is used to test sensitivities of these parameters, and the preliminarily results from two case studies are presented. The method compares the length frequency of the catch to the expected equilibrium length frequency for different levels of SPR, and uses a control rule to adjust fishing effort (a proxy for F) to stabilise the fishery around the target SPR. This method has potential to provide a tool for rapid, cost-effective, and conservative assessment of data-poor fisheries.