Targets or Limits for Management of Fisheries?

Abstract

A simple probability framework for precautionary risk assessment is suggested for fisheries management. The approach distinguishes between the concepts of target reference points (TRPs) and limit reference points (LRPs) in fisheries management. The use of LRPs formed a basis for international concensus at the 1995 United Nations Conference on Straddling Fish Stocks and Highly Migratory Fish Stocks. We demonstrate a methodology using the example of fishing mortality (F). An LRP such as the fishing mortality at maximum sustainable yield (FMSY) is set by management as an upper acceptable limit to the rate of exploitation. The acceptable level of risk, expressed as a probability that this LRP will be exceeded, also is chosen by managers. In addition, given some level of uncertainty as to the current estimate of F (expressed as its coefficient of variation), a “safe” TRP, in this case less than FMSY, can be calculated. We explore the implications of both normal and lognormal uncertainty in setting such a target fishing mortality. At the lower acceptable risks typical of statistical tests, such as 5%, the normal and lognormal cases diverge, but when the same coefficient of variation is assumed for the uncertainty in current F and probabilities of exceeding the LRP (FMSY) are in the range of 10–30%, the two statistical distributions yield similar results. For the normal distribution of uncertainty in F (or other reference point variable), we provide a closed-form analytical solution for calculating the TRP Other distributions require numerical solutions for the TRP to be calculated under the probability integral, and we carry them out for the lognormal case. This simple and direct approach to accommodating uncertainty permits calculation of an approximate “safe” target control value for any variable used to monitor exploitation rate. Requirements are a preset LRP, an estimate of current exploitation rate and a guess at its variance, and an agreement on the acceptable risk of exceeding the LRP The approach also provides a simple way of visualizing risk analysis when fisheries management is based on limit reference points.