Yield–mortality models: a precautionary bioeconomic approach

Abstract

This paper develops a bioeconomic approach to yield–mortality models. The usefulness of the conceptual background of these models, and the application of the resulting reference points (RPs), are analysed in a precautionary management framework. Quantification of uncertainty by bootstrapping provided a more realistic comparison of the relative performance of RPs for management advice. Sensitivity analysis, performed under wide initial variations in the natural mortality coefficient M, displayed two important features of this bioeconomic model: (1) the yield at maximum economic yield ( Y MEY) and the associated mortality levels Z MEY and F MEY were always the most conservative RPs. This takes utmost importance at low M values, where maximum sustainable yield (MSY) and the yield at maximum biological production ( Y MBP) tended to coincide, whereas MEY and mortality-related RPs were substantially lower, thus constituting useful target RPs for precautionary management, (2) The RPs derived from the biological production curve were always lower than those derived from the sustainable yield curve. The former could be used as target RPs, whereas both MSY and F MSY should be considered as upper exploitation limits. Sensitivity analysis conducted with the economic input of the system, the unit cost of effort ( c), revealed consistency of the bioeconomic performance of the model with accepted theory, i.e., Y MEY approaches MSY with decreasing c values. A simple approach to the formulation of risk-averse management strategies was explored using decision theory. For this purpose, the maximax, maximin and minimax regret criterion were used to evaluate alternative management decisions under uncertainty, without the need for explicit statements of probabilities on alternative hypotheses. Results confirmed the need to consider economic and MBP related RPs for precautionary management. Guidelines for future work are suggested.