Sustainable recruitment: the bottom line

This paper develops a single-species perspective on the limits of sustainable fishing. Given uncertain data and assessments, the problem is to determine lower confidence limits for the minimum fishing mortality rate F-tau that, if maintained indefinitely, would drive the stock to extinction. For non-depensatory stocks, F-tau is determined by the slope of the stock-recruitment curve at the origin. The paper reviews three different approaches to making inferences about F-tau: recruit-per-biomass relationships (e.g. F-high), parametric curves (e.g. Ricker), and nonparametric curves (e.g. LOWESS fits; Cook 1998). Of these, nonparametic curves are potentially the best, but existing implementations suffer from operational difficulties and do not always give biologically sensible fits. These problems can be circumvented by two new nonparametric methods, CONCR (Concave Recruitment) and DIMPOS (Diminishing returns, Positive recruitment), which are described and applied to several ICES stocks. Finally, the paper explores issues surrounding the use of sustainable recruitment measures in assessment and management.