Implementing Harvest Rate Objectives by Directly Monitoring Exploitation Rates and Estimating Changes in Catchability

Optimal harvest rates for a species can be roughly approximated from longevity and age at entry. Systems that estimate stock sizes and set a total allowable catch based on an optimal harvest rate are vulnerable to errors in estimating current stock size. The approach of regulating fishing effort by limiting number of fishing days assumes that fishing mortality is proportional to fishing effort and often fails because catchability changes over time. A third approach, independent of stock-size estimates, is to monitor exploitation rates and calculate catchability coefficients as a function of fishing mortality rate and fishing effort. Tracking changes in catchability over time requires a direct estimate of exploitation rate, for example from tagging programs, area-swept estimates, or monitoring of changes in total mortality from mean body size and/or age-composition data. We describe 'closed-loop' simulation models for evaluating management systems that rely on annual estimates of stock size and for those that monitor exploitation rates. Simulation studies suggest that intensive tagging programs, or extensive tagging combined with annual survey data, are a more cost-effective and risk-averse method of fisheries management. Survey information for fisheries that lack time-series data are greatly improved when combined with direct estimates of exploitation rate from tagging data.