Evaluating empirical indicators and reference points for fisheries management: application to the broadbill swordfish fishery off eastern Australia

Calls to use target, limit and threshold reference points to achieve precautionary management goals have increased substantially in recent years. For many fisheries there is little chance of conducting formal stock assessments, and indicators of stock status have to be based on changes in measurable quantities such as catch rates and percentiles of the length distribution of the catch. Use of such empirical indicators for management decisions also requires a choice of appropriate reference or trigger levels for the indicators. Monte Carlo simulation can be used to examine the performance of alternative indicators and associated reference points in terms of their ability to correctly identify the biological conditions that they were designed to measure. This study examines the performance of several potential empirical indicators for broadbill swordfish off eastern Australia, taking into account uncertainties in stock structure, current status, and future trends in fishing effort. Indicators based on catch rates are shown to be potentially very misleading. In contrast, indicators based on the mean length or mean weight of the catch perform better, because these quantities change in a more predictable manner with abundance. Nevertheless, reference points based on these quantities are frequently triggered either too early or too late.