An initial evaluation of management strategies for the southern bluefin tuna fishery

Abstract

Within the Commission for the Conservation of Southern Bluefin Tuna there has not been agreement on a mechanism for adjusting quotas for the resource. It has been proposed that development of a decision-rule-based management strategy is an issue of high priority, indeed a matter of urgency. Results are presented from an initial application of a Monte Carlo management strategy evaluation (MSE) approach aimed at developing a rebuilding strategy for the stock. Southern bluefin tuna (Thunnus maccoyii) provides a substantial test of the power of this approach to develop a decision framework that can cope with large uncertainties about the current state of the stock and conflicting information from the fishery that is used in stock assessment. The results indicate that a robust feedback decision-making framework can be developed which ensures fast rates of recovery across the broad range of interpretations about stock dynamics and current status of the resource. Among the harvest strategies examined, the F0.2, F0.1, and SSB/R strategies were clearly the most robust and consistent in terms of recovery and sustainability. Fast recovery rates were only achieved with strategies that had catches substantially below 1998 levels. Within the strategies examined, little scope exists for improvement in yield without a direct loss in recovery performance. For the MSE approach to succeed, the operating, sampling, and conditioning model components must incorporate the full uncertainty about the real stock and the sampling processes appropriately. There is clearly scope for improving and broadening the way uncertainties have been handled in this initial examination. In particular, uncertainties associated with the catch-at-age, the relationship between the c.p.u.e, and true abundance, and the conditioning of the historical population sizes in the face of inconsistent historical data, need further development. As the MSE approach requires conditioning to historical data, full and appropriate incorporation of uncertainty into stock assessment and conditioning would lead to a better basis for developing and evaluating management procedures that will work. (C) 1999 International Council for the Exploration of the Sea.