The design of a rebuilding strategy for southern buefin tuna: from theory to practice

PARMA, A.M.

Warnemünde/Rostock, Alemania

Simposio; ICES/PICES/UNCOVER Symposium on Rebuilding Depleted Fish Stocks –Biology, Ecology, Social Science and Management Strategies; 2009

ICES-PICES

Although the severe depletion of the southern bluefin tuna stock was long recognized and a rebuilding target was established by the Commission for the Conservation of Southern Bluefin Tuna in 1994, no agreement about quotas could be reached for many years. A plan to design a management procedure was finally initiated in 2002, which involved scientists from member countries proposing and simulation-testing decision rules using software developed by the Scientific Committee. The process shifted attention away from inconclusive arguments about abundance estimates towards discussion of testing protocols, uncertainty axes to explore in stock projections, plausibility of alternative hypotheses, and trade-offs between robustness and optimality in the selection process. Candidate decision rules established by the Commission for the Conservation of Southern Bluefin Tuna in 1994, no agreement about quotas could be reached for many years.A plan to design a management procedure was finally initiated in 2002, which involved scientists from member countries proposing and simulation-testing decision rules using software developed by the Scientific Committee. The process shifted attention away from inconclusive arguments about abundance estimates towards discussion of testing protocols, uncertainty axes to explore in stock projections, plausibility of alternative hypotheses, and trade-offs between robustness and optimality in the selection process. Candidate decision rules were tuned to achieve a range of rebuilding levels to facilitate comparisons. The philosophy was that the Scientific Committee would evaluate consequences of alternative rules, and the Commission would define the operational goals to be achieved.In practice, several iterations were needed to present management choices in terms of attainable goals and trade-offs. A consensus recommendation was made in 2005 which called for an immediate reduction of catches followed by adoption of a management procedure with an associated high probability of stock rebuilding. The first substantial quota cut in many years was taken, but the management procedure was not implemented because substantial under-reporting of historical catches became apparent, which warranted reexamination of the models. Lessons learned should facilitate the next round of simulation-testing, which is about to start. The main difficulties remain in the need to have reliable indicators of stock trends to drive decision rules.