ICCAT SCRS_2019_130.pdf (939.55 kB)

Application of “fixed proportion” candidate management procedures for North Atlantic bluefin tuna using operating model package version 5.2.3.

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posted on 12.03.2021, 08:20 by Doug Butterworth, Melissa Jacobs, Rebecca A Rademeyer, Mitsuyo Miyagawa
Two adjustments are made to the simplest form of the fixed proportion CMPs developed earlier. Caps are placed on the TACs for both the West and the East area so as not reduce resource abundance unduly in circumstances where regime shifts occur. In addition, the TAC for the West area can be reduced further if an index, based on results from the Gulf of Mexico larval survey, drops below a specified threshold; this is necessary to prevent undue depletion in circumstances where the current abundance of the stock of tuna of western origin is low. Results for two variants of this new CMP (FXP_1 and FXP_2) are presented for the interim grid and primary robustness test Operating Models (OMs) (OM1-OM15 of Package version 5.2.3). These reflect more and less conservative approaches, and are intended as initial examples of this form of CMP; they are NOT intended as final candidates. Rather their purpose is to provide rough initial bounds on what variants might ultimately be considered to provide acceptable CMP performance. The results point to the importance of the assignment of plausibility to the scenario reflected by the primary robustness test of lower current western stock abundance. Assigning high plausibility to this can necessitate a reduction in average annual catches of some 1,000 t in the West area and about 10,000 t in the East area (at least as far as control rule parameter variants have been able to be explored – this has certainly been a limited exercise only to date). Results for further robustness tests are provided in an Annex. Suggestions for further exploration of control rule variations are made.

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Department of Mathematics and Applied Mathematics, University of Cape Town