The implementation of a management procedure approach to  set catch limits for the Southern Bluefin Tuna and the  Atlantic Bluefin Tuna

Jacobs, Melissa

The implementation of a management procedure approach to set catch limits for the Southern Bluefin Tuna and the Atlantic Bluefin Tuna

Master Thesis

2020

Publisher

University of Cape Town

Department

Department of Mathematics and Applied Mathematics

Faculty

Faculty of Science

Abstract

The aim of this thesis is to develop, test and evaluate candidate management procedures (CMPs) for the implementation of the Management Procedure (MP) approach to set catch limits for two bluefin tuna species: Southern bluefin tuna (SBT - Thunnus maccoyii ) and Atlantic bluefin tuna (ABFT - Thunnus thynnus). The thesis reviews possibilities and advances proposals for the implementation of this approach in two international tuna Commissions: the Commission for the Conservation of Southern Bluefin Tuna (CCSBT), and the International Commission for the Conservation of Atlantic Tunas (IC- CAT) in relation to stocks of bluefin tuna. These approaches aim to provide a simple and widely agreed way to calculate annual catch limits that will lead to meeting fishery management objectives such as attaining or maintaining reasonably large catches in the future, while at the same time avoiding reduction of the resource's abundance to a level which would put the stock and its future productivity at risk. The SBT component leads to the development, simulation testing and comparative analysis of five final candidate management procedures. The first three, called DMRMCPUE, DMRMGT and DM- RMCKMR, each use only CPUE, gene-tagging (GT) and close-kin mark-recaptures (CKMR) indices of abundance respectively. These are followed by DMRcomb1 and DMRcomb2, which are weighted combinations of the first three CMPs. Each CMP is tuned to two different recovery objectives set by the CCSBT: to achieve a median spawning stock biomass (SSB) which is either 30% of its pristine value by 2035, or 35% of this value by 2040. This must be achieved over a weighted set of different Operating Models (OMs) for the resource, which serve as a reference set. Each CMP is applied to the reference set (basel8), a CPUE variable squares robustness test (cpuew0) which reflects a resource of worse current status, and a low recruitment robustness test (reclow5) which includes an extended period of poor recruitment in the future. Simulation testing of DMRMCPUE, DMRMGT and DMRMCKMR, together with an equally weighted combination MP DMRcomb1, indicated the need to focus on improved levels for the lower percentile for SSB depletion, and that this can be achieved by placing a higher weight on the GT component in a combined CMP. The DMRcomb2 then does this by placing a 60% weight on the GT component of the MP, and 20% weight on the CPUE- and CKMR-components each. Even though this results in lower stock risk, it also leads to lower catch limits which is a common trade-off. Subsequent to these analyses, a final MP for SBT was chosen. This CMP was selected on a broader (international) basis where four final CMPs were evaluated and compared, with DMRcomb2 being one of these. These four final CMPs are compared briefly. Although the performance of DMRcomb2 compared favourably with the others, eventually a Management Procedure (MP) called RH13 developed by an Australian scientist was preferred. The primary basis on which RH13 was considered to outperform DMRcomb2 was that it provided a greater probability of catches and SSB continuing to increase after the 2035 recovery target had been attained.

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