Browsing by Author "Jacobs, Melissa"

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    Further exploratory investigations of some simple candidate management procedures for southern bluefin tuna
    (2018) Butterworth, Doug S; Miyagawa, Mitsuyo; Jacobs, Melissa
    Target type CMPs for SBT presented to the Seattle meeting are refined and extended to include the input of CKMR information through use of the associated TRO index developed by Hillary. Essentially CMPs, tuned to median recovery to 30% of the pristine TRO in 2035 are developed for each of the CPUE, gene tagging and CKMR data sets alone, and then weighted combinations of these are considered. A subset of the robustness tests which show the gre atest differences in performance compared to the base/reference c ase (RC) OM are selected. Overall the CMP based on the CKMR data only seems to perform best for the RC, but when the selected robustness test results are also taken into account, a variant based on a weighted combination of CMPs using all three data types seems marginally preferable.
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    Initial exploratory investigations of some simple candidate management procedures for southern Bluefin tuna.
    (2018) Butterworth, Doug S; Miyagawa, Mitsuyo; Jacobs, Melissa
    Simple constant proportion and target based empirical candidate management procedures are applied to the basic grid operating model and a low recruitment robustness test for SBT . The first two approaches , DMM1 and DMM2 , respectively use CPUE index data only, while DMM3 ad ds gene tagging data to the DMM2 approach. The key results are that the DMM2 target based approach substantially outperforms the constant proportion DMM1 one in terms of smoothness of the TAC trajectories, and that (at least as far as investigations have b een possible to date) the addition of gene tagging data offers little improvement to depletion statistics in instances where low recruitment has occurred . Performance under DMM2 is unusually good, but this approach still needs to be subjected to the other robustness tests, and further attempts need to be made to seek more improvement in performance when gene tagging data are used.
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    The implementation of a management procedure approach to set catch limits for the Southern Bluefin Tuna and the Atlantic Bluefin Tuna
    (University of Cape Town, 2020) Jacobs, Melissa; Butterworth, Doug S
    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.