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Results for initial explorations of simple candidate “fixed proportion” MPs for Atlantic bluefin tuna based on the operating models package circulated.

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posted on 20.09.2021, 09:46 by Doug ButterworthDoug Butterworth, Mitsuyo Miyagawa, Melissa JacobsMelissa Jacobs
In an initial exploratory exercise, simple fixed proportion MP control rules are applied using composite abundance indices for the East and West areas, where these composites take weighted averages over standardised values of the agreed indices and are then averaged over the last three years for which they would be available. These candidate MPs (CMPs), which also impose a 20% cap on biennial TAC changes, show ready ability to achieve median depletion close to the MSY spawning biomass for each stock within a 30-year projection period for a number of members of the Reference Set of Operating Models (OMs). Two insights from the analyses are first that discussion is needed regarding the most appropriate statistic to use to measure resource depletion in circumstances where some OMs allow for changes in stock recruitment relationships at some time during the projection period considered. The second is that resource depletion can at times be too great for the OM for which the historical abundance of the East stock shows a large increase over recent years. Typical TAC changes are also greater than desirable for adequate stability from an industrial viewpoint. Suggestions are made for further work towards improving MP performances in these respects.

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Department of Mathematics and Applied Mathematics, University of Cape Town