Browsing by Subject "Atlantic bluefin tuna"

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    Open Access
    Can the wide range of resource behaviours evident across the ABFT MSE interim grid of OMS be “tamed” by the feedback control provided by a CMP?
    (ICCAT, 2020) Butterworth, Doug; Rademeyer, Rebecca
    The interim grid of OMs is used to explore the 30-year projection behaviour for catches and the status of the eastern and western ABFT stocks (expressed in terms of their abundance relative to dynamic Bmsy by the Br30 statistic) for both constant future catches and some simple “Fixed Proportion” CMPs. If current TACs continue unchanged, both stocks are rendered extinct for about 20% of the 96 OMs of the interim grid. Fortunately, however, this undesirable feature can be “tamed” through the feedback control mechanism of the CMPs, which can prevent occurrences of extinction. The trade-off between catches and final abundance across the OMs, as the control parameters of the CMPs are varied from lower to higher harvesting intensities, are illustrated. The need to focus on CMP refinement to reduce the spread of the Br30 distributions across the OMs is stressed. Priorities for future work are listed; these include the use of these CMP results to indicate which of the uncertainty axes in the current grid have the greater impacts on MP performance. The interim grid provides a useful framework to continue this work, even though (together with advice on final management objectives and desired trade-offs) it still awaits finalisation.
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    Open Access
    Refinements of the BR CMP as of April 2022
    (2022) Butterworth, Douglas; Rademeyer, Rebecca
    The BR CMP is adjusted in a few respects, most importantly by allowing limited temporal dependence in the values of the control parameters over the first few years of management, to allow for smoother transitions in the TACs from 2022 to 2023. This was necessitated especially by the now higher West area TAC for 2022 included in the updated package. Results are provided for the four basic development tunings, plus one variant for one of those tunings where the default maximum TAC decrease constraint is reduced from 30% to 20%. Suggestions are made of areas for possible improvement in performance, which would require some further refinements of this CMP.