Browsing by Author "Butterworth, Douglas"

Now showing 1 - 7 of 7
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    An approach to weighting the various Operating Models in a Reference Set in inverse relation to the similarity of their results
    (2017) Vrancken, Candysse; Butterworth, Douglas
    How to weight results from different OMs in getting a “best” representation across their differing results is a problem not only in fisheries but also in Climate Change. We attempt here to borrow an approach from the latter field and compare it to the conventional likelihood (or AIC) basis sometimes used in fisheries, which gives higher weights to models in a Reference Set (RS) that fit the data better. In contrast, a major problem perceived in Climate Change analyses, when averaging over an ensemble of models, is how to avoid “bias” through including too many models which scarcely differ amongst each other – one therefore downweights models on the basis of “nearness” of their results to each other. Here we apply a multi-dimensional scaling (MDS) approach which has been applied for weighting different Climate Change models to the RS developed for selecting the current OMP used to recommend TACs for the South African hake fishery. It is found that the MDS and AIC weights are very different, which begs the question of how then to “average” across these two distinct bases for model preference to perhaps obtain some combined weight. It was nevertheless found that in all the cases considered, the weighted RS provided a higher spawning biomass projection for M. paradoxus than the equally weighted RS used to select the current hake OMP in 2014. This suggests that, had some unequal weighting approach been used in 2014, it might have led to a slightly less conservative OMP, which allowed for greater catches to be taken, being selected.
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    Claim by Pichegru et al. that marine no-take zone benefits penguins remains premature
    (2010) Butterworth, Douglas; Brandao, Anabela; de Moor, Carryn; Robinson, William
    Pichegru et al. (2010), and a follow up comment by Ryan et al. (2010), make strongly stated claims that closures to pelagic fishing around islands with penguin breeding colonies lead to rapid improvements in those penguin populations. These claims are contested for a variety of reasons, including problems with the analysis method used. Claims that the pelagic fishing stocks are in poor condition, and that this is because of an absence of spatial management, are also contested, as are some other sweeping statements. The data currently available are too limited to be able to confirm or deny the claims made at this stage.
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    Developing informative prior distributions for the bias of hydro-acoustic survey estimates of the biomass of the South African round herring population
    (2022) de Moor, Carryn; Coetzee, Janet; Butterworth, Douglas
    Prior distributions for the bias of the hydro‐acoustic survey estimates of the biomass of the South African round herring are developed by considering the bias associated with the individual sources of error
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    Effect of missing values from the Canadian spring and fall surveys of NAFO Divisions 3LNO on the calculation of the TAC using the Greenland halibut HCR
    (2022) Regular, P M; Butterworth, Douglas; Rademeyer, Rebecca
    To test the impact of ignoring recent missing abundance indices for Greenland Halibut in NAFO divisions 2+3KLMNO on applying the accepted HCR for this population to provide a TAC recommendation for 2023, the impact of similar exclusions in the past is examined and found to be small. To further test of the impact of the missing 2021 index from the Canada Fall 3LNO survey, a range of pessimistic to optimistic abundance index values were assumed to assess the plausible range of impact this one value might have on the TAC computation. The range of the resultant TACs is small, and the difference of the impact of TACs at either end of this range on exploitable biomass projections for the next year is found to be negligible. Hence, it is argued, the minimalist and straightforward approach of simply ignoring the missing 2021 Canadian Fall 3LNO result in the four-survey version of the HCR agreed last year would be a defensible and appropriate approach to the required adjustment of the implementation of this HCR to provide a TAC recommendation for 2023.
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    On the development of a roadmap for new formal harvest strategies for SIOFA
    (2022) Butterworth, Douglas
    The Consultants past experience with conducting assessments of and/or providing management advice for SIOFA fish stocks has indicated that a key problem has been the lack of background information on the data available and how they relate to the way the fishery operates. That missing information is a key input to the assessment process, and its ability to provide reliable results. The International Whaling Commission’s “harvest strategy roadmap” is reviewed. Their first step for any stock of a “pre-assessment” process to compile the data to be used in the harvest strategy analyses and how they should be interpreted, is suggested to be an essential component of any similar SIOFA roadmap. This process should be put into practice by the appointment, for any stock for which a harvest strategy is to be developed, of a Technical Sub-Committee which would meet separately from the SIOFA Scientific Committee and report back to it. This Sub-Committee would include persons with the relevant expertise about the stock to provide this missing information and to develop ToR’s for the basis on which the harvest strategy development should proceed. Overview comments are provided about the process that would then follow. An important decision to be made is whether the harvest strategy for a specific stock is to be based on the “best assessment plus harvest control rule” approach or on Management Strategy Evaluation (MSE). A table is provided summarising the details associated with this “Technical Sub-Committee” pre-assessment component of a harvest strategy development roadmap.
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    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.
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    Summary of issues pertinent to a decision on possible adjustment of the maximum directed sardine catch allowance west of Agulhas for 2016
    (2016) Coetzee, Janet; Butterworth, Douglas; van der Lingen, Carl; de Moor, Carryn
    This paper summarises proposals on the many and varied issues that need to be addressed in reaching a decision on a possible increase in the maximum directed sardine catch that may be taken west of Cape Agulhas during 2016. It also includes a list of associated issues on which further information is required, both for the short and the longer term.