Browsing by Subject "Simulation testing"

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    Developing and refining a joint management procedure for the multispecies South African pelagic fishery
    (Oxford University Press, 2004) De Oliveira, J A A; Butterworth, Doug S
    Pilchard (sardine) and anchovy are the main targets of South Africa's pelagic fishery. This fishery is the country's second most valuable in monetary terms, and produces the highest annual yield in terms of landed mass (in recent years, a combined catch of the order of 400 000 t). It is the most dynamic of South Africa's main commercial fisheries, because the species targeted are relatively short-lived, often occur in mixed shoals, and experience large fluctuations in abundance. Mixed shoaling causes operational problems for the fishery, because of the inevitability of juvenile pilchard bycatch (of more value as adults for canning) in the anchovy-directed fishery. This operational interaction implies a trade-off between allowable catches for the two species, and hence necessitates that they are managed together. The development of a joint “management procedure” (sensu IWC) for the two species is described. This provides a framework for quantifying this trade-off, subject to the constraint that acceptable levels of risk of “collapse” are not exceeded for either resource. Important new features incorporated in a revision of the procedure implemented in 2002, which have made appreciably enhanced catches from the resources possible, are described.
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    Simulation testing of the sabbatical model estimator for the assessment of Southern Hemisphere humpback whale breeding Stock C and its component Sub-stocks
    (2009) Johnston, Susan J; Butterworth, Doug S; Muller, A
    Johnston and Butterworth (2009) implement four models (Resident, Sabbatical, Tourist and Migrant) to estimate parameters for the C1 and C3 substocks, including the probability of interchange between them, using a Bayesian approach which takes account of capture-recapture information from photo-id data. This estimator generally captures the underlying parameter values reasonably, though with a tendency to estimate r too low and K too high. Interchange rates are also reasonably estimated, both when the true rates are low and high. In the latter case, abundance estimates in terms are lower. Here a range of Operating Models (OMs) are defined and used to test the Sabbatical estimator
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    Simulation testing of two estimators for the assessment of Southern Hemisphere humpback whale breeding Stock C and its component Sub-stocks
    (2009) Johnston, Susan J; Butterworth, Doug S; Muller A
    This paper develops operating models of the C1 and C3 substocks of humpback whales in the western Indian Ocean which allow interchange between the two on the basis of the Sabbatical model for this mixing process. These operating models are used to compare the performance of the Sabbatical and Resident estimators, in what is intended as a preliminary exercise whose primary aim is to illustrate this simulation testing approach in the context of the substocks of breeding stock C of Southern Hemisphere humpback whales., .
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    Why a management procedure approach? Some positives and negatives
    (Oxford University Press (OUP), 2007) Butterworth, Doug S
    The origin of the management procedure (MP) approach (sometimes termed management strategy evaluation), with its simulation testing of feedback-control algorithms as a necessary and structured basis for dealing with the inevitable uncertainties associated with fisheries assessments, is briefly reviewed. Also discussed are the advantages that overcome some of the difficulties of the “traditional” approach of coupling an annual “best” assessment to some harvest control rule, such as a failure to consider longer-term trade-offs properly. The MP approach does, however, also have disadvantages, such as the length of time typically required for its development and an argued inflexibility after implementation. Solutions that have been developed to overcome some of these difficulties are discussed.