Browsing by Author "Edwards, Charles T T"

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    Development of a boundary setting algorithm based on migration rates estimated using BayesAss and its preliminary application to TOSSM datasets
    (2007) Edwards, Charles T T; Butterworth, Doug S
    A Boundary Setting Algorithm (BSA) is developed that utilises migration rates estimated by the program BayesAss. It is subjected to preliminary testing through application of the BSA to a subset of TOSSM datasets from Archetypes I and II. Two Fully Internally Mixed Areas (FIMAs) are considered, connected by varying rates of migration. The objective of the BSA, implemented through the run.tossm control program, is to decide whether to manage FIMAs as one or two units. Performance is assessed using the depletion and catch statistics produced by run.tossm. Although BayesAss has the potential to provide information relevant to management, performance of the BSA was found to be poor, apparently due to inconsistent and unreliable estimation of the migration rate.
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    Investigating the consequences of Marine Protected Areas for the South African deep-water hake (Merluccius paradoxus) resource
    (Oxford University Press (OUP), 2009) Edwards, Charles T T; Rademeyer, Rebecca A; Butterworth, Doug S; Plagányi, Éva E
    Protected areas are often touted as important management tools to mitigate the uncertainty inherent in marine ecosystems, and thereby improve the long-term prospects for sustainable resource use. However, although they certainly play an important role in conservation, their usefulness in improving fishery yields is contentious. We present a simulation model that explores spatial closure options, and apply it to the demersal hake trawl fishery off South Africa. The model is based on the age-structured approach used for current assessments, representing the dynamics of the deep-water hake Merluccius paradoxus within a zonally disaggregated spatial system. Fitting the model to two zones, which demarcate a potential closed area from the remaining fished area, we investigate the consequences that such a protected area could have for the fishery. Our model suggests that area closures would have a negligible benefit for the fishery, regardless of the level of hake movement between areas. This is likely the result of the model's simplicity, and we suggest additional factors that should be considered to quantify the impact of Marine Protected Areas on the fishery more reliably.
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    Participatory assessment of the South African abalone resource and its impact on predicted population trajectories
    (Academy of Science of South Africa, 2008) Edwards, Charles T T; Plagányi, Éva E
    Illegal harvesting is a cause for concern in many of the world's fisheries. Over the last decade, the abalone resource in South Africa has come under severe fishing pressure, largely because of increased and unmitigated levels of poaching. The unquantified illegal exploitation of this resource is a major impediment to management, because understanding of abalone population dynamics is affected. Incorrect assessments of population abundance could lead to inadequate attempts by management to stem the decline. Here, population trends along the west coast of South Africa are investigated. A simple discrete-time logistic model was used to estimate parameters within a maximum likelihood statistical framework by fitting to available catch rate data. To address the problem of unknown levels of illegal catch, interview data were collected on non-commercial catch trends and the model was structured to allow this catch to be estimated during the fitting process. The results show that such a participatory approach to stock assessment can lead to an improved understanding of resource dynamics, illustrating the benefit this approach may have for management.
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    Preliminary data for the stock assessments of lobster fisheries around Inaccessible, Gough and Nightingale islands
    (2008) Edwards, Charles T T; Butterworth, Doug S
    This document presents the catch and nominal catch per unit effort (CPUE) data available for the lobster (Jasus tristani) fisheries on the outer islands (Inaccessible, Gough and Nightingale). Data sources, calculation of the CPUE indicies and changes to the fishery that may impact consistency of the CPUE time series are also briefly described.
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    Reconciliation of data from the lobster fisheries on Inaccessible, Nightingale and Gough Islands
    (2007) Edwards, Charles T T; Glass, James P
    For the years between 1997 and the present, a number of different data sources exist for the Tristan lobster (Jasus tristani) fisheries on the islands surrounding Tristan da Cunha . In an accompanying document (MARAM/Tristan/07/Dec/05), the sources of fisheries related data for Inaccessible, Nightingale and Gough, and the limitations of that data are described in detail. Catch and effort values from these sources had been shown previously to exhibit major differences. Here we relate progress made on reconciling these differences.
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    Towards standardisation of the longline CPUE series from the lobster fisheries around Inaccessible, Nightingale and Gough islands
    (University of Cape Town, 2008) Edwards, Charles T T; Butterworth, Doug S
    Two CPUE indicies exist for the tristan lobster (Jasus tristani ) fisheries surrounding Inaccessible, Nightingale and Gough islands. Here the nominal indicies are presented, alongside descriptions of how they will be standardised to ensure they each provide a more accurate index of population density.