Browsing by Author "Geromont, Helena F"

Now showing 1 - 9 of 9
  • Thumbnail Image
    Item
    Restricted
    Complex assessments or simple management procedures for efficient fisheries management: a comparative study
    (Oxford University Press, 2015) Geromont, Helena F; Butterworth, Doug S
    Complex stock assessment methods are data- and expertise-hungry, with the annual updates of catch-at-age data and models typically seen as an essential requirement for sound management. But are the heavy commitments of resources required for this level of annual intervention really necessary to achieve efficient long-term fishery management? This question is addressed through a retrospective analysis of management performance over the last 20 years for four North Atlantic fish stocks. The assessments for two of these stocks have exhibited fairly strong retrospective patterns. The actual assessment advice for these stocks was provided based on complex assessment methods making use of age data. The outcomes are compared with what could have been achieved with much simpler catch control rules based upon age-aggregated survey indices alone. Even for the stocks whose assessments exhibit retrospective patterns, these simple rules can achieve virtually equivalent catch and risk performance, with much less interannual TAC variability, compared with what actually occurred over the past 20 years.
  • Thumbnail Image
    Item
    Restricted
    Does selectivity matter? A fisheries management perspective
    (Elsevier, 2014) Butterworth, Doug S; Rademeyer, Rebecca A; Brandão, Anabela; Geromont, Helena F; Johnston, Susan J
    The authors’ experiences in relation to the estimation of selectivity and its impact on the formulation of management advice are summarised for 14 different resources. These include instances where advice (generally in the form of a catch limit) is based on an assessment, as well as cases where a Management Procedure is developed (applying MSE) for this purpose. Relative paucity of older fish in either or both of catches or surveys is a frequent occurrence which has a number of alternative explanations, including doming in selectivity, and estimates of many biological reference points are not robustto this uncertainty. However, when the impacts of uncertainties in estimating selectivities are considered relative to those of the other uncertainties which also influence the calculation of catch limits, the former usually (although not always) seem to be of somewhat lesser importance.
  • Thumbnail Image
    Item
    Restricted
    Generic management procedures for data-poor fisheries: forecasting with few data
    (Oxford University Press, 2015) Geromont, Helena F; Butterworth, Doug S
    The majority of fish stocks worldwide are not managed quantitatively as they lack sufficient data, particularly a direct index of abundance, on which to base an assessment. Often these stocks are relatively “low value”, which renders dedicated scientific management too costly, and a generic solution is therefore desirable. A management procedure (MP) approach is suggested where simple harvest control rules are simulation tested to check robustness to uncertainties. The aim of this analysis is to test some very simple “off-the-shelf” MPs that could be applied to groups of data-poor stocks which share similar key characteristics in terms of status and demographic parameters. For this initial investigation, a selection of empirical MPs is simulation tested over a wide range of operating models (OMs) representing resources of medium productivity classified as severely depleted, to ascertain how well these different MPs perform. While the data-moderate MPs (based on an index of abundance) perform somewhat better than the data-limited ones (which lack such input) as would be expected, the latter nevertheless perform surprisingly well across wide ranges of uncertainty. These simple MPs could well provide the basis to develop candidate MPs to manage data-limited stocks, ensuring if not optimal, at least relatively stable sustainable future catches.
  • Thumbnail Image
    Item
    Restricted
    Generic management procedures for data-poor fisheries:forecasting with few data
    (Oxford University Press, 2015) Geromont, Helena F; Butterworth, Doug S
    The majority of fish stocks worldwide are not managed quantitatively as they lack sufficient data, particularly a direct index of abundance, on which to base an assessment. Often these stocks are relatively “low value”, which renders dedicated scientific management too costly, and a generic solution is therefore desirable. A management procedure (MP) approach is suggested where simple harvest control rules are simulation tested to check robustness to uncertainties. The aim of this analysis is to test some very simple “off-the-shelf” MPs that could be applied to groups of data-poor stocks which share similar key characteristics in terms of status and demographic parameters. For this initial investigation, a selection of empirical MPs is simulation tested over a wide range of operating models (OMs) representing resources of medium productivity classified as severely depleted, to ascertain how well these different MPs perform. While the data-moderate MPs (based on an index of abundance) perform somewhat better than the data-limited ones (which lack such input) as would be expected, the latter nevertheless perform surprisingly well across wide ranges of uncertainty. These simple MPs could well provide the basis to develop candidate MPs to manage data-limited stocks, ensuring if not optimal, at least relatively stable sustainable future catches.
  • Thumbnail Image
    Item
    Restricted
    Generic MPs for data-poor fisheries: Southern hemisphere collaboration.
    (2010) Butterworth, Doug S; Geromont, Helena F
    The focus of the Southern Hemisphere Collaboration amongst Australia, New Zealand and South Africa is the Management Procedure approach, also known as Management Strategy Evaluation (MSE), as a better basis to manage fisheries. The primary aim of the Collaboration is basic research into the design, development and evaluation of a set of generic Management Procedures (MPs) for data-poor fisheries, for which fishery-specific MPs would be too costly and time-consuming to develop, so as to set the management of such fisheries on a sounder and sustainable basis.
  • Thumbnail Image
    Item
    Restricted
    Initial investigation of generic management procedures for data-poor fisheries
    (2010) Geromont, Helena F; Butterworth, Doug S
    The vast majority of fish stocks in South Africa are not managed quantitatively as there is not sufficient data (such as an index of abundance) on which to base a resource assessment. In addition, these stocks are relatively “low-value”, which renders dedicated scientific management too costly, and a generic approach is therefore required. The aim of this work is to design and test some very simple “off-the-shelf” management procedures (MPs) that can be applied to groups of data-poor fisheries that share some key characteristics in terms of demographic parameters. For this initial investigation, a selection of empirical MPs is simulation tested on a wide range of operating models (OMs) representing the underlying dynamics of the resource in order to ascertain how well the different MPs perform. While the data-rich MPs perform somewhat better than the data-poor ones, as would be expected, it seems that the very simple data-poor MPs are surprisingly robust to a wide range of uncertainty for key parameters and could well be candidates to manage the South African data-poor stocks, ensuring perhaps not optimum, but at least relatively stable sustainable future catches.
  • Thumbnail Image
    Item
    Open Access
    An initial reality check on estimates related to black-browed albatross kills in the trawl fishery
    (2010) Butterworth, Doug S; Geromont, Helena F
    The SASSI Kingklip assessment makes certain key statements about the kills of black-browed albatross (the avian species most at risk of those they consider) in the SA trawl fishery: an annual kill of 5000 in 2006 (all birds 18000) a subsequent reduction to 2200 through the introduction of tori lines (all birds 8000) (assuming proportional reduction) a necessary bycatch reduction to less than 2% of current levels needed for the population to recover. The document conducts an initial reality check on these assertions. To do so it makes use of results in Thomson et al. (2009).
  • Thumbnail Image
    Item
    Restricted
    Performance review of simple management procedures
    (International Council for the Exploration of the Sea, 2015) Carruthers, Thomas R; Kell, Laurence T; Butterworth, Doug S; Maunder, Mark N; Geromont, Helena F; Walters, Carl; McAllister, Murdoch K; Hillary, Richard; Levontin, Polina; Kitakado, Toshihide; Davies, Campbell R
    Using a management strategy evaluation approach, we compare a range of new and established management procedures (MPs) for setting catch-limits in fisheries. Performance is evaluated with respect to fish life history type, level of stock depletion, data quality, and autocorrelation in recruitment strength. We quantify the robustness of each MP with respect to the various observation processes. Methods using observations of absolute biomass or stock depletion offer the best overall performance and this is consistent across life history types, data qualities, and stock depletion levels. Simple MPs can outperform conventional data-limited methods and data-rich assessments that use time-series of catch and effort data. MP performance is most sensitive to biases in catch data. Our results indicate that often tuning MPs for specific stocks is important, though this may not be viable in data-poor assessment scenarios because of insufficient data and analysis resources.
  • Thumbnail Image
    Item
    Restricted
    Preliminary assessment of the western and south-western Cape Carpenter resource using an age-structured production model
    (2011) Geromont, Helena F; Butterworth, Doug S
    An age-structured production model (ASPM) is used to assess the Carpenter resource off the south western coast of South Africa. The model is fitted to standardized CPUE and length frequency linefish data. Problems encountered when attempting to fit the model to these data are explored. Reasons may include conflicting data, or complexities in the resource dynamics that are not incorporated in the simple density-dependent ASPM dynamics. Plausible parameter estimates are only achieved when fixing or imposing penalty functions for key parameters. However, this leads to deterioration in the fit to the data with systematic trends in the residuals which render results suspect and management advice based on such assessments dubious. Indeed, a management procedure approach may be better suited to circumstances when a “best assessment” is problematic, as seems to be the case here.