Browsing by Author "Butterworth, Doug"

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    2020 sole dynamic Schaefer production model results
    (2020) Glazer, Jean; Butterworth, Doug
    The sole resource is modelled by a dynamic Schaefer production model which allows for a drop in the value of the intrinsic growth rate parameter from 2000 onwards. The model is fit to the available CPUE and survey abundance indices. These data are not sufficiently informative to be able to distinguish amongst fairly wide ranges of pre- and post-2000 intrinsic growth rate parameters. Nevertheless, all suggest that the sole resource has never been substantially depleted (being well above its MSY level), and furthermore that the current replacement yield and MSY are reasonably robustly estimated in the ranges of 275-350 and 490-720 mt respectively. Given that this update 2020 assessment indicates slightly lower values for resource productivity than in 2019, consideration should be given to some reduction in the current TAC of 502 mt.
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    Age-Structured Production Model (ASPM) assessments of the Alfonsino (Beryx splendens) resource in the SIOFA area of the Southern Indian Ocean
    (2020) Brandão, Anabela; Butterworth, Doug; Johnston, Susan
    An Age-Structured Production Model is applied to assess the Alfonsino (Beryx splendens) resource in the West and East SIOFA areas of the Southern Indian Ocean. Data limitations restrict these applications to deterministic variants, which assume no variation in annual recruitment about the predictions from a stock-recruitment relationship. The models are fitted to the CPUE series and single year of commercial catch length distribution information available. Both West and East stocks are estimated to be at about 60% of their pre-exploitation spawning stock biomass levels, and well above the levels corresponding to MSY (MSYL). These results are insensitive to all sensitivities explored, except for changes in the value assumed for natural mortality (M). For the Base case assumption of M=0.2, projections under constant annual catches up to 40% above the 2018 levels remain above MSYL for the next two decades. However, if a less productive situation is assumed (M=0.15), there are some cases of spawning biomass dropping below MSYL within 10 years for both areas, including even for continuation of the 2018 catch for the East area.
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    Calculating an informative prior distribution for the bias of hydro‐acoustic survey estimates of the biomass of the South African anchovy
    (2020) de Moor, Carryn; Coetzee, Janet; Butterworth, Doug
    A prior distribution for the bias of the hydro‐acoustic survey estimates of the biomass of the South African anchovy is developed by considering the bias associated with the individual sources of error.
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    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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    Initial results from the development of a new OMP 2020 for Tristan da Cunha island rock lobster resource
    (2020) Johnston, Susan; Butterworth, Doug
    Development of a revised OMP (OMP 2020) for the Tristan island rock lobster resource first leads to a recommendation to include the annual Edinburgh/GS CPUE and the biomass survey index as well as the Tristan powerboat CPUE index as inputs to the formula to provide an annual TAC recommendation. Following an extensive comparison exercise, two options are put forward as candidates for the revised OMP: CMP1 and CMP2. Both reflect slightly less risk of undue resource depletion than under the current OMP 2016. CMP1 is very similar to that current OMP, and is expected to maintain annual TACs close to 120 MT. In contrast, under CMP2 TACs would be expected to increase slightly over the next decade, accompanied by a slight decrease in the catch rate compared to that under CMP1.
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    OMP 2020 candidates for the Nightingale rock lobster fishery
    (2020) Johnston, Susan; Butterworth, Doug
    The performances of a number of OMP candidates for the fishery for rock lobster at Nightingale are reported. These performances do not show much variation amongst the various candidates. For this and other reasons it is recommended that CMP9 be adopted as the new OMP for Nightingale. This CMP has a TAC ceiling of 100 MT. Very little further resource protection (which is in any case already certainly adequate) is provided by a CMP with a lower TAC ceiling. All the CMPs have appropriate feedback, so that if the resource declines in the future, the future TAC will do so too. An added precautionary feature defines an Ilim catch rate below which the TAC can decline faster than the baseline constraint of 5% pa given the occurrence of specified “Exceptional Circumstances”; this feature is built into the CMPs.
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    Re-analysis of the island closure experiment results to implement the suggestions of the December 2020 International Panel
    (2021) Ross-Gillespie, Andrea; Butterworth, Doug
    The suggestions of the December 2020 International Panel for further analysis of the results from the island closure experiment, in particular to use the same data within a common framework to facilitate comparisons, and to include month as a covariate, are implemented for all response variables except chick survival. Broadly, the results indicate that aggregated and disaggregated approaches give the same results for estimates and variances of the island closure effect (see Figure 2). This is especially a consequence of implementing the disaggregated approach with a nesting structure as advocated by the Panel, which is shown to be statistically justified. Inclusion of the month co-variate does impact results to some extent, the more so for foraging data for the west coast islands (see Figure 3). The sensitivities investigated generally make little difference to results; these checks include extensions to incorporate data prior to 2008. As to be expected, REML based estimates of CIs are somewhat wider than those based on MLE.
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    Results of the progression towards a proposed Management Procedure for the Toothfish (Dissostichus eleginoides) Resource in the Prince Edward Islands vicinity
    (2020) Brandão, Anabela; Butterworth, Doug
    Brandão and Butterworth (2019a) investigated various target-based Candidate Management Procedures (CMPs) for the Prince Edward Islands toothfish resource; however, their performances under some operating models were not satisfactory so that further investigation/adjustment of these CMPs was needed. An adjusted form of CMP(mean+tag) of Brandão and Butterworth (2019a), which incorporated trends in the cumulative number of recaptured tags as well as the recent mean of the trotline CPUE, was considered to have good potential as a CMP. This paper documents the various iterations of results as this CMP was refined and the results examined by a Task Team. The “final” MP now proposed for the consideration of the Demersal Working Group considers an initial smoothing of the TAC trajectory, is tuned to a target of 40% of the median final depletion under OM10 and constrains the TAC to a maximum inter-annual change of 10%. This MP performs satisfactorily under most of the OMs, in that median catches increase for most of the projection period while catch rates keep increasing and the median final depletion remains above the specified target value under OM10. The application of an initial TAC smoothing largely eliminates the pattern of an initial increase before a later drop in TACs.
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    A summary of results for the island closure experiment
    (2021) Butterworth, Doug; Ross-Gillespie, Andrea
    A simple summary is offered of the results of the island closure experiment reported in Ross-Gillespie and Butterworth (2021), which implemented the suggestions of the December 2020 International Panel. A single result in selected for each island/response variable combination, and reasons are provided for preferring the aggregated data approach for this, with increases made to the CI estimates so as to correspond to the unbiased REML method. Integration across the results is problematic for various reasons, but a coarse summary indicates little evidence for any impact (in either direction) of fishing in the neighbourhood of island colonies on penguin population growth rates. Given that the islands in the experiment have been closed for 50% of the period since 2008, some coarse predictions for the extent of improvement in annual population growth rates (on average over time) were these islands to be closed to fishing every year in the future are: Dassen -0.5% (i.e. no improvement), Robben +0.25%, Bird 0% and St Croix +0.5%.
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    Updated extension of a photo-identification based assessment model to southern right whales in South African waters to allow for the possibility of an early abortion of the calf in the model
    (2021) Brandao, Anabela; Ross-Gillespie, Andrea; Vermeulen, Els; Butterworth, Doug
    This paper updates analyses (Brandão et al., 2020) of an extension of the model developed by Brandão et al. (2019) so as to now include sightings data for 2019 and 2020. Brandão et al. (2020) had been modified to include the possibility of an early abortion so that a pregnant (receptive) whale in year y can again be pregnant in year y+1 (the “delta-loop”). This was to be able to account for an increase in calving intervals that are dependent on environmental conditions, and in a way that differs from a change in the value of the β (whale rests for another year) or the y (late abortion) parameters; this was to be able to explain the low number of sightings of females with calves observed over the 2015 to 2017 period. The further data now available show that following a large number of cow-calf pairs sighted in 2018, there were again low numbers of sightings for 2019 and 2020. From initial work, it has become clear that the low sighting probabilities estimated for 2015 to 2017 and for 2019 to 2020 are not eliminated by the incorporation of this “delta-loop” in the model. A weighted penalty function for the sighting probabilities is necessary to obtain recent sighting probability values in the region of earlier ones; this seems necessary for realism in circumstances where there has not been any marked reduction in the survey sighting effort over these recent years. A weight of 5.0 is able to achieve estimates close to the average of previous sighting probabilities for both periods of low sightings. Thus, low numbers of sightings of females with calves for five of the last six years can be explained by changes in reproduction-related demographic parameters without the need to postulate an increase in the adult mortality rate. Changing environmental (particularly feeding) conditions seem the likely cause, and to be associated with a changed distribution.