Browsing by Author "Ross-Gillespie, Andrea"

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    The 2018 Operation Management Procedure for the South African Merluccius paradoxus and M. capensis Resources
    (2019) Ross-Gillespie, Andrea; Butterworth, Doug S; Glazer, Jean; Fairweather, Tracey
    Specifications and projection results for the 2018 Operational Management Procedure used for setting South African hake Total Allowable Catches are provided along with various background information, including details of the metarule processes.
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    The 2018 Operational Management Procedure for the South African Merluccius paradoxus and M. capensis resources.
    (2018-10) Ross-Gillespie, Andrea; Butterworth, Doug S; Glazer, Jean; Fairweather, Tracey
    Specifications and projection results for the 2018 OMP used for setting SA hake TACs are provided along with various background information, including details of the metarule process.
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    Application of a photo-identification based assessment model to southern right whales in South African waters, now including data up to 2012
    (2013) Brandão, Anabela; Butterworth, Doug S; Ross-Gillespie, Andrea; Best, Peter B
    This paper extends the analyses of Brandão et al. (2012) which applied the three-mature-stages (receptive, calving and resting) model of Cooke et al. (2003) to photo-identification data available from 1979 to 2010 for southern right whales in South African waters, by taking two further years of data into account. The 2012 number of parous females is estimated to be 1 321, the total population (including males and calves) 5 062, and the annual population growth rate 6.6%. This reflects a small reduction to the 6.8% increase rate estimated previously; this is a result of slightly lesser numbers in recent years than estimated previously. Information from resightings of grey blazed calves as adults with calves allows estimation of first year survival rate of 0.850, compared to a subsequent annual rate of 0.988. This information also suggests that 7% (s.e. 6%) of grey blazed calves are not recognised as such when adults; this estimate is much less than that of 27% obtained previously by Brandão et al. (2012), and is more compatible with the relative proportions of grey blazed animals amongst calves and amongst calving adults which suggest a value of 10% (s.e. 8%).
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    Assessment results for humpback breeding stocks D, E1 and Oceania following recommendations from SC 65a
    (University of Cape Town, 2014) Ross-Gillespie, Andrea; Butterworth, Doug S; Johnston, Susan J
    Following recommendations made at IWC 65a, 2013, a single-stock BSD (Breeding Stock D, West Australia) model has been run for a range of Antarctic catch boundaries, and some two-stock BSE1 (Breeding Stock E1, East Australia)+BSO (Breeding Stock Oceania) models have been explored. The single-stock BSD model excluded the Hedley et al. (2011) absolute abundance estimate from the model fits, and instead utilised an uninformative uniform prior on the log of the target abundance estimate. The minimum value for this prior was based on calculations by Hedley of a minimum absolute abundance indicated by the 2005-2008 survey (Hedley et al. 2011). These changes markedly improve the fit to the BSD relative abundance series. The two-stock models considered consist of one model with fixed Antarctic boundaries that allowed for a proportion of each of the BSE1 and BSO stocks to feed in a common feeding ground between 170°E and 170°W, and a second model in which there was no overlap between the two stocks, but a range of different Antarctic catch boundaries have been explored. Results of these models showed that (a) the BSE1 growth rate remained virtually at 0.106 yr-1 (the demographic boundary imposed by the model), (b) fits to the BSE1 mark-recapture data were relatively poor and (c) the Nmin constraint remained problematic for BSO. Further two-stock runs, as well as a three-stock run, have not been included in this paper, but the authors aim to provide the results as an addendum to this paper at the meeting.
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    Bridge-building between the 2017 and 2018 hake RC assessment models
    (2018-11) Ross-Gillespie, Andrea; Butterworth, Doug S
    A step-by-step set of results is given to indicate the impacts of various changes made to the hake assessment Reference Case model over the since May of 2017
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    Bridge-building between the 2017 and 2018 hake RC assessment models
    (2018-05) Ross-Gillespie, Andrea; Butterworth, Doug S; Durholtz, Deon
    A step-by-step set of results is given to indicate the various impacts of various changes made to the hake assessment Reference Case model over the past 12 months.
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    Conditioning of the hake OMP2018 Reference Set models
    (2018-05) Ross-Gillespie, Andrea; Butterworth, Doug S
    An associated brief background and the first set of results are presented for the conditioning of the OMP2018 Reference Set (RS) models.
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    Conditioning of the hake OMP2018 Reference Set models with the Model A6b input data.
    (2018-07) Ross-Gillespie, Andrea; Butterworth, Doug S
    Results are provided for the reconditioning of the hake OMP 2018 Reference Set with “data” forthcoming from the species splitting algorithm Model A6b. The results are similar to the ones provided previously in FISHERIES/2018/MAY/SWG-DEM/21. Models starting in 1978 have been excluded at this point in time as some issues have arisen in fitting these that require further investigation. It is preliminarily proposed to remove these models from the Reference Set and consider them instead for robustness tests only.
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    Constant catch projections for the RS for the 2018 hake OMP review
    (2018-08) Ross-Gillespie, Andrea; Butterworth, Doug S
    The RS Operating Models are projected forward under a range of constant future annual TACs from 130 000t to 160 000t to provide an initial scale on what TACs may maintain the M. paradoxus resource above BMSY. A basis for projecting the future ratio of the two hake species in catches is put forward. A constant catch of 140 000t would maintain the M. paradoxus resource above BMSY at the 5% probability level. A number of questions are listed for response to aid in the further development of the OMP.
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    Details of updates made to the Reference Case specifications from the time of the 2017 MSC audit.
    (2018-07) Ross-Gillespie, Andrea; Butterworth, Doug S
    Details are provided of the changes that have been made to the South African hake Reference Case assessment model since the time of the 2017 MSC audit.
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    Does parasite infection definitely increase for sardine aged 2 and above on the south coast
    (2014) Ross-Gillespie, Andrea; Butterworth, Doug S
    The trend with length in parasite prevalence in sardine on the South Coast is modelled in a manner that isolates behaviour at larger lengths to enable a determination of whether the trend continues to increase from age one to ages of two and above. The results indicate that a continued increase is robustly confirmed at the 5% level of significance, and hence that there must be some movement of sardine of ages greater than one in at least one direction between the West and the South Coasts under the hypothesis that infection by the parasite can occur only on the West Coast.
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    Evaluating the accuracy of hake abundance index predictions using different smoothing techniques
    (2013) Butterworth, Doug S; Robinson, William M L; Ross-Gillespie, Andrea
    Alternative projection approaches based on linear and quadratic smoothing are applied retrospectively to abundance indices for hake to ascertain whether they can provide more accurate predictions of resource status one and two years ahead, in line with ideal needs for the empirical OMP used to set hake TACs. The results suggest that there is probably little if anything to be gained from attempting more complex formulations than three-year averages of abundance indices for input to a target-based empirical OMP for SA hake.
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    Exploration of a safe-guard criterion for OMP2018 in the eventuality that the M. capensis CPUE and survey indices of abundance drop too low
    (2018) Ross-Gillespie, Andrea; Butterworth, Doug S
    A value is sought for the M. capensis combined CPUE and survey index of abundance J which would constitute the threshold below which additional (supra-OMP) management measures would need to be taken (probably in the form of moving the distribution of offshore trawling to deeper waters) to safeguard this resource in circumstances where its abundance had dropped too low. A simple approach suggests that a threshold value of 0.6 would be appropriate to identify and achieve some reasonable response to a recruitment failure, whilst limiting instances of responses to false positives where there was in fact no problem.
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    Exploration of a safe-guard criterion for OMP2018 in the eventuality that the M. capensis CPUE and survey indices of abundance drop too low
    (2018-11) Ross-Gillespie, Andrea; Butterworth, Doug S
    A value is sought for the M. capensis combined CPUE and survey index of abundance J which would constitute the threshold below which additional (supra-OMP) management measures would need to be taken (probably in the form of moving the distribution of offshore trawling to deeper waters) to safeguard this resource in circumstances where its abundance had dropped too low. A simple approach suggests that a threshold value of 0.6 would be appropriate to identify and achieve some reasonable response to a recruitment failure, whilst limiting instances of responses to false positives where there was in fact no problem
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    First set of robustness tests conducted for OMP2018
    (2018-11) Ross-Gillespie, Andrea; Butterworth, Doug S
    Results are presented for a total of ten robustness tests. The first five robustness tests investigate alternative assumptions regarding future surveys and commercial catchability. Further robustness tests involve testing alternative mortality-at-age vectors and sensitivity to data arising from different species splitting algorithms. The final robustness tests explored in this document decrease past or future carrying capacity, simulating a situation where expected recruitment fails. For all robustness tests, simulations suggest that OMP2018 is able to provide adequate management advice for the situations simulated. For robustness tests simulating predictable situations (e.g. if it were known that surveys would be discontinued in the future), the OMP needs to be re-tuned to avoid adverse impacts on TACs and/or resource status. The Appendix reports the results of three additional robustness tests concerning future surveys requested by the Panel during the course of the workshop, none of which indicated that OMP2018 would not be able to provide adequate management advice.
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    First set of robustness tests conducted for OMP2018
    (2018) Ross-Gillespie, Andrea; Butterworth, Doug S
    Results are presented for a total of ten robustness tests. The first five robustness tests investigate alternative assumptions regarding future surveys and commercial catchability. Further robustness tests involve testing alternative mortality-at-age vectors and sensitivity to data arising from different species splitting algorithms. The final robustness tests explored in this document decrease past or future carrying capacity, simulating a situation where expected recruitment fails. For all robustness tests, simulations suggest that OMP2018 is able to provide adequate management advice for the situations simulated. For robustness tests simulating predictable situations (e.g. if it were known that surveys would be discontinued in the future), the OMP needs to be re-tuned to avoid adverse impacts on TACs and/or resource status. The Appendix reports the results of three additional robustness tests concerning future surveys requested by the Panel during the course of the workshop, none of which indicated that OMP2018 would not be able to provide adequate management advice.
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    Full set of results for the CMP recommended to be adopted for OMP2018, including for the robustness tests conducted thus far.
    (2018-10) Ross-Gillespie, Andrea; Butterworth, Doug S
    This document either summarises previous reported performance statistics and plots, or extends these, to provide a comprehensive consolidation of such results for the OMP candidate to be recommended for the next hake OMP (control parameters b increased by 5% and a TAC cap of 160 thousand tons - these being the key changes to OMP-14).
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    Implementing the 2014 three-stock model for Southern Hemisphere Breeding Stocks E1, Oceania and G
    (2015) Ross-Gillespie, Andrea; Butterworth, Doug S
    The three stock model approach with mixing on feeding grounds, which was implemented in 2014 to assess the BSD, BSE1 and BSO breeding stocks, is applied in a similar manner to the BSE1, BSO and BSG breeding stocks. This is intended as a first step in applying this approach consecutively around the globe to check for consistency of results in circumstances of uncertainty in the allocation of feeding ground catches. Compared to earlier assessments of the BSE1, BSO and BSG breeding stocks in partly separate analyses, there is an appreciable change in that the pre exploitation level is estimated higher for BSE1and lower for BSO; correspondingly, BSE1 is estimated as less and BSO as more recovered towards those pre-exploitation levels. Comparable likelihoods do not differ greatly for data for these stocks used in both the 2014 BSD+BSE1+BSO and the 2015 BSE1+BSO+BSG assessments, suggesting that the data used here do not contain sufficient information to distinguish these rather different results
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    Incorporating cannibalism and inter-species predation effects into the hake assessment model: Methods and some preliminary results
    (2013) Ross-Gillespie, Andrea; Butterworth, Doug S
    Hake cannibalism and inter-species predation is modelled explicitly using a Type II functional response. Because of the fast dynamics of predation compared to other processes, the standard hake assessment model is converted from a yearly to a monthly time-step. Convergence of model fits is difficult to achieve both because of providing good estimates for starting values and of the tendency of the model towards oscillating behaviour. Thus far satisfactory fits have been obtained only from a somewhat reduced level of predation compared to that thought to occur. These results suggest that both hake species are at higher levels relative to pristine than for the standard assessment.