Browsing by Author "Furman, Liam B"

Now showing 1 - 8 of 8
  • Thumbnail Image
    Item
    Open Access
    The 2014 Operational Management Procedure for the South African horse mackerel resource
    (2015) Furman, Liam B
    This document specifies the 2014 Operational Management Procedure (OMP) to provide Precautionary Upper Catch Limit (PUCL) recommendations for horse mackerel bycatches in the South African pelagic fishery and Total Allowable Catch (TAC) recommendations for targeted horse mackerel catches in the South African midwater trawl fishery. The basis for the associated computations for each fishery are set out in separate sections below. Appendix A gives the details for the GLM procedure used to produce a horse mackerel CPUE series for the midwater fishery, and Appendix B describes Exceptional Circumstances (ECs) that could lead to deviations from OMP recommendations.
  • Thumbnail Image
    Item
    Open Access
    Analyses of the results from the island closure feasibility study for the Dassen/Robben and St Croix/Bird Island Pairs
    (2014) Robinson, William M L; Butterworth, Doug S; Furman, Liam B
    The results from the Island Closure Feasibility Study are analysed using the GLMs as set out at the 2010 international stock assessment workshop. Estimates of residual variance for a random year effects GLM for the various penguin response variables are considered to be sufficiently precise to enable power analyses to be conducted to contribute to the evaluation of whether to transition to a full experimental closures programme, so that the feasibility study may be considered successfully concluded. For the Dassen and Robben Islands about 80% of the estimates of the fishing effect parameter λ are positive, with this same proportion maintained for those (about one sixth) of these estimates which are significant at the 5% level. Thus the preponderance of the evidence from these analyses is that the impact of fishing around these islands has been positive. The rather fewer instances available to analyse for the Eastern Cape colonies suggest a weakly positive effect at Bird Island, but a somewhat stronger negative effect at St Croix. The power analyses suggest that in cases for Dassen and Robben Islands where further data collection might render currently non-significant λ estimates significant at the 5% level within the next two decades, the likely period required for such further collection would typically be in the vicinity of five years. The advantage provided by continuing the closure programme itself seems however to be slight, as the natural variation over time in normal catches would be sufficient to provide the contrast to achieve such results with only typically two years’ extension to those five years. For the two Eastern Cape islands, it seems that results which are statistically significant at the 5% level seem unlikely to be achieved in the foreseeable future – a result which may be a consequence of the relatively low levels of sardine catches typically taken close to those islands.
  • Thumbnail Image
    Item
    Open Access
    A preliminary evaluation of the potential use of pelagic survey data in setting the horse mackerel PUCL
    (2011) Furman, Liam B; Butterworth, Doug S
    The existing population model for the horse-mackerel resource is extended to take commercial catch-at-length information from the mid-water trawl fishery and demersal surveys into account to allow the estimation of recruitment variations, which are correlated with the results for horse-mackerel abundance (considered to primarily reflect recruits) from the November pelagic surveys. Correlation is better with results for the West coast only from these surveys, rather than with those for the assessment area as a whole. At this stage the conclusion is either that these November survey estimates are (for whatever reason) a relatively weak predictor of incoming horse-mackerel recruitment strength, or that there are problems with the modelling of the demersal/midwater data which require further attention.
  • Thumbnail Image
    Item
    Restricted
    Preliminary results from a GLM standardised CPUE series for the South Coast midwater horse mackerel fishery
    (2011) Furman, Liam B; Butterworth, Doug S
    This paper provides results for a preliminary GLM standardisation analysis of the CPUE data for the South African horse mackerel (Trachurus trachurus capensis) midwater fishery from commercial trawl data. The series is to be used as part of an updated horse mackerel assessment. The data, provided by Jan van der Westhuizen (pers. commn), cover a variety of vessels and fisheries. However, the Desert Diamond accounted for the vast majority (81 percent) of the horse mackerel caught. Therefore, this GLM uses only data recorded by the Desert Diamond, a midwater trawl vessel, which covers the years 2003-2010.
  • Thumbnail Image
    Item
    Open Access
    Proposed catch control rules for horse mackerel bycatch in the small pelagics fishery and directed catch in the midwater trawl fishery
    (MARAM, University of Cape Town, 2012) Furman, Liam B; Butterworth, Doug S
    This document summarises the results of recent work on the development of rules for setting limits on horse mackerel catches. Section 1 describes the development of a revised Precautionary Upper Catch Level (PUCL) Rule for the horse mackerel bycatch in the small pelagics fishery. A specific proposal for implementation is put forward. Section 2 considers the directed catch allocation in the midwater trawl fishery, and investigates a number of simple experimental Management Procedure (MP) control rules , based on future CPUE and survey results, that would allow cautious increase of the current constant TAC for this sector of the fishery, with the aim of benefitting from the current likely underutilization of the resource. Results are provided for a number of candidates MPs amongst which a choice needs to be made. Appendix A provides results from an updated assessment and projection model, while Appendix B sets out the mathematical details of these models.
  • Thumbnail Image
    Item
    Open Access
    Testing time-varying selectivity models of the South Coast rock lobster with data generated by a model with spatial structure
    (2011) Furman, Liam B; Butterworth, Doug S
    Both gear selectivity and availability combine to form selectivity. When selectivity is allowed to change with time, it is difficult to determine the source of the variation. If availability changes with time, perhaps as a consequence of changing spatial distributions of fishing effort, selectivity-at-age will appear to vary over time. It then becomes unclear how to renormlise selectivity so that the catchability q which relates CPUE to underlying abundance does not change. This study uses a model with spatial structure in the age distribution of the population together with changes in the spatial distribution of fishing over time to simulate variations in availability. The model is used to generate data for simulation testing of two different approaches to modeling time-varying selectivity.
  • Thumbnail Image
    Item
    Open Access
    Updated analyses of the results from the Island Closure Feasibility Study for the Dassen/Robben and St Croix/Bird Island pairs given revised data and responses to matters raised in documents
    (2014) Butterworth, Doug S; Moosa, N; Furman, Liam B; Robinson, William M L; Johnston, Susan J
    Repetition of earlier analyses of the data from the island closure feasibility study, given some corrected and extended data series, makes little change to results for Robben and Dassen Islands, except that the estimated power of foraging–related response variables to achieve statistically significant results decreases appreciably. However for St Croix Island, the inference of a negative impact of fishing is strengthened. The rationales offered in MARAM/IWS/DEC14/Peng/A3 for using closure instead of catch as a covariate in the analysis, and for restricting data to the years from 2008 onwards, are questioned. Comparisons conducted by applying the MARAM/IWS/DEC14/Peng/B4 approach, which uses annual means of response variables, indicate that the use of closure rather than catch as the covariate generally results in poorer precision and fewer statistically significant estimates of the fishing effect parameter λ. Furthermore when catch is used as the covariate, appreciably better precision for estimates of λ is generally achieved by including all years in the analyses, rather than by restricting them to the period from 2008 onwards. Importantly comparative estimates of λ from the Peng/B4 approach are shown to achieve better precision generally than those from the more complex and data-intensive Peng/A3 approach, thus negating the assertions in Peng/A1 and Peng/A2 that estimates from the former are compromised by their dependence on response variable means alone. The failure of Peng/A3 to report the variance estimates needed for input to the power analysis required for the feasibility study is noted. Furthermore Peng/A3 offers no specification of the simulation studies necessary to carry out a power analysis for the estimators which it proposes, so that it has failed to address this key first step in this overall closure study process. Peng/A3 has prodived some strongish evidence that closures may benefit penguins, but for the Eastern Cape colonies only. However it has failed to address the primary aim of the feasibility study itself to ascertain for how long an experimental closures programme would need to continue for reliable determination of the impact of fishing in the near vicinity of island colonies on penguin reproductive success. Use of the Peng/B4 approach indicates that this period is appreciably lengthened if data for analyses are to be restricted to the years from 2008 onwards only, and particularly so if closure replaces catch as a covariate.
  • Thumbnail Image
    Item
    Open Access
    Updated assessments and projections under alternative future catch levels for the horse mackerel resource
    (2011) Furman, Liam B; Butterworth, Doug S
    The age-structured production model assessment of Johnston and Butterworth (2007) is updated to take account of further catch and survey data. In addition length-frequency data from surveys and the Desert Diamond, and a GLM standardised CPUE series from the Desert Diamond are now included when fitting the model to the data. The assessments do indicate an increase in abundance of about 20% over the last five years, primarily as a result of good recruitment. However, long term projections under different levels of future catches remain fairly similar to those of Johnston and Butterworth (2007).