OpenUCT :: Browsing by Author "Butterworth, Doug S"

Browsing by Author "Butterworth, Doug S"

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2003 updated assessment for the Merluccius paradoxus hake resource off the south and west coasts
(2003) Rademeyer, Rebecca A; Butterworth, Doug S
This document presents an updated assessment of the M. paradoxus hake resource off the south and west coasts of South Africa. The previous assessment of this resource is described in Rademeyer and Butterworth (2002).
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The 2004 age-structured production model assessments and projections for the South Coast rock lobster resource
(2004) Johnston, Susan J; Butterworth, Doug S
The assessment conducted in 2003 has been routinely extended, taking account of a further year’s catch, CPUE and catch-at-age data. CPUE shows a continuation of the increase that commenced in 1998. Results are generally more optimistic than those for the 2003 assessment; this is shown to be a result of both the new abundance data now available as well as the revision of the historic catch series, with the former having slightly the larger effect. The Reference Case (RC) scenario suggests that a TAC of 360 MT or less would be appropriate to prevent biomass decline in the future. Other scenarios suggest either higher or lower values than this. If the catch-at-age data are down-weighted, then this appropriate level for the TAC is increased to 390 MT. On the other hand, the scenario which assumes the 1995+ recruitment to be equal to the average of the previous 10 years is more pessimistic and suggests an appropriate TAC level of only some 300 MT
Open Access
The 2004 re-assessment of the South African sardine and anchovy populations to take account of revisions to earlier data and recent record abundances
(National Inquiry Services Centre (NISC), 2009) de Moor, Carryn L; Butterworth, Doug S
Hydroacoustic surveys off the coast of South Africa over the early years of the 21st century indicated that both the sardine Sardinops sagax and anchovy Engraulis encrasicolus populations had simultaneously reached record abundances. The South African pelagic fishery is regulated using an Operational Management Procedure (OMP). The OMP in use at that time had been developed using data from the two populations prior to this rapid and substantial increase in abundances. This paper documents the revised assessments that were urgently required to provide a basis to update the OMP. These assessments resulted in a changed perception of the status and productivity of these populations. In particular, estimates of the stock-recruitment relationships and the extent of variation about them, which play a key role in evaluating risk when developing OMPs, altered substantially from estimates derived from earlier assessments.
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The 2005 Age-Structured Production Model assessments and constant catch projections for the south coast rock lobster resource
(2005) Johnston, Susan J; Butterworth, Doug S
The 2005 assessment was routinely extended, taking account of a further year’s catch, CPUE and catch-at-age data. CPUE shows a continuation of the increase that commenced in 1998. However, sustainable yield estimates are generally less than those for the 2004 assessment, although estimates of current biomass levels relative to K increase. The Reference Case (RC) scenario suggests that a TAC of about 360 MT or less would be appropriate to prevent biomass decline in the future. Other scenarios suggest either higher or lower values than this. If the catch-at-age data are down-weighted, then this 360 MT level for the TAC is increased to 390 MT. On the other hand, the scenario which assumes the 1996+ recruitment to be equal to the average of the previous 10 years is more pessimistic and suggests an appropriate TAC level of only some 300 MT or less to prevent biomass decline. A model (Model 2) which allows for time-varying selectivity is presented and shows promising results. Model 2 is better able to reproduce the recent CPUE trend. Preliminary results for a model that fits to catch-at-length rather than catch-at-age data, using a selectivity-at-length rather than selectivity-at-age function, are presented, but these do not as yet reflect satisfactory fits to the data so that they should not be considered reliable in the context of stock status estimates. These different scenarios reflect very different interpretations of the recent increase in CPUE for the resource. If the catch-at-age data are down-weighted, the model fit essentially ignores them and suggests a recent increase in abundance. However, under either the effort saturation or the time-varying selectivity approaches, spawning biomass is estimated to have decreased further over recent years.
Open Access
The 2006 age-structured production model assessments and projections for the South Coast rock lobster resource-routine update
(2006) Johnston, Susan J; Butterworth, Doug S
The assessment conducted in 2006 has been routinely extended, taking account of a further year’s catch, CPUE and catch-at-age data. CPUE shows a continuation of the increase that commenced in 1998. The sustainable yield estimates are generally very similar to those for the 2005 assessment, although estimates of current biomass levels relative to K increase. The Reference Case (RC) scenario suggests that a TAC of about 360 MT or less would be appropriate to prevent biomass decline in the future. Other scenarios suggest either higher or lower values than this. If the catch-at-age data are down-weighted, then this 360 MT level for the TAC is increased to 390 MT. On the other hand, the scenario which assumes the 1996+ recruitment to be equal to the average of the previous 10 years is more pessimistic and suggests an appropriate TAC level of only some 300 MT or less to prevent biomass decline.
Open Access
2006 assessment of the toothfish (Dissostichus eleginoides) resource in the Prince Edward Islands vicinity
(2006) Brandão, Anabela; Butterworth, Doug S
The ASPM assessment of the Prince Edward Islands toothfish resource by Brandão and Butterworth (2005) that permitted annual fluctuations about a deterministic stockrecruitment relationship is updated to take account of further catch, GLM standardised CPUE and catch-at-length information that has become available for the years 2005 and 2006. The assessment allows for a second fleet to accommodate data from a pot fishery that operated in 2004 and 2005. Updated biological parameter values for Subarea 48.3 are incorporated and lead to less optimistic results. The resource is estimated to be at about 40% of its average pre-exploitation level in terms of spawning biomass. It is suggested that it would be prudent to restrict annual legal catches to 500 tonnes or less, unless a large proportion of the catch is to be taken by pots (which avoid the cetacean predation associated with longlining). Specific issues raised at WGFSA-SAM 2006 about this assessment are addressed.
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The 2007 Age-Structured Production Model assessments and projections for the South Coast rock lobster resource-routine update using model fitting to catch-at-age data
(2007) Johnston, Susan J; Butterworth, Doug S
The assessment conducted in 2006 (WG/06/06/WCRL3) has been routinely extended, taking account of a further year’s catch, CPUE and catch-at-age data. The observed CPUE shows a slight decrease for 2005 (2005/06 season). The sustainable yield estimates are generally very similar to those for the 2006 assessment, although estimates of current biomass levels relative to K increase. The Reference Case (RC) scenario suggests that a TAC of about 330 MT or less would be appropriate to prevent biomass decline in the future. The other two scenarios reported suggest higher values than this. If the catch-at-age data are down-weighted, then this 360 MT level for the TAC is increased to 390 MT.
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The 2007 age-structured production model assessments and projections for the South Coast rock lobster resource-routine update using Pope's approximation model fitting to catch-at-age data including scenarios for time-varying selectivity
(2007) Johnston, Susan J; Butterworth, Doug S
The assessment conducted in 2006 (WG/06/06/WCRL3) has been routinely extended (except that the Baranov equation has been replaced by Pope’s approximation), taking account of a further year’s catch, CPUE and catch-at-age data. The observed CPUE shows a slight decrease for 2005 (2005/06 season). The sustainable yield estimates are generally very similar to those for the 2006 assessment, although estimates of current biomass levels relative to K increase. The Reference Case (RC) scenario suggests that a TAC of a little less than 330 MT or less would be appropriate to prevent biomass decline in the future. The other four scenarios reported suggest higher values than this, ranging from 350 MT to 405 MT. Spawning biomass trends over the last 10 years are downward for all the models considered.
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2007 assessment of the toothfish (Dissostichus eleginoides) resource in the Prince Edward Islands vicinity
(2006) Brandão, Anabela; Butterworth, Doug S
The ASPM assessment of the Prince Edward Islands toothfish resource by Brandão and Butterworth (2006) that permitted annual fluctuations about a deterministic stock-recruitment relationship is updated to take account of further catch, GLM standardised CPUE and catch-at-length information that has become available for the years 2006 and 2007. The assessment allows for a second fleet to accommodate data from a pot fishery that operated in 2004 and 2005. Biological parameter values adopted for Subarea 48.3 are used. The resource is estimated to be at about 37% of its average pre-exploitation level in terms of spawning biomass. It is suggested that it would be prudent to restrict annual legal catches to 500 tonnes or less, unless a large proportion of the catch is to be taken by pots (which avoid the cetacean predation associated with longlining).
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2007 Routine update of the South African hake baseline assessment
(2007) Rademeyer, Rebecca A; Butterworth, Doug S
This paper presents a routine update of the South African hake assessment, including new commercial (catches and CPUE) and survey (abundance estimates and catch-at-age) data. Only one scenario (M1-H1-C1-SR2) of the Reference Set, which is that with the highest likelihood, has been run – it is termed the ‘baseline’ assessment.
Open Access
2008 routine update of the South African hake resource baseline assessment
(2008) Rademeyer, Rebecca A; Butterworth, Doug S
This paper presents a routine update of the South African hake assessment, including new commercial (catches and CPUE) and survey (abundance estimates) data. This update is for a “baseline” assessment, chosen to correspond to the scenario amongst the 2006 Reference Set which had the highest likelihood (M1-H1-C1-SR2).
Open Access
2009 updated stock assessments for West Coast rock lobster
(2009) Johnston, Susan J; Butterworth, Doug S
The west coast rock lobster assessment of 20061 based on data to 2004 is updated to include data up to 2008. Over the last four years the exploitable biomass trend is upwards for Areas 7 and 8 and the resource as a whole, but downwards for Areas 5+6 and almost level for Areas 1+2 and 3+4. The overall increase since 2006 is significant at the 5% level. While better than median projections at the time the current OMP developed, the increase remains within the 95% probability intervals calculated at the time.
Open Access
2010 Estimates of sustainable rock lobster yield for the four islands of the Tristan da Cunha group
(2010) Johnston, Susan J; Butterworth, Doug S
A simple replacement yield model is fitted to CPUE and annual catch data from 1997 for the four islands of the Tristan da Cunha group. The results suggest that the current TAC be maintained, with a slight increase (perhaps 5 tons) to the allocation to Gough, and the same decrease to the allocation to Tristan.
Open Access
The 2010 operational management procedure for the South African Merluccius paradoxus and M.?capensis resources
(2010) Rademeyer, Rebecca A; Fairweather, T; Glazer, Jean Patricia; Leslie, R L; Butterworth, Doug S
The algorithm for the 2010 Operational Management Procedure (OMP) to provide TAC recommendations for the South African Merluccius paradoxus and M. capensis resources is empirical. It combines an increase or decrease of the TAC in relation to a) the magnitude of recent trends in CPUE and survey abundance estimates for both species and b) the relative level of recent CPUE and survey abundance estimates compared to a target level. The basis for the associated computations is set out below, with the tuning parameters given in Table 1. Details of the computation procedures for the CPUE and catch data are provided in Appendix A, and for the survey estimates of Biomass in Appendix B.
Open Access
2012 South coast rock lobster operating models-some initial results
(2012) Johnston, Susan J; Butterworth, Doug S
Initial updated assessments are developed for the South Coast rock lobster resource incorporating the recently revised estimates of somatic growth rate by area developed by OLRAC, and fitting to area-specific CPUE and scientific catch-at-length data. The initial focus is on a model structure which splits the resource into areas A1E, A1W and A2+3. A problem arises because the slow growth rate estimate for A1E results in an unrealistically high estimate of preexploitation biomass, and some approaches to circumvent this are explored. The pack-category-based catch-at-length data are not consistent with these results and it is suggested that they be excluded from future model fits. Allowing selectivity at length to vary over time makes little difference to stock trajectory estimates, but scarcely improves non-random patterns in the catchat-length residuals, so that the investigation of alternative functional forms for this variation is suggested. An alternative model structure based on areas A1, A2 and A3 yields an estimate of the current depletion of the spawning biomass which is appreciably less than in previous assessments which were based on this structure; the reasons for this need to be identified.
Open Access
2013 Update of the Reference Set of Operating Models used in testing Candidate OMPs for the South African hake resource
(2013) Rademeyer, Rebecca A; Butterworth, Doug S
A Reference Set (RS) of 12 scenarios was selected as the primary basis to be used to simulation test candidates for OMP-2011 (Rademeyer and Butterworth, 2010). This paper presents an update of this RS, based on updated commercial data (catches, trawl and longline length distribution and trawl CPUE), some revisions of biological parameter values, and new survey data (biomass estimates and length distributions) until January 2012.
Open Access
2013 update of the South African hake reference case assessment
(2013) Rademeyer, Rebecca A; Butterworth, Doug S
The Reference Case assessment of the SA hake resource is updated using unchanged methodology from that used in developing the existing OMP, but with revised and updated data sets. The only change of particular note is the inclusion of new longline catch-at-length data subsequent to 2000 results in a somewhat reduced estimate of current the M. paradoxus spawning biomass.
Open Access
2015 updated South Coast Rock Lobster assessment results
(2015) Johnston, Susan J; Butterworth, Doug S
The 2013 assessment of the resource is updated given three further years of data now available. Recruitment is estimated to have been poor over the further three seasons for which this can now be estimated. The spawning biomass trajectory has flattened after a preceding increase, and is now showing a slight downward trend.
Open Access
The 2016 Horse Mackerel Assessment Model
(2016-09) Johnston, Susan J; Butterworth, Doug S
Furman (2014) developed and described the horse mackerel assessment model for his MSc thesis. This document extracts pertinent sections from his thesis which describe the population assessment model, and provides a description of assumptions made for projecting the resource into the future. The data, as updated in 2016, are also provided.
Open Access
2016 Horse Mackerel projections
(2016-10) Johnston, Susan J; Butterworth, Doug S
The projections conducted in 2015 are updated to include the further data now available. The results are somewhat more positive than previously as a result of the CPUE for 2015, although still low, being higher than that for 2014. Nevertheless, given the range of interpretations possible for these low CPUE data, the projections continue to point to the need for an effort restriction to be applied to the midwater trawl fishery.