OpenUCT :: Browsing by Author "Rademeyer, Rebecca A"

Browsing by Author "Rademeyer, Rebecca A"

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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).
Open Access
The 2006 operational management procedure for the South African Merluccius paradoxus and M. capensis resources
(2006) Rademeyer, Rebecca A; Glazer, Jean Patricia
The algorithm for the 2006 Operational Management Procedure (OMP) to provide TAC recommendations for the South African Merluccius paradoxus and M. capensis resources is empirical, increasing or decreasing the TAC in relation to the magnitude of recent trends in CPUE and survey abundance estimates for both species. The basis for the associated computations is set out below.
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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
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
2011 Routine update of the South African hake base reference case assessment
(2011) Rademeyer, Rebecca A
As required under the annual OMP review process, this paper presents a routine update of the South African hake Reference Case assessment (RS1) (Rademeyer and Butterworth, 2010), including new commercial (catches and CPUE) and survey (abundance estimates) data. This analysis does not include updated length distribution data.
Open Access
2012 Routine update of the South African hake reference case assessment
(2012) Rademeyer, Rebecca A
This paper presents a routine update of the South African hake Reference Case assessment (RS1) (Rademeyer and Butterworth, 2010), including new commercial (catches and CPUE) and survey (abundance estimates and length distribution) data. This analysis does not include post-2009 commercial length distribution data whose finalisation is still in progress.
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
The abundance of blue whales on the Madagascar Plateau, December 1996
(International Whaling Commission, 2003) Best, Peter B; Rademeyer, Rebecca A; Burton, Chris; Ljungblad, Don; Sekiguchi, Keiko; Shimada, Hiroyuki; Thiele, Deborah; Reeb, Desray; Butterworth, Doug S
As part of the International Whaling Commission’s SOWER blue whale research programme, two sighting vessels, the Shonan Maru and the Shonan Maru No.2 , surveyed the Madagascar Plateau between 25° and 35°S, 40° and 45°E, in December 1996. A total of 95 sightings of 110 blue whales (assigned in the field as pygmy blue whales – see discussion), 14 sightings of 21 blue whales (subspecies un determined) and 12 sightings of 13 ‘like blue’ whales was made in 23 days. In the first half of the survey, the whole research area was cov ered in a mainly pre-determined zigzag search pattern, and the associated sightings and effort have been used to derive density estimates for bl ue whales for the area. Sightings in the second half of the survey, where effort was directed at blue whale concentrations, have only bee n used to provide supplementary data for calculation of the effective search half-width and mean school size. The resulting population es timate is 424 (CV = 0.42), or 472 (CV = 0.48) whales when ‘like blue’ sightings are included. Dive times and surfacing behaviour recorded in just over 21h of monitoring suggest that the assumption that all groups on the trackline were seen ( g (0) = 1) is reasonable. As the geographical extent of the survey area was substantially less than that of past catches of blue whales in the region in December, this estim ate must refer to only a portion (possibly about one third) of the total population. Some evidence of feeding on euphausiids in the region was detected, possibly as a consequence of a localised upwelling cell at the southern tip of Madagascar
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ADDENDUM: further SCAA/ASPM assessments of Gulf of Maine Cod including data for 2007 and exploring the impact of age-dependence in natural mortality
(2008) Butterworth, Doug S; Rademeyer, Rebecca A
Deadlines for the submission of the main text of this paper did not allow time for the computation of Frebuild statistics. This statistic is reported here for the one case amongst the 16 assessment options of the main paper for which the estimate of spawning biomass for 2007 was less than B*sp MSY : BevertonHolt D (the “default” case with flat survey selectivity and M = 0.2), together with an explanation of the methodology used for the computation. Further, the main text indicated that rather than considering the mechanisms of domed survey selectivity, a higher age-independent M, or M increasing with age in isolation to account for the relative paucity of older cod in the surveys and fishery, this could be similarly achieved by considering these in combination. To illustrate this possibility, the results for one such combination (assessment Ricker I defined below) are presented here.
Open Access
Age-reading error matrices for Merluccius paradoxus and M. capensis
(2009) Rademeyer, Rebecca A
The revised methodology for hake assessments currently under development requires inputs on the relative biases amongst different readers of hake ages from otoliths, and the extent of variability of these readings. This document details the methodology applied to determine these ageing error matrices.
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Alternative stock structure hypotheses for the southern African hake resources
(2006) Butterworth, Doug S; Rademeyer, Rebecca A
This document summarises alternative stock structure hypotheses for the southern African hakes. The alternatives put forward could apply to either M. capensis or M. paradoxus. In the scenarios to be considered, the stock structure as modelled for M. capensis and M. paradoxus does not need to be the same for both species. Some of the hypotheses illustrated for age-structured movement patterns are, however, species specific.
Open Access
And yet further candidate management procedure testing for the South African hake resource
(2010) Rademeyer, Rebecca A; Butterworth, Doug S
This document reports progress on hake OMP revision issues identified at the last DWG meeting as requiring further attention. In particular it updates the Reference Case CMP in terms of the agreements reached at that meeting. Suggestions are made for the final set of calculations needed before a selection is made amongst CMPs at the following DWG meeting. In addition, proposals are made concerning revision of the hake-specific sections of the document governing “exceptional circumstances” for overriding an OMP TAC recommendation or bringing forward an OMP review. These include provisions relating the MSC condition concerning specification of Target and Limit Reference Points for M.paradoxus.
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Application of an Age-Structured Production Model to the Georges Bank Yellowtail Flounder. GARM-III working paper 2
(2008) Butterworth, Doug S; Rademeyer, Rebecca A
A Reference Case application of ASPM to the Georges Bank yellowtail flounder, together with three sensitivies, is presented. Strong residual patterns in the fits to survey indices of abundance, particularly to that for the Canadian DFO Spring survey, raise concerns about the compatibility of the population model and these indices. The model fits strongly favour domed over asymptotically flat selectivity. Selectivity assumptions are key to estimates of stock status, with fits to a fully flexible selectivity parameterization indicating the resource to be effectively at its MSY level sp BMSY , whereas imposing asymptotically flat selectivity sees the stock estimated to be below the “overfishing” threshold of 0.5 sp BMSY .
Open Access
Assessment of and management procedures for the hake stocks off southern Africa
(2003) Rademeyer, Rebecca A; Butterworth, Doug S
The hake fishery off southern Africa is a highly valuable and important fishery in terms of both revenue and local employment. This thesis considers the current assessment and management of the Cape hakes stocks. It commences with a brief review of the biology of the Cape hakes and the history of the fishery and the management of the resource. The data available for the assessments are described, giving some details as to how they are collected and analysed.
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Assessment of the South African Hake resource taking its two-species nature into account
(National Inquiry Services Centre, 2008) Rademeyer, Rebecca A; Butterworth, Doug S; Plagányi, Éva E
The commercially valuable hake fishery off South Africa consists of two morphologically similar species, the shallow-water Cape hake Merluccius capensis and the deep-water Cape hake M. paradoxus. Because catch-and effort statistics collected from the fishery are not species-disaggregated, previous published quantitative assessment methods have treated the two hake species as one. Furthermore, recent evidence suggests that (although treated as two separate populations in past assessments) the South and West coasts components of each species form a single stock. This paper describes the development of the first fully species-disaggregated coast-wide baseline assessment of the South African hake resource. M. paradoxus is estimated to be currently at <10% of its pre-exploitation level whereas M. capensis is estimated to be well above its maximum sustainable yield level. By taking into consideration the primary sources of uncertainty in this assessment, a Reference Set of 24 operating models is developed to be used in Operational Management Procedure testing.
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A Bayesian assessment of the South Atlantic population of albacore which explicitly models changes in targeting
(International Commission for the Conservation of Atlantic Tuna (ICCAT) Secretariat, 2004) Rademeyer, Rebecca A; Butterworth, Doug S; Penney, Andrew J
The primary assessment method applied to South Atlantic albacore during the October 2000 ICCAT SCRS assessment session was an age-structured production model which assumed a fleet-aggregated selectivity pattern with a single change only (in 1969), and estimated model parameters using only CPUE data. A particular problem of this approach is that the Japan CPUE series is not considered comparable over the complete period for which it is available because of changed targeting practices, so that this series needs to be split into three separate segments when estimating model parameters. This paper refines that 2000 assessment by treating the Japanese, Taiwanese and Brazilian fisheries all as initially primarily albacore directed fisheries which have shifted over time to bigeye tuna targeted fisheries, taking albacore as a decreasing bycatch. The transitions over time are explicitly estimated, as are separate selectivities for the two components of each national fishery by also including catch-at-age information in the data fitted by the model. Compared to the high levels of uncertainty in past assessments, the refined model provides a reasonable to good fit to all available information, and suggests that the productivity of the resource is somewhat higher than estimated previously.
Open Access
Building a bridge between the May and the September Reference Cases for the South African resource and related matters
(2016-09) Rademeyer, Rebecca A; Butterworth, Doug S
An investigation of the reasons for the changes in particularly M. paradoxus assessment results from the May to the September Reference Case (RC) assessments shows these to be almost entirely a consequence of the changed formulations for selectivities, with updating of and further years’ data having little impact. The probability of a TAC drop of greater than 5% under the current OMP is not high, and in terms of the September RC is not forecast to occur with more than 5% probability before the end of the decade.
Open Access
Building a bridge between the May and the September Reference Cases for the South African resource and related matters
(2016-09) Rademeyer, Rebecca A; Butterworth, Doug S
An investigation of the reasons for the changes in particularly M. paradoxus assessment results from the May to the September Reference Case (RC) assessments shows these to be almost entirely a consequence of the changed formulations for selectivities, with updating of and further years’ data having little impact. The probability of a TAC drop of greater than 5% under the current OMP is not high, and in terms of the September RC is not forecast to occur with more than 5% probability before the end of the decade.