Browsing by Author "de Moor, C L"

Now showing 1 - 11 of 11
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    A summary of the South African sardine (and anchovy) fishery. MARAM/IWS/2019/Sardine/BG1
    (University of Cape Town, 2019) Coetzee, J C; de Moor, C L; Butterworth, Doug S
    The sardine fishery is an important component of the South African purse-seine fishery, the largest commercial fishery in South Africa (by landed mass). This fishery, initially established on the West Coast, but with some subsequent infrastructure development on the South Coast, is currently under pressure because of recent low biomass levels, reduced Total Allowable Catches (TACs) and frequent changes in the spatial distribution of the resource. The current low biomass followed from prolonged poor recruitment, whereas the distributional changes are plausibly linked to processes related to spatial structuring of the population, which is now hypothesized to comprise multiple components (western, southern and eastern), with interchange amongst them. Given the predominantly west-coast-based location of sardine processing infrastructure, exploitation levels on the western component have been high relative to other components, particularly when most of the biomass is located on the south coast. This has necessitated the implementation of a form of spatial management to promote both a healthy ecosystem and a more soundly managed resource. This document summarises the history of the fishery, the current status of the resource and data used in its assessment and management.
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    The Agreed Method to Determine the Maximum Sardine Catch West of Cape Agulhas During 2017
    (2017-06) de Moor, C L
    This document details the method agreed at the Small Pelagic Scientific Working Group meeting on 27th June 2017 to be used to inform the maximum directed >14cm sardine catch to be recommended to be caught west of Cape Agulhas during 2017. The agreement involves setting the maximum directed >14cm sardine catch west of Cape Agulhas for 2017 to the average of the maximum catch calculated from two different methods. Note that this maximum may also not be more than the total directed >14cm sardine TAC for 2017 as calculated by OMP-14.
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    Assessing the South African sardine resource: two stocks rather than one?
    (National Inquiry Services Centre, 2015) de Moor, C L; Butterworth, Doug S
    Sardine Sardinops sagax distributed off the west and south coasts of South Africa have traditionally been assumed to comprise a single well-mixed stock for assessment and management purposes. New research, however, lends weight to the possibility of two stocks in this region. A precautionary management approach thus needs to consider the impact of management decisions on the hypothesised two individual stocks as well as on the resource as a whole. As a first step in this process, Bayesian assessments of South African sardine are presented, which compare results for the traditional single-stock hypothesis with those that follow from a new two-mixing-stock hypothesis. Recruits from the west stock are assumed to move to and remain part of the south stock in annual pulses of varying size. This movement is estimated to be appreciable, and to take place from a substantially more productive west stock to the south stock. This immigration makes a greater contribution to the south-stock biomass than do years of above-average south-stock recruitment. Importantly, this two-mixing-stock hypothesis is shown to be consistent with the data available. Further alternative sardine stock-structure hypotheses suggested by the most recent data are discussed.
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    Assessment of the South African sardine resource using data from 1984-2015: Results at the joint posterior mode for the single stock hypothesis
    (2016-07) de Moor, C L; Butterworth, Doug S
    The assessment of the South African sardine resource has been revised and updated using data available up to November 2015. Two primary hypotheses regarding the sardine stock structure have been agreed for investigation. The first considers sardine distributed off the west and south coasts of South Africa to form a single homogeneous “stock” (or “population”). The second considers the sardine to consist of a western stock and southern stock with some mixing between the two. While there is growing evidence supporting the existence of sub-population structure amongst sardine distributed off the west and south coasts of South Africa (e.g. Coetzee et al. 2008, van der Lingen et al. 2009, 2015, van der Lingen 2011, Weston et al. 2015), the single stock hypothesis continues to be modelled as it allows for easy comparison to past assessments and, in particular, to past risk statistics and previous Operational Management Procedures. It also reflects a limiting case of the mixing model as the extent of mixing becomes very large. This document presents results at the joint posterior mode for the single stock hypothesis only.
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    Assessment of the South African sardine resource using data from 1984-2015: Results at the joint posterior mode for the two mixing-stock hypothesis.
    (2016-07) de Moor, C L; Butterworth, Doug S
    The Operating Model (OM) for the South African sardine resource has been updated from that used to develop OMP-14 given four more years of data. The model has been altered from previous assessments to now model infection of sardine by a “tetracotyle”-type metacercarian parasite, and includes data on this parasite prevalence-by-length in the likelihood. A Hockey-Stick stock recruitment relationship is used for this OM, with a different median recruitment and higher variability estimated for the west stock during “peak years”. Time-invariant natural mortality is assumed to be 1.0 year-1 for juveniles and 0.8 year-1 for adults as before. The total resource abundance is estimated to be 688 thousand tons in November 2015, with the west stock consisting of 142 thousand tons and the south stock consisting of 546 thousand tons. These biomasses are well below the long term average of 1 039 and 492 thousand tons for the total resource and west stock respectively, and near the long term average of 547 thousand tons for the south stock. The west stock has experienced below average recruitment in 11 out of the 12 most recent years.
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    Discussion of OMP-17 simulation projection framework in respect of sardine
    (2016-09) de Moor, C L; Butterworth, Doug S; Coetzee, J C
    The simulation projection framework to be used to test Candidate Management Procedures for OMP-17 is yet to be developed (cf de Moor and Butterworth (2013a) for the framework used when developing OMP-14). However, given experience gained during the testing of OMP-14, some initial assessment results (de Moor and Butterworth 2016a,b) and discussions in the SPSWG, we can begin to consider some alternative key assumptions to be used in the simulation projection framework to be used for developing OMP-17.
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    A proposal for determining the final desirable maximum catch of directed sardine west of Cape Agulhas during 2017
    (2017-06) de Moor, C L
    The maximum directed >14cm sardine catch recommended to be caught west of Cape Agulhas during 2017 was initially set at 21 400t (DAFF 2016) based on the method set out in de Moor and Butterworth (2016). The mid-year revision in the South African sardine and anchovy TACs and TABs for 2017 is to be based on OMP-14, as OMP-17 is still under development. The maximum directed >14cm sardine catch recommended to be caught west of Cape Agulhas thus requires some revision to take account of further work on the underlying sardine Operating Model and information resulting from the 2017 recruit survey.
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    A proposal for determining the initial desirable maximum catch of directed sardine west of Cape Agulhas during 2017, with suggestions on how this might be achieved
    (2016-12) de Moor, C L; Butterworth, Doug S
    Butterworth (2016) outlined a draft process for setting the initial directed >14cm sardine TAC for 2017. This document expands on the detail of that draft process, with some changes/updates. Further, it provides initial projections from November 2015 to November 2017, assuming that the May 2016 survey estimate of recruitment was observed without error. Ideally the SWG-PEL will select a low probability for November 2017 projected 2+ biomass being below the inflection point of the hockey-stick stock recruitment curve at the meeting on 7th December, based on the initial results presented below. In addition, the SWG-PEL may need to select a percentage increase above the recommended west coast catch limit that could be used to recommend an initial directed >14cm sardine TAC. This latter step seems to need consideration because formal spatial restrictions will not at this stage be included on the permits with the initial TAC allocation. Thus, as a worst case scenario, the majority of this initial TAC may be caught on the west coast. The remaining steps in this process are proposed to be undertaken after the November 2016 hydroacoustic survey estimate of abundance becomes available.
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    Refined estimates of South African pelagic fish biomass from hydro-acoustic surveys: quantifying the effects of target strength, signal attenuation and receiver saturation
    (2008) Coetzee, J C; Merkle, D; de Moor, C L; Twatwa, N M; Barange, M; Butterworth, Doug S
    The biomass of small pelagic fish species off the coast of South Africa has been monitored since 1984 using hydro-acoustic survey techniques. These time-series of spawner biomass and recruitment estimates form the basis for management of both the South African sardine Sardinops sagax and anchovy Engraulis encrasicolus resources and are central to the setting of annual total allowable catch levels. However, these survey estimates have, for the most part, been treated as relative indices as there are several biases inherent in acoustic survey methodology that remain difficult to quantify. Advances in acoustic technology together with an improved understanding of the major sources of survey errors have enabled estimation of and correction for biases such as receiver saturation, acoustic signal attenuation and target strength. Incorporation of these corrections over the entire time-series has resulted in an improved accuracy of acoustic survey estimates and substantial changes to the biomass estimates of both species, without jeopardising the requirement that the time-series remains comparable throughout its duration. Furthermore, the resultant decrease in the level of uncertainty associated with these abundance estimates has rendered improved utilisation of these resources possible.
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    TABs in OMP-17
    (2017-02) de Moor, C L; de Goede, J
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    The two mixing stock hypothesis for South African sardine without an assumed stock-recruit relationship
    (2016-11) de Moor, C L
    The two mixing stock hypotheses for South African sardine presented in de Moor and Butterworth (2016) and de Moor (2016) assumed a Hockey-Stick stock-recruit relationship, with the stock-recruit parameters estimated to differ by “effective spawning stock”. In addition, recruitment to the west stock during the “peak” 2000-2004 years was estimated to vary about a different median level. In this document the two-mixing stock hypothesis is re-run without any assumed stock-recruit relationship.