Browsing by Author "Coetzee, J C"
Now showing 1 - 11 of 11
Results Per Page
Sort Options
- ItemOpen AccessA 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 SThe 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.
- ItemOpen AccessAbrupt environmental shift associated with changes in the distribution of Cape anchovy Engraulis encrasicolus spawners in the southern Benguela(2007) Roy, C; van der Lingen, C D; Coetzee, J C; Lutjeharms, J R ECape anchovy Engraulis encrasicolus spawners in the southern Benguela showed an eastward shift in their distribution on the Agulhas Bank that occurred abruptly in 1996 and has since persisted. We assessed whether this shift was environmentally mediated by examining sea surface temperature data from different regions of the Agulhas Bank, which showed that in 1996 the inner shelf of the Agulhas Bank to the east of Cape Agulhas abruptly became 0.5°C colder than in previous years and has since remained that way. In addition, signals, coherent with the 1996 shift recorded in sea surface temperatures, were also found in atmospheric surface pressure and zonal wind data for that region; interannual coastal SST variability is also shown to be correlated with zonal wind-stress forcing. As a result, increased wind-induced coastal upwelling east of Cape Agulhas is proposed as the main driver of the observed cooling in the coastal region. The synchrony between the environmental and biological signals suggests that the eastward shift in anchovy spawner distribution was environmentally mediated and arose from a change in environmental forcing that altered the relative favourability for spawning between regions to the west and east of Cape Agulhas. The results highlight how a relatively minor change in environmental conditions can lead to a drastic spatial reorganisation of the life history of one species in an ecosystem.
- ItemRestrictedAlternative hypotheses of two mixing stocks of South African sardine: initial testing(2014) de Moor, Carryn L; Butterworth, Doug S; van der Lingen, C D; Coetzee, J CA new assessment of sardine based on data from 1984 to 2014 is planned to commence in a few months’ time. This assessment will eventually form the operating model which will be used to project the sardine population forward in time under alternative future catch scenarios – i.e. it will be used in the development of the next OMP to likely be finalised in 2016. In preparation for this assessment, some initial potential alternative two mixing stock hypotheses are considered using the previous assessment based on data from 1984 to 2011 (de Moor and Butterworth In Review) which has been used in the development of OMP-14. These tests would hopefully highlight any hypotheses that could have a substantial influence on model results and those which might not, and thus aid in prioritisation of future research. This document is a first stage of this process, firstly outlining the alternative hypotheses currently considered plausible, and then refitting the previous assessment at the joint posterior mode for some of these hypotheses.
- ItemRestrictedCan anchovy age structure be estimated from length distribution data collected during surveys?(National Inquiry Services Centre (NISC), 2013) De Moor, Carryn L; Butterworth, Doug S; Coetzee, J CHistorically, a time-series of proportions-at-age 1 from annual November hydro-acoustic surveys has been used to inform the assessment of, and in particular the choice of appropriate values for juvenile and adult natural mortality for, the South African anchovy Engraulis encrasicolus. However, information from direct ageing is limited and almost two decades old. A new method is developed to estimate the annual proportions of anchovy at age 1 directly from the population length distributions estimated from samples taken during the November surveys. This method involves modelling the annual length distributions of age 1 and age 2+ anchovy. The analysis provides a new time-series of proportions-at-age 1, together with associated standard errors, for input into assessments of the resource. The results also caution against the danger of scientists reading more information into data than is really there.
- ItemOpen AccessComparative harvest rates for sardine and anchovy stocks elsewhere(2016-08) Butterworth, Doug S; Coetzee, J CA risk analyses conducted by de Moor and Butterworth (2016a) in which they consider the risk to the sardine resource in terms of harvest proportion has prompted discussion about acceptable harvest proportions elsewhere for other small pelagic fish stocks.
- ItemOpen AccessDiscussion of OMP-17 simulation projection framework in respect of sardine(2016-09) de Moor, C L; Butterworth, Doug S; Coetzee, J CThe 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.
- ItemRestrictedFurther information provided by the Small Pelagic Working Group in respect of the IFFO FISHERY ASSESSMENT REPORT of anchovy and redeye round herring(2012) Coetzee, J C; Butterworth, Doug S“The South African management regime from the evidence assessed is robust and provides a solid framework for the management for the pelagic species within this fishery. The scientific evaluations of this stock and associated by catch stock are on-going and are used when management decisions on the total allowable catch (TAC) are made. The evidence of compliance to the set TAC to this scientific advice were positive and in compliance to the precautionary approach adopted within these pelagic fisheries. However, information on the fishing gear interaction with TEP species and what is recorded by the fishers was not available. The level of management enforcement was adequate but it effectiveness could also not be determined.”
- ItemOpen AccessInitial exploration of available data to estimate sardine recruitment on the south coast(2014) De Moor, Carryn L; Coetzee, J C; Butterworth, Doug SAnnual May/June survey estimates of sardine recruitment east of Cape Infanta are low in comparison to those west of Cape Infanta and in comparison to the numbers of recruits needed to result in the recent peak in 1+ biomass on the south coast. The assessment model for two sardine stocks (split east and west of Cape Agulhas) consequently estimates the majority of the “south” stock biomass to originate from “west” stock recruits, at least across the turn of the century (de Moor and Butterworth 2013). Although sardine are known to spawn throughout the year, this winter recruit survey is timed to measure the recruits emanating from the postulated spawning peak between September – March (van der Lingen and Hugget 2003). However, in contrast to observations on the west coast of South Africa, high sardine egg concentrations have also been observed in winter on the south coast (van der Lingen et al. 2005). This could potentially give rise to local south coast recruitment, which would not be detected during the winter recruit surveys. This document explores the extent to which winter spawning on the south coast and subsequent successful local recruitment there may have an impact on the “south” stock biomass with the data currently available, and proposes future work to explore this possibility further.
- ItemRestrictedRefined 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 SThe 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.
- ItemRestrictedRefined estimates of South African pelagic fish biomass from hydro-acoustic surveys:quantifying the effects of target strength, signal attenuation and receiver saturation(National Inquiry Services Centre (NISC), 2008) Coetzee, J C; Merkle, D; De Moor, Carryn L; Twatwa, N M; Barange, M; Butterworth, Doug SThe 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.
- ItemOpen AccessRevised estimates of abundance of South African sardine and anchovy from acoustic surveys adjusting for echosounder saturation in earlier surveys and attenuation effects for sardine(National Inquiry Services Centre (NISC), 2008) De Moor, Carryn L; Butterworth, Doug S; Coetzee, J CHydro-acoustic surveys have been used to provide annual estimates of May recruitment and November spawner biomass of the South African sardine Sardinops sagax and anchovy Engraulis encrasicolus resources since 1984. These time-series of abundance estimates form the backbone of the assessment of these resources, and consequently the management of the South African sardine and anchovy is critically dependent on them. Upgrades to survey equipment over time have resulted in recent surveys providing more accurate estimates of abundance, yet in order to maintain comparability across the full time-series, estimates of biomass mimicking the old equipment were used for a number of years. In this paper we develop a method to revise the earlier part of the time-series to correct for receiver saturation in the older generation SIMRAD EK400 and EKS-38 echo sounders and to account for attenuation in dense sardine schools. This is applied to provide a revised time-series of biomass estimates for the South African sardine and anchovy resources with associated variance–covariance matrices. Furthermore, the time-series presented here are based on updated acoustic target strength estimates, making this the most reliable time-series currently available for both resources.