Browsing by Subject "abalone"

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    Preliminary analysis of catch-per-unit-effort data for abalone in Zones A-D
    (2008) Plagányi, Éva E
    The General Linear Model (GLM) approach used last year needs to be updated for use in standardising the commercial abalone CPUE time series (1980 – 2006) of abalone for the influence of other factors on the CPUE apart from resource abundance. The raw data were supplied by A. Mackenzie and G. Maharaj (MCM). The number of new data entries added to the database is 483 (all from Zone B), bringing the total number of data entries to 41432. The comparability of the new data with the older data needs to be discussed. Note that the recent nominal CPUE for Zone B is the lowest yet.
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    A spatial- and age-structured assessment model to estimate the impact of illegal fishing and ecosystem change on the South African abalone Haliotis midae resource
    (National Inquiry Services Centre, 2010) Plagányi, Éva E; Butterworth, Doug S; ;
    The management of abalone stocks worldwide is complicated by factors such as illegal fishing combined with the difficulties of assessing a sedentary (but not immobile) resource that is often patchily distributed. The South African abalone Haliotis midaefishery is faced with an additional problem in the form of a relatively recent movement of rock lobsters Jasus lalandii into much of the range of the abalone. The lobsters have heavily reduced sea urchin Parechinus angulosus populations, thereby indirectly negatively impacting juvenile abalone which rely on the urchins for shelter. A model is developed for abalone that is an extension of more standard age-structured assessment models because it explicitly takes spatial effects into account, incorporates the ecosystem change effect described above and estimates the magnitude of substantial illegal (‘poached’) catches. The model is simultaneously fitted to catch per unit effort and Fishery-Independent Abalone Survey abundance data, as well as to several years of catch-at-age (cohort-sliced from catch-at-size) data for the various components of the fishery and different spatial strata. It constitutes the first quantitative approach applied to the management of this commercially valuable resource in South Africa and has provided a basis for management advice over recent years by projecting abundance trends under alternative future catch levels.
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    Summary of estimates of recent trends in abalone poaching
    (2011) Maharaj, G; Butterworth, Doug S
    Preliminary analyses using three separate indices to estimate the trend in poaching suggest that the target of a 15% per annum reduction commencing October 2009 in poaching has not been reached. In fact, all indices suggest an increase in poaching. The 15% per annum decline was the key criterion for rebuilding the abalone resource and was amongst the conditions set by the Minister associated with reopening the fishery.
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    A summary of the assessment and management approach applied to South African abalone (Haliotis midae) in Zones?A-D
    (2007) Plagányi, Éva E
    The management of abalone stocks worldwide is complicated by factors such as poaching combined with the difficulties of assessing a sedentary (but not immobile) resource that is often patchily distributed. The South African abalone Haliotis midae fishery is faced with an additional problem in the form of a movement of rock lobsters Jasus lalandii into much of the range of the abalone. The lobsters have dramatically reduced sea urchin Parechinus angulosus populations, thereby indirectly negatively impacting juvenile abalone, which rely on the urchins for shelter. The model developed for abalone is an extension of more standard age-structured assessment models because it explicitly takes spatial effects into account, incorporates the ecosystem change effect described above and formally estimates illegal catches using a novel index, the Confiscations Per Unit Policing Effort (CPUPE). The model is simultaneously fitted to CPUE and Fishery-Independent Abalone Survey (FIAS) abundance data as well as several years of catch-at-age (cohort-sliced from catch-at-length) data for the various components of the fishery as well as for different strata. A basic tenet of fisheries modelling is to not go beyond the information content of the data. The model developed involves the efficient use of data to allow a model of greater complexity (as was essential in this instance) than usual. It has provided the basis for management advice over recent years by projecting abundance trends under alternative future catch levels.
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    Summary of the GLM used to standardize abalone catch-per-unit-effort data for Zones A-D over the period 1980-2006
    (2007) Plagányi, Éva E; Edwards, Charlie
    The General Linear Model (GLM) approach used last year is here updated for use in standardising the commercial abalone CPUE time series (1980 – 2006) of abalone for the influence of other factors on the CPUE apart from resource abundance. The raw data were supplied by A. Mackenzie (MCM). The explanatory variables included in the GLM are as in previous years. The same is true for the interaction terms in the model. The purpose of refitting a GLM to the abalone data is to update the results of Plaganyi (2006) using the data available for the 2006 Model year (October 2005 – September 2006). The method described by Plaganyi (2006) has been replicated as far as possible.
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    Was overfishing of predatory fish responsible for a lobster-induced regime shift in the Benguela?
    (Elsevier, 2014) Blamey, Laura K; Plagányi, Éva E; Branch, George M
    The top-down effects of predators have been demonstrated for terrestrial, freshwater and marine systems and their removal can cause a shift in ecosystem state. In many cases, the depletion of top-predators occurred long before humans began monitoring these systems, but models can elucidate likely ecosystem changes. In this paper we use a multispecies model to demonstrate that the abundance of predators can induce different ecosystem states, emphasising the importance of sustainable harvesting. Our model is founded on empirical data documenting a regime-shift that took place in the kelp-forest ecosystem along the south-west coast of South Africa following an ‘invasion’ by the West Coast rock lobster Jasus lalandii in the early 1990s. Formerly dominated by herbivores and encrusting corallines, the ecosystem became dominated by lobsters and macroalgae. A combination of illegal fishing of abalone Haliotis midae and the lobster-induced extermination of the urchin Parechinus angulosus (which shelters juvenile abalone) depleted abalone stocks severely. In this currently over-fished ecosystem where predators of lobsters are scarce, the regime-shift appears irreversible. Using an intermediate complexity model of lobster, urchin and abalone relationships, we simulated the effects of predatory fish at various population levels on the course of development of the currently lobster-dominated system. Our results indicate that current levels of fish biomass (<10% of the pristine population) would have been too small to significantly affect the lobster population. However, at pristine levels of fish abundance (or even 50% less), the invasion of lobsters would have been controlled by linefish, and urchin and abalone populations would not have collapsed. The effect of the lobster invasion had a greater influence on abalone abundance than illegal fishing, but together they severely depleted the abalone. Ecosystem effects of historical exploitation of top predators are of central concern, and our ecosystem model indicates that depletion of linefish had severe consequences for lobsters, urchins and abalone, illustrating how overfishing of top predators may precipitate regime shifts, with profound socio-economic consequences.