Browsing by Subject "stock assessment"

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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.
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    An initial attempt at a spatially structured stock assessment for the South African hake resource including movement based on a gravity model.
    (2014) Rademeyer, Rebecca A
    The movement model for assessing the South African hake populations which was presented to last year’s review workshop is simplified by applying the “gravity” method to model movement. This reduces the numbering of movement parameters estimated from 234 to 68. This results in recent biomass estimates which are slightly more precise and somewhat lower in both absolute terms and relative to estimated pre-exploitation levels. Areas for possible future research are discussed briefly.
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    Life history traits that predispose South African linefishes to overexploitation
    (2018) Haupt, Meghan; Kerwath, Sven; Parker, Denham; Winker, Henning
    Globally, the status of many fish stocks remains unknown, of which the majority fall under data-limited small-scale fisheries. Management decisions in most of these fisheries are difficult due conflicting objectives and views from fisheries managers and scientists. In South Africa, the traditional boat-based ‘linefishery’ provides such an example of a small-scale, multi-species fishery with a long history. The historical de facto open access nature of this fishery resulted in continuous declines in catches of many linefish species, and in 2000 the fishery was declared to be in a state of emergency. This led to a reduction of up to 70% within the fishery, among other measures, such as introductions of size and bag limits. Assessing the status of linefish species is difficult due to a lack of reliable long-term data for the majority of species. The aims of this study were therefore: (1) to quantify the stock status for all linefish species with available life history and size composition information, (2) compare current and historical stock levels to ascertain if the reduction in effort facilitated any recovery in linefish species and (3) correlate the current stock status estimates to life history traits to identify simple indicators of resilience to exploitation. For this purpose, length frequency data from 1988-1990 and 2008-2010 and biological parameters sourced from literature were used to conduct per-recruit analysis to estimate spawner biomass depletion (SBD) for both time periods. The majority of the 26 species analyzed, (68%) showed improvements in spawner biomass between the two time periods, with 12 species undergoing a change in stock status (i.e. improving from collapsed or overexploited). Specifically, increases in length-at-capture (Lc) as well decreases in fishing mortality (F) facilitated recovery for many species. Asymptotic length (L∞), as well as the ratio between Lc / L∞ and Lc / Lopt (where Lopt is the optimum length) were found to be significantly correlated to spawner biomass depletion. Kruskal Wallis analyses revealed that only movement pattern had a significant relationship to SBD, more specifically, migratory species were significantly more depleted than resident ones. This study identifies simple indicators that, in the absence of conventional stock assessments, provide fisheries managers with a fundamental understanding of a species’ susceptibility to overexploitation – offering another decision making tool for use in data poor fisheries such as the South African linefishery.
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    Sensitivity of the west coast rock lobster length-based stock assessment to a consistently higher somatic growth rate
    (2005) Johnston, Susan J; Butterworth, Doug S
    A recent study by Dubula et al. (2005) on the effect of tagging on the subsequent growth rate of rock lobsters has shown that there may be an appreciable reduction in the growth rate of male rock lobsters as a result of tagging. The amount of somatic growth reduction that might be occurring is of the order 2-3 mm per annum. A key question then to consider is what implications this might have for the sustainable productivity from the resource.
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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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    Underlying assumptions for the area-disaggregated stock assessment of west coast rock lobster
    (2007) Johnston, Susan J; Butterworth, Doug S
    The West Coast resource is divided into the following “super” areas: Area 1-2: The most Northern region – only a very small tonnage is currently caught in this area. Area 3-4 (Areas 3 and 4 only) Area 5-6 (Areas 5 and 6 only) Area 7 (Dassen Island), and Area 8+ (areas to the south and east of Dassen Island, which include Area 8 (Cape Point), Area 10 (Hout Bay), Area 11 (False Bay), and Areas 12-14 (East of Hangklip). Figure 1 illustrates the West Coast rock lobster fishing zones and areas. Various methods have been used to collate data from Areas 3, 4, 5 and 6 to produce values for the “super” Area 3-6. Note that the convention used here for referring to split seasons is to quote only the first of the two years for each split season, i.e. the 1992/93 season is referred to here as “1992”. A size-structured modelling approach (as described in RLWS/DEC05/ASS/7/1/2) is used to model the dynamics of each super-areas independently. The area-disaggregated modelling approach assumes that recruitment from each area remains in that area and that there is no immigration or emigration between areas (of larvae or adults)