Indicators of sustainable fishing for South African sardine Sardinops sagax and anchovy Engraulis encrasicolus
- Authors: Fairweather, T P , Van der Lingen, C D , Booth, Anthony J , Drapeau, L , Van Der Westhuizen, J J
- Date: 2010
- Language: English
- Type: text , article
- Identifier: http://hdl.handle.net/10962/124778 , vital:35691 , https://doi.10.2989/18142320609504215
- Description: Six indicators were investigated for South African sardine Sardinops sagax and anchovy Engraulis encrasicolus: mean length of catch, length-at-50% maturity, total mortality, exploitation rate, ratio of bycatch, and centre of gravity of commercial catches. Sardine length-at-50% maturity is the most promising as a descriptive indicator because it is positively correlated with population size for an extensive time period (1953–2005). The remaining indicators were limited by shorter data-series (1984–2005). However, mean length of catch, ratio of bycatch and exploitation rate were found to be useful when considered in conjunction with other indicators. The centre of gravity of commercial sardine catches has shown a significant eastward shift from the West Coast, whereas that of anchovy has remained off that coast. Ratio of bycatch indicates that school composition is a reliable descriptive indicator of relative abundance in the two species. Fewer indicators for anchovy were useful, which is attributed to this species’ flexible life-history pattern.
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Shark fishing effort and catch of the ragged-tooth shark Carcharias taurus in the South African competitive shore-angling fishery
- Authors: Dicken, Matthew L , Booth, Anthony J , Smale, Malcolm J
- Date: 2010
- Language: English
- Type: text , article
- Identifier: http://hdl.handle.net/10962/126944 , vital:35937 , https://doi.10.2989/18142320609504209
- Description: In South Africa, Carcharias taurus is commonly known as the ragged-tooth shark or raggie. The species is also referred to as the sand-tiger shark in North America and as the grey-nurse shark in Australia. It is a long-lived species with an estimated longevity of up to 40 years (Goldman 2002). Female sharks reach sexual maturity at approximately 10 years (Goldman 2002), and they exhibit a biennial reproductive cycle (Branstetter and Musick 1994, Lucifora et al. 2002, G Cliff, Natal Sharks Board, unpublished data). Intra-uterine cannibalisation results in a maximum fecundity of two pups per litter after a gestation period of approximately 9–12 months (Bass et al. 1975, Gilmore et al. 1983). These life-history characteristics make this species particularly susceptible to overexploitation.
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Spatial and seasonal distribution patterns of the ragged-tooth shark Carcharias taurus along the coast of South Africa
- Authors: Dicken, Matthew L , Booth, Anthony J , Smale, Malcolm J
- Date: 2010
- Language: English
- Type: text , article
- Identifier: http://hdl.handle.net/10962/127008 , vital:35943 , https://doi.10.2989/18142320609504210
- Description: Off South Africa, the ragged-tooth shark Carcharias Taurus has been occasionally reported from the West Coast, but it is more commonly found along the East and South coasts from Cape Town to northern KwaZulu-Natal (KZN) (Bass et al. 1975, Smale 2002). Mating is thought to occur off the south coast of KZN from October to late November (G Cliff, Natal Sharks Board, unpublished data). Pregnant females then move northward to spend the early part of their gestation in the warmer waters of northern KZN and possibly southern Moçambique. During July and August, the near-term pregnant females begin to move southwards towards the cooler waters of the Eastern Cape (Wallett 1973, Bass et al. 1975, G Cliff, unpublished data), where they give birth from September to November (Smale 2002). After parturition, many of the females migrate back to KZN. The whereabouts of mature males outside of the mating season is unclear. These broadscale distribution and migratory habits for C. taurus have been inferred from limited catch records obtained for only parts of its range along the South African coast.
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Spatial description of hake-directed fishing activity off the west coast of South Africa
- Authors: Fairweather, T P , Booth, Anthony J , Sauer, Warwick H H , Leslie, Rob W
- Date: 2010
- Language: English
- Type: text , article
- Identifier: http://hdl.handle.net/10962/127031 , vital:35945 , https://doi.10.2989/18142320609504129
- Description: Historically, the two species of Cape hakes Merluccius capensis and M. paradoxus off South Africa were commercially exploited exclusively by demersal trawling. In 1994, hake-directed demersal longline was introduced on an experimental basis, and in 1998 was initiated as a commercial fishing sector. The effect of a combined fleet composed of both trawlers and longliners on the Cape hake resource is not fully understood. Analysis of fishing intensity and catch-rate data revealed that the highest catch rates were found around the 400m and 500m isobaths for the trawl and longline fisheries respectively. Catch rates from both fishing sectors were also noted to be higher over sediments with a sand component. Differences between areas of the highest fishing intensity and highest catch rates were noted. In addition to other factors, it is suggested that a ‘friction of distance’ effect applies — vessels will trade-off higher catch rates with the increased costs associated with fishing in deeper waters.
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Maximum likelihood and Bayesian approaches to stock assessment when data are questionable
- Authors: Booth, Anthony J , Quinn II, T J
- Date: 2006
- Language: English
- Type: Article
- Identifier: vital:6764 , http://hdl.handle.net/10962/d1007962
- Description: This study examines the use of age-structured maximum likelihood and Bayesian approaches for stock assessment of the Namibian monkfish, Lophius vomerinus, resource with questionable data, in which time series are short, abundance indices are variable, and research data conflict with commercial data. Bayesian approaches with both noninformative and informative priors are investigated to determine if they enhance estimation stability. Three data scenarios are assessed: commercial and research survey data, research survey data only, and commercial data only. Both statistical approaches show that resource abundance has decreased with exploitable biomass estimated at approximately 44% of pristine levels. The maximum likelihood and the Bayesian approach with noninformative priors result in similar estimates. As the abundance data contained little information pertaining to possible density dependence within the stock–recruit relationship, only a Bayesian approach with informative priors reduces uncertainty in the steepness parameter h. Estimated management quantities are sensitive both to the set of data sources and whether prior information was informative or not. The strengths of the Bayesian approach include the integration of prior information with uncertain data, the exploration of data conflicts, and the ability to show the uncertainty in estimates of management parameters. Its weakness is that estimation stability is dependent on the choice of priors, which alters some posterior distributions of management quantities.
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Predictor variables for moggel (Labeo umbratus) biomass and production in small South African reservoirs
- Authors: Potts, Warren M , Booth, Anthony J , Hecht, Thomas , Andrew, Timothy G
- Date: 2006
- Language: English
- Type: text , article
- Identifier: http://hdl.handle.net/10962/125763 , vital:35815 , https://doi.10.2989/16085910609503877
- Description: South Africa has approximately 3 100 registered reservoirs, ranging in size from 1–1 000 hectares, with a surface area totalling 84 439 hectares (SADC Surface Water Body Database, unpublished data). Within southern and eastern Africa, Lindqvist (1994) estimated the number of small reservoirs to be between 50 000 and 100 000. Given Bernacsek’s (1986) estimate of the total fishery potential of small reservoirs in Africa at between 1 and 2.3 million tons, this number of reservoirs clearly could provide fishery opportunities for rural communities.
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Preliminary observations of tag shedding, tag reporting, tag wounds, and tag biofouling for raggedtooth sharks (Carcharias taurus) tagged off the east coast of South Africa
- Authors: Dicken, Matthew Laurence , Booth, Anthony J , Smale, Malcolm John
- Date: 2006
- Language: English
- Type: text , article
- Identifier: http://hdl.handle.net/10962/125774 , vital:35816 , https://doi.10.1016/j.icesjms.2006.06.009
- Description: Mark-recapture models do not distinguish how ‘‘deaths’’ accrue to marked animals in the population. If animals lose their tags, then recaptures will be fewer than expected and estimates of survival will be underestimated (Arnason and Mills, 1981; McDonald et al., 2003). Similarly, if the non-reporting rate is unknown and assumed to be negligible, as is the case in some tagging studies (e.g. Cliff et al., 1996, for white sharks Carcharodon carcharias), the probability of capture can be underestimated. The effects of both these problems, inherent in cooperative tagging programmes, lead to too few tagged fish being recovered, with a positive bias on the estimation of population size. These effects are most pronounced when capture probability is low and fewer tags are available for recapture (McDonald et al., 2003).
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Spatio-temporal patterns in maturation of the chokka squid (Loligo vulgaris reynaudii) off the coast of South Africa
- Authors: Olyott, L J H , Sauer, Warwick H H , Booth, Anthony J
- Date: 2006
- Language: English
- Type: Article
- Identifier: vital:6763 , http://hdl.handle.net/10962/d1007930
- Description: Knowledge of the temporal and spatial characteristics of chokka squid (Loligo vulgaris reynaudii) biology in South African waters is limited, so the possibility of there being a geographically fragmented stock was examined by investigating the distribution of maturity patterns for the species, covering all known spawning areas and using both historical and recent data. Gonadosomatic indices (GSI) varied between year-round consistency and apparent seasonal peaks in both summer and winter; there was no clear spatial pattern. Monthly percentage maturity provided further evidence for two peak reproductive periods each year, although mature squid were present throughout. Sex ratios demonstrated great variability between different areas and life history stages. Male-biased sex ratios were only apparent on the inshore spawning grounds and ranged between 1.118:1 and 4.267:1. Size at sexual maturity was also seasonal, squid maturing smaller in winter/spring than in summer/autumn. Also, squid in the east matured smaller than squid in the west. Although the results from the present study do not provide conclusive evidence of distinct geographic populations, squid likely spawn over a significantly larger area of the Agulhas Bank than previously estimated, and squid on the west coast of South Africa may return to spawn on the western portion of the Agulhas Bank. It remains likely, however, that the east and west coast populations are a single stock and that migration of juveniles to the west coast and their subsequent return as sub-adults is an integral but non-essential and variable part of the life history.
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