Cape Town, South Africa
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Pieterse A.,Stellenbosch University | Pitcher G.,Fisheries Research and Development | Pitcher G.,University of Cape Town | Naidoo P.,Institute for Animal Production | Jackson S.,Stellenbosch University
Journal of Shellfish Research | Year: 2012

The Pacific oyster Crassostrea gigas is cultured at 8 commercial farms in South Africa. Worldwide, environmental-specific intensive selection on the species optimizes commercially beneficial traits, but its performance has not been studied in South Africa. From May 2010 to March 2011, we compared 2-mo measurements of growth rate, condition, and survival of 3 cohorts of different origin in longline culture at 3 different South African environments: 2 sea-based farms located in Saldanha Bay (Western Cape) and Algoa Bay (Eastern Cape) and a land-based farm at Kleinzee (Northern Cape). Overall, Saldanha Bay was cooler (mean sea surface temperature of 16.0°C; CV, 16.2%) than the other 2 localities, which did not differ significantly from one another (Kleinzee: 18.6°C; CV, 20.4%; Algoa Bay: 17.8°C; CV, 8.9%). The high variability at Kleinzee reflected stronger summer warming than at the other 2 farms. Saldanha Bay had higher phytoplankton biomass (mean, 14.3 mg chlorophyll a/m3; CV, 54.2%; May 2010 to March 2011) than did Algoa Bay (mean, 5.3 mg chlorophyll a/m3; CV, 81.0%; September 2010 to March 2011). The 3 cohorts showed similar trends in growth and condition. Growth rates, expressed as live or dry mass gains, were 2-10 times those reported elsewhere in the world, and dry weight condition indices were also high. High live mass growth rates in Algoa Bay, despite its relatively low phytoplankton biomass, seem to reflect a similar phenomenon to that reported in other relatively phytoplankton-poor grow-out environments, such as the Mediterranean Thau Lagoon in France. Dry meat mass gain and condition were highest for oysters in Saldanha Bay, with high food availability offsetting the thermal advantages of the warmer Algoa Bay site. Oysters in the bottom layers of the cages grew significantly faster than those in the top layers, particularly in Saldanha Bay, possibly reflecting fine-scale vertical differences in phytoplankton biomass. Saldanha Bay is the best of the 3 locations to produce market-ready oysters. Algoa Bay yields faster growth but leaner oysters and is a good nursery location, as is Kleinzee, which yields overall slow growth but good shell quality in winter and early spring. Copyright © 2013 BioOne.


Pitcher G.C.,Fisheries Research and Development | Pitcher G.C.,University of Cape Town | Probyn T.A.,Fisheries Research and Development | Du Randt A.,Fisheries Research and Development | And 5 more authors.
Journal of Geophysical Research: Oceans | Year: 2014

Acquisition of high resolution time series of water column and bottom dissolved oxygen (DO) concentrations inform the dynamics of oxygen depletion in St Helena Bay in the southern Benguela upwelling system at several scales of variability. The bay is characterized by seasonally recurrent hypoxia (<1.42 ml l-1) associated with a deep pool of oxygen-depleted water and episodic anoxia (<0.02 ml l-1) driven by the nearshore (<20 m isobath) decay of red tide. Coastal wind forcing influences DO concentrations in the nearshore through its influence on bay productivity and the development of red tides; through shoreward advection of the bottom pool of oxygen-depleted water as determined by the upwelling-downwelling cycle; and through its control of water column stratification and mixing. A seasonal decline in bottom DO concentrations of 1.2 ml l-1 occurs with a concurrent expansion of the bottom pool of oxygen depleted water in St Helena Bay. Upwelling of this water into the nearshore causes severe drops in DO concentration (<0.2 ml l-1), particularly during end-of-season upwelling, resulting in a significant narrowing of the habitable zone. Episodic anoxia through the entire water column is caused by localized degradation of red tides within the confines of the shallow nearshore environment. Oxygenation of the nearshore is achieved by ventilation of the water column particularly with the onset of winter mixing. No notable changes are evident in comparing recent measures of bottom DO concentrations in St Helena Bay to data collected in the late 1950s and early 1960s. Key Points Phenology of coastal upwelling influences bay productivity and red tides Bay is subject to seasonally recurrent hypoxia and episodic anoxia No change in deep pool of seasonally hypoxic water over past 50 years © 2014. American Geophysical Union. All Rights Reserved.


Mirimin L.,Stellenbosch University | Mirimin L.,Mayo Institute of Technology, Galway | Kerwath S.,Fisheries Research and Development | Kerwath S.,Stellenbosch University | And 7 more authors.
Fisheries Research | Year: 2016

Silver kob (Argyrosomus inodorus) is a sciaenid fish that has been targeted for more than 150 years by commercial and recreational fisheries in the coastal waters of Namibia and South Africa, where four main stocks have been hypothesised. In the present study, recently developed molecular markers were utilized to evaluate levels of genetic diversity and population structuring among such stocks. Moderate levels of genetic diversity and signatures of bottlenecking were found in the South African stocks (South Western Cape, Southern Cape and South Eastern Cape). Significant population genetic differentiation was detected between Namibia and South Africa, but not among the three putative South African stocks, indicating that the main oceanographic barrier for silver kob populations is to be found in the cool-temperate Benguela region, corroborating recent genetic studies. The lack of genetic structuring in South African waters indicates strong gene flow among the 3 South African putative stocks, which are characterized by distinct age and growth patterns. Thus, in order to guarantee a successful recovery of silver kob stocks, genetic findings should be used in conjunction with life-history and behavioral data to tailor future management measures aimed at mitigating the effects of different fisheries throughout the species' distribution range. © 2016.


Mirimin L.,Stellenbosch University | Mirimin L.,Mayo Institute of Technology, Galway | Macey B.,Fisheries Research and Development | Kerwath S.,Fisheries Research and Development | And 7 more authors.
Marine and Freshwater Research | Year: 2016

Overfishing has led to the collapse of many marine fish stocks along the South African coast, particularly species characterised by predictable distribution patterns and vulnerable life-history traits. Dusky kob (Argyrosomus japonicus) is an estuarine-dependent sciaenid fish that has been severely depleted by overfishing and is currently managed by suboptimal management measures aggravated by poor enforcement and compliance to regulations. The present study utilised microsatellite markers to evaluate levels of genetic diversity and population structuring of dusky kob along the South African coast, including five estuarine areas. Results showed signatures of bottlenecking, low and declining trends of effective population size and weak differentiation among samples collected along 2000km of coastline, indicating that dusky kob continues to be subject to the detrimental influence of fishing pressure. Additionally, because dusky kob is a key species in the emerging South African aquaculture sector, parentage analyses were used to successfully identify the origin of first-generation (F1) individuals from a commercial hatchery, hence allowing traceability of farmed products and discrimination between wild and farmed fish. Findings from the present study provided essential information to aid future management of wild populations, as well as to establish sustainable fish farming. © CSIRO 2016.


Pitcher G.C.,Fisheries Research and Development | Pitcher G.C.,University of Cape Town | Probyn T.A.,Fisheries Research and Development
Harmful Algae | Year: 2011

Oxygen deficiency in the southern Benguela has a pronounced negative impact on living marine resources and within the greater St Helena Bay anoxia is the cause of large episodic mortalities of the rock lobster Jasus lalandii. These impacts have motivated further investigation, specifically of the role of high biomass dinoflagellate blooms, commonly known as red tides, in the development of anoxic conditions. A high resolution time series of dissolved oxygen concentrations obtained from a bottom mooring off Elands Bay, located within the greater St Helena Bay region, is examined in relation to the development of an exceptional bloom of the dinoflagellate Ceratium balechii and an anoxia-induced mass mortality. A clear seasonal trend is evident in bottom dissolved oxygen concentrations, initiated in spring by upwelling events that advect low oxygen waters across the shelf. Increased deposition of organic carbon derived from primary production maxima in summer and autumn, together with the development of an increasingly stratified environment exacerbate dissolved oxygen deficits leading to a progressive decline in dissolved oxygen concentrations in the cold bottom layer. Within this seasonal timeframe episodic anoxia may occur throughout the water column of shallow inshore regions following the decay of red tides accumulated within these environments under conditions of persistent downwelling. Anoxia within these shallow non-stratified nearshore regions is dependent on exceptional organic loading of the water column as afforded by the decay of red tide and to the absence of wind-induced mixing or wave action. These requirements contribute to the local and transient character of these events of anoxia. With the onset of winter strong mixing results in reduced primary production and increased ventilation of bottom waters causing an increase in dissolved oxygen concentrations. © 2011 Elsevier B.V.

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