Introduced Pacific oystersCrassostrea gigasin South Africa: demographic change, genetic diversity and body condition

Abstract

Successful aquaculture species are often chosen for their fast growth rates and fecundity, which are also characteristics of invasive species. The Pacific oyster Crassostrea gigas, which constitutes 80% of global oyster trade, has been confirmed as invasive in 17 of the 66 countries where it is cultured. The single study of its status in South Africa reported populations in six South Coast estuaries in 2001, dropping to three sites in 2003. We resurveyed these estuaries, visited others in the Eastern and Western Cape provinces, and sampled oyster tissue for molecular analyses of population structure. Pacific oysters have disappeared from Knysna and, following our collections, possibly also from the GouKou Estuary. Between 2003 and 2012, the Breede Estuary population decreased by 87%, from an estimated 184 206 to 23 760 individuals. Within this estuary, oysters 12 km upriver had denser shells and higher body condition indices than did those within 1.4 km of the river mouth, presumably reflecting higher availability of suspended organic matter. However, low salinity over most of the species' range in the estuary probably inhibits recruitment. New populations of Pacific oysters in the Swartkops and Kaaimans estuaries urgently require monitoring and eradication. Haplotype (h) and nucleotide (π) diversities across all oyster populations sampled (h = 0.2300 [SD 0.0595], π = 0.0006 [SD 0.0007]) were lower than those of co-occurring indigenous Cape rock oysters Striostrea margaritacea from the GouKou and Breede estuaries (h = 0.9076 [SD 0.0386], π = 0.00589 [SD 0.00347]). Pacific oysters either have been introduced to South African estuaries infrequently, or have experienced genetic bottlenecks following river floods or human exploitation, or both. Populations growing outside culture infrastructure are restricted to estuaries in the Eastern and Western Cape provinces, with no evidence of occurrence in fully marine shelf environments. Given the species' considerable socio-economic importance, estuarine and coastal surveillance coupled with aquaculture zoning are required to integrate biodiversity and food security considerations.

Socioecological factors related to aquaculture introductions and production of Pacific oysters (Crassostrea gigas) worldwide

“…Where C. gigas has established, effects have been studied in countries with average GDPpc of US$45,772 (SE $19,497, N = 11), twice the amount relative to countries without studies of impacts (US$24,061, SE $21,831, N = 21; t 1,30 = 6.1, p = 0.02). Removal efforts have occurred in restricted areas of Norway, Sweden, 21 UK, 52 Australia 53 and South Africa 54 …”

Construction and evaluation of a high-density SNP array for the Pacific oyster (Crassostrea gigas)

“…Sustainable factory farming of oysters requires the development of high-quality oyster strains with rapid growth rates and high disease and stress resistance. Recent studies of the Pacific oyster industry have primarily focused on growth and yield [ 7 – 10 ], summer mortality [ 11 – 14 ], germplasm diversity [ 15 – 18 ], and viral infection [ 14 , 19 – 21 ]. In the last decade, studies have mapped important traits [ 12 , 22 – 24 ]and whole genome sequencing of C .…”

Comparative Description and Analysis of Oyster Aquaculture in Selected Atlantic Regions: Production, Market Dynamics, and Consumption Patterns

“…The aquaculture value amounted to USD 5 million [31]. The introduction of the Pacific oyster to South Africa occurred in 1973, while the current market for native oysters relies entirely on wild harvesting [54]. Wild oysters are either collected by subsistence fishers in the Eastern Cape province for their own consumption or are sold for income [55], or by licensed commercial harvesters in the southern Cape for sale to the restaurant trade.…”