History and status of oyster exploitation and culture in South Africa, and the role of oysters as vectors for marine alien species
In South Africa, both wild and cultivated oysters are consumed. Edible wild oysters include Striostrea margaritacea, Saccostrea cucullata, Ostrea atherstonei and 0. algoensis and all occur along the South and East coasts. These oysters were, or are, exploited commercially, recreationally and via subsistence fishers with S. margaritacea being the most targeted species. The commercial harvesting areas are along the Southern Cape coast and in KwaZulu-Natal. The Southern Cape coast is the largest harvesting area with 102 of the 145 pickers employed in the region. Commercial and recreational harvesting is managed by the Marine and Coastal Management Branch of the Department of Environmental Affairs and Tourism. Data on the total annual catch of oysters in these provinces are minimum estimates, as collectors do not always comply with the harvesting regulations. Subsistence harvesting is largely unmanaged, except in KZN, and is particularly rife in the Eastern Cape Province. The culture of oysters is dependent on importing Crassostrea gigas spat mostly from Chile. Oyster production statistics are only available since 1985, but approximately two million Crassostrea gigas oysters were produced annually throughout the seventies and early eighties. Since then, production has fluctuated over the years with an approximate increase of six million between 1985 and 1991, a decrease of five million between 1991 and 1998, and is presently stable. The establishment and closure of a highly productive farm in the late eighties and early nineties respectively, as well as improved production in recent years, has resulted in these trends. Although the market for oysters has grown, production has not kept up with demand, due to a lack of suitable locations for mariculture purposes. Finding suitable sites for oyster cultivation along the Northern Cape coast and establishing local oyster hatcheries for C. gigas oysters is suggested as the way forward. The latter would also prevent associated marine alien species from being imported with spat. Globally, oysters are well known vectors of marine alien species and despite oyster imports as early as 1894 into South Africa, this topic has been afforded little or no local attention. A visit to various oyster farms in South Africa resulted in the discovery of four newly-recorded alien species: the black sea urchin Tetrapygus niger, from Chile, the European flat oyster Ostrea edulis, thought to be locally extinct following its intentional introduction into South Africa in 1946, Montagu's crab Xantho incisus, from Europe, and the brachiopod Discinisca tenuis, from neighbouring Namibia. Oyster imports are suggested as their most likely vector into South Africa and the biological attributes of some emphasizes the possible threat and the need to limit or prevent their spread. Local or intraregional translocation of C. gigas and associated species, including aliens colonizing the area, may aid in this spread. Oysters host a diverse community of epi-and infaunal fouling taxa, which can be accidentally translocated along with their hosts in the course of commercial oyster trade. Thus, the types and quantities of fouling taxa occurring on farmed Crassostrea gigas were examined. How effectively these taxa are removed by standard cleansing techniques and whether those that persist after washing, survived intraregional translocation, were also examined. Cleaning and translocating oysters significantly reduced both the quantity (by more than 30 and 40 times respectively) and variety of fouling taxa. Although the mean abundance (A) or biomass (B) of taxa in uncleansed oysters (A: 79.48±233.10 (SD), B: 0.034±0.314 (SD)) were greatly reduced in cleansed oysters (A: 2.30±7.65 (SD), B: 0.0003±0.002 (SD)), small quantities still managed to survive translocation (A: 1.87± 7.43 (SD), B: 0.006±0.020 (SD)). Thus, the effectiveness of exposing oysters to freshwater or heated seawater as a more thorough cleansing regimen, to prevent the translocation of such taxa, were examined. Results indicated that oysters were able to survive for a longer time in freshwater (0% mortalities after 18 h) than in heated seawater (26.7% mortalities after 40 sec), but most taxa were eliminated more effectively by the latter treatment (e.g. 88.5% of the mudworm Polydora hoplura died after 20 sec compared to 97.5% after 18 h in freshwater). However, only a single reproductive individual of an alien species may be required for a successful introduction, and soaking for 20 sec in heated seawater would still be ineffective. An alternative treatment of 18 h in freshwater and 20 sec in heated seawater or freshwater, is suggested as a more effective treatment.
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