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OYSTER TRADE CONTRIBUTES TO THE INTRODUCTION OF ALIEN SPECIES IN SOUTH AFRICA

Bringing in oyster baskets on cultured ropes at Saldanha Bay oyster farm.

After decades of attempting to establish successful oyster farms in South Africa using native oysters, the decision was made as early as 1894 to import oysters from foreign lands. Several oyster species have been introduced to our shores from as far as France, Chile and the United Kingdom. They have however not arrived unaccompanied. Other organisms often hitch-hike inside or on the shells of oysters and are translocated to new locations. Non-native oyster species may also become established locally after relocation. Several surveys in oyster farms were undertaken by C·I·B-funded student Tanya Haupt and colleagues in 2007 and 2008 to identify alien species associated with the oyster culture. The results were published in a paper entitled “Oysters as vectors of marine aliens, with notes on four introduced species associated with oyster farming in South Africa” in the journal African Zoology (Volume 45, pp. 52-62). Four newly-recorded alien species were found: the black sea urchin Tetrapygus niger from Chile; the European flat oyster Ostrea edulis (previously thought to be locally extinct following its intentional introduction into South Africa in 1946); Montaguís crab Xantho incisus from Europe; and the brachiopod Discinisca tenuis from Namibia. Oyster imports are the most likely vector for these alien species given that their occurrence is tightly associated with non-native oyster beds. Some of their biological attributes such as fast reproductive rates and larvae dispersal ability and their success as invaders in other parts of the world emphasize their potential threat for South African shores. Of particular importance is the need to limit or prevent their spread within South Africa. However, the current translocation and local exchange between oyster farms, nurseries, retailers and customers in South Africa could lead to a further spread of these aliens. Before oysters are relocated, their shells are cleansed manually of any fouling organisms (such as mussels, crabs, polychaetes, anemones, sponges). Then they are jet-blasted with seawater. Cleaning and translocating oysters significantly reduces both the quantity (by more than 30 and 40 times, respectively) and the variety of fouling taxa, but small quantities of organisms survive in their new location. A way to improve the process would be to implement a more thorough cleansing regime, such as exposing oysters to freshwater or heated seawater. Perhaps, in the meanwhile, efforts should be concentrated on eradicating these newly recorded alien species and preventing the introduction of alien oyster species until new regulations are in place. A closer relationship between scientists and industry, as well as a more informed public would be beneficial in developing early warning networks, and a shared responsibility.

Appearance of (a) Tetrapygus niger and (b) Ostrea edulis both found at Alexander Bay oyster farm, (c) Xantho incisus at Kleinsee oyster nursery and (d) Discinisca tenuis at Saldanha Bay oyster farm.

Oysters are manually cleansed of fouling organisms and debris (A) after which they are jet-sprayed with sea water (B), before being packed in polystyrene boxes for relocation.