| Summary: | Farmed oysters host a diverse community of epifaunal and infaunal fouling taxa, including alien species, and these are easily translocated in the course of commercial oyster trade. We document the diversity and densities of fouling taxa associated with farmed oysters Crassostrea gigas in South Africa, how effectively these are removed by conventional cleansing techniques, and whether those that remain after cleansing survive intra-regional translocation. Over 40 invertebrate species belonging to 11 major taxa were found living on farmed oysters. Both mean abundance (A) and biomass (B) of invertebrate taxa associated with uncleansed oysters (A: 79.48, SD 233.10; B: 0.034 g, SD 0.314) were greatly reduced following cleansing (A: 2.30, SD 7.65; B: 0.0003 g, SD 0.002), but mall numbers survived even after translocation (A: 1.87, SD 7.43; B: 0.006 g, SD 0.020). We examined the effectiveness of exposing oysters to either fresh water or heated seawater as a more thorough cleansing regimen to prevent the translocation of such taxa. Oysters survived soaking in fresh water (0% mortality after 18 h) better than immersion in heated seawater (26.7% mortality after 40 s), but associated organisms were more effectively eliminated by the latter treatment. However, as some taxa survived both types of treatment, translocation of oysters would still pose some bio-security risks, even following such treatments. Centre of Invasion Biology — a joint venture between the National Research Foundation and Department of Science and Technology
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