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Routine shellfish aquaculture practices in British Columbia (BC) result in cultured Pacific oysters (Crassostrea gigas (Thunberg, 1793)) being moved from tunicate infested culture areas to non-infested areas for processing, thereby posing a potential risk of spreading the colonial ascidian Didemnum vexillum Kott, 2002 and other nuisance species, to new areas. Three intervention points (IPs; i.e., stages of the processing regime) were identified in existing aquaculture practices where clusters of C. gigas received manipulation or stress that could alter the cover of epibionts, notably D. vexillum. These IPs were: IP1) harvesting, IP2) transportation (from harvesting areas to processing plants), and IP3) processing (shucking of the oysters). The percentage coverage of D. vexillum on oyster clusters was evaluated at each IP for products originating from two aquaculture sites in BC, Lemmens Inlet and Okeover Inlet. A total of 60 clusters were sampled from Lemmens Inlet and 46 from Okeover Inlet. Results demonstrated a significant loss (P < 0.05) of D. vexillum coverage on C. gigas clusters from IP1 to IP3 for both sites. Although variations existed between the sites, the mean percentage coverage decreased from 48% post-harvest to 30% post-transportation and 17% post-shucking. Since shucked oyster shells still have a substantial cover of D. vexillum on them and are disposed of in areas exposed to tidal waters of un-infested bays, the risk of secondary introduction related to shellfish aquaculture practices remains high. Based on these results, thresholds could be set to reduce the risk of introducing D. vexillum into new areas, and new research is recommended to determine the risk of dispersal of D. vexillum should it be inadvertently introduced via shellfish movements.

Didemnum vexillum: invasion potential via harvesting and processing of the Pacific oyster (Crassostrea gigas) in British Columbia, Canada