Mitigating non-indigenous species movements: effects of pressure-washing intensity and duration on the removal of biofouling and mobile invertebrates from cultured Pacific oysters (Crassostrea gigas (Thunberg, 1793))

The inadvertent movement of non-indigenous species (NIS) poses a significant risk to marine ecosystems. The present study examined the interactive effects of pressure-washing intensity and duration on removal of biofouling and various mobile invertebrate species on string-cultured Pacific oysters ( Crassostrea gigas (Thunberg, 1793)). Six pressure-washing treatments were established by combining two intensities (2000 and 3000 PSI) and three durations (10, 20, and 30 s). These were compared with controls of no washing and simple dunking, which most likely are the current industry practices. Oysters in the various pressure-washing treatments had significantly less total biofouling compared to the no-washing and dunk controls. Significantly less biofouling remained when pressure washing was applied for longer periods of time (20 and 30 s) than for shorter periods (10 s), regardless of intensity. Dunking oysters repeatedly in seawater had no significant effect on the amount of biofouling when compared with the no-wash control, although it did lead to significantly fewer shrimp. Regardless of the faunal group assessed (i.e. total biofouling community, NIS tunicates, or various mobile invertebrate species), individuals remained on the oysters after every experimental washing treatment, suggesting none are 100% effective. In addition, the number of oysters remaining on the strings and their shell condition were significantly reduced after pressure washing, suggesting a potential cost to growers. The results have implications both for oyster farming and mitigation of NIS movement.