Assessing potential control options for the invasive tunicate Didemnum vexillum in shellfish aquaculture

Globally, invasive tunicates continue to plague shellfish aquaculture operations by fouling both cultured bivalves and cultivation gear. Recently, in British Columbia, Canada the colonial tunicate Didemnum vexillum has become established at various oyster aquaculture sites and on some tenures it is the dominant fouling species. In the present study we examined potential chemical, mechanical, and biological treatments to control D. vexillum and botryllid tunicates (Botrylloides violaceus and Botryllus schlosseri) fouling Pacific oysters (Crassostrea gigas) grown in suspended tray culture. Fouled oysters were deployed from July to November 2009 with chemical (4% hydrated lime exposure) and mechanical (manual removal via scrubbing) treatments applied in July prior to deployment and in September following sampling, and a biological treatment (10 green sea urchins, Strongylocentrotus droebachiensis, per tray) applied in August. Oysters were measured and photographed monthly to determine changes in oyster shell growth and percent coverage by D. vexillum and the two botryllid tunicates. Both chemical and mechanical treatments reduced D. vexillum fouling on oysters by 85 to 96%, however, the reduction in D. vexillum fouling created free space that allowed botryllid fouling to increase. The sea urchin S. droebachiensis was not successful as a biological treatment at controlling tunicate fouling. Our results suggest that both lime and mechanical treatments may be viable options for controlling D. vexillum fouling Pacific oysters, although a decrease in percent survival was observed for lime-treated oysters, suggesting that the concentration and duration of exposure should be investigated further.