Assessing potential control options for the invasive tunicate Didemnum vexillum in shellfish aquaculture S.E. Switzer a , T.W. Therriault a, ⁎, A. Dunham a, b, 1 , C.M. Pearce a, b a Fisheries and Oceans Canada, Pacific Biological Station, 3190 Hammond Bay Road, Nanaimo, BC, Canada, V9T 6N7 b Fisheries and Aquaculture Department, Vancouver Island University, 900 Fifth Street, Nanaimo, BC, Canada, V9R 5S5 abstract article info Article history: Received 17 December 2010 Received in revised form 19 April 2011 Accepted 21 April 2011 Available online 10 May 2011 Keywords: Didemnum vexillum Invasive tunicates Shellfish aquaculture Crassostrea gigas Hydrated lime treatment Botryllid tunicates 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. Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved. 1. Introduction Non-indigenous species pose an enormous risk to native biodi- versity and can compromise ecosystem function in both marine and freshwater environments (Sala et al., 2000). In shellfish aquaculture, non-indigenous tunicates have become a global concern because they can displace native fouling species and foul both aquaculture gear and product (Blum et al., 2007; Lambert and Lambert, 2003; LeBlanc et al., 2007; Mazouni et al., 2001). In many cases, these tunicates become prolific as suspended bivalve aquaculture operations provide an abundance of substratum suitable for sessile organisms to occupy, thus creating conditions amenable for these tunicates to become a major industry fouling problem with significant impacts (Carver et al., 2003; McKindsey et al., 2007; Rodriguez and Ibarra-Obando, 2008). Over the last decade, the blue mussel farming industry on the east coast of Canada has experienced significant challenges dealing with the colonial tunicates Botrylloides violaceus (violet tunicate) and Botryllus schlosseri (golden star tunicate), and the solitary tunicates Styela clava (club tunicate) and Ciona intestinalis (vase tunicate) (Carver et al., 2003; LeBlanc et al., 2007; Morris et al., 2009; Paetzold and Davidson, 2010). On the west coast of Canada, in British Columbia (BC), each of these species has been found with the exception of C. intestinalis, which has been reported from Puget Sound, WA, USA but has yet to be confirmed in Canadian waters (Therriault and Herborg, 2008). However, to date, none of these tunicates have proliferated in BC to the same extent as on the east coast. More recently, the global invader Didemnum vexillum – a species that has spread from Japan (Lambert, 2009) to Europe (Gittenberger, 2007), New Zealand (Kott, 2002), and both coasts of North America (Bullard et al., 2007; Daniel and Therriault, 2007) – has been detected in BC, including at several shellfish aquaculture sites. The shellfish aquaculture industry has long battled fouling and predatory organisms. Fouling by algae can reduce water flow and increase siltation, epibiontic filter feeders can be effective competitors for resources, while predatory organisms can destroy shellfish product. Early in the last century, liming bays to reduce impacts associated with sea-star predation was a common practice (Wood, 1908 cited in Locke et al., 2009) and quicklime has been used to kill fouling organisms to create spat settlement space (MacKenzie, 1977). Mitigating the impacts associated with invasive tunicate fouling specifically is relatively new and no clear cost-effective, environmentally-friendly methods for controlling tunicates have been documented to date. A number of Aquaculture 318 (2011) 145–153 ⁎ Corresponding author. Tel.: + 1 250 756 7394; fax: + 1 250 756 7138. E-mail address: thomas.therriault@dfo-mpo.gc.ca (T.W. Therriault). 1 Previously published as A. Epelbaum. 0044-8486/$ – see front matter. Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.aquaculture.2011.04.044 Contents lists available at ScienceDirect Aquaculture journal homepage: www.elsevier.com/locate/aqua-online