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Research Document - 2007/063

Risk Assessment for Two Solitary and Three Colonial Tunicates in Both Atlantic and Pacific Canadian Waters

By Therriault, T.W. and L.-M. Herborg


Non-indigenous tunicates have become a global concern, especially for the aquaculture industry. Some of these tunicate species have rather extensive invasion histories dating back several decades while others are much more recent. In order to determine the potential risk posed by these non-indigenous tunicate species to Canadian waters, including both the Atlantic and Pacific coasts, a formal risk assessment was undertaken. This risk assessment included two solitary tunicates, the club tunicate, Styela clava, and the vase tunicate, Ciona intestinalis; and three colonial ones, the golden star tunicate, Botryllus schlosseri, the violet tunicate, Bottryloides violaceus, and Didemnum sp. Regardless of species, most global introductions of tunicates have been attributed to hull fouling or aquaculture-related activities suggesting these non-indigenous tunicates are good hitchhikers. Three of these tunicate species already exist in Canadian waters on both coasts (golden star tunicate, violet tunicate, and club tunicate), while vase tunicate is present on the Canadian Atlantic coast and Didemnum sp. is present on the Canadian Pacific coast. There is considerable concern about the potential ecological and genetic impacts if these tunicates spread in Canada. Life history characteristics largely preclude long-distance natural dispersal or dispersal via ballast water but a number of other potential vectors exist, especially the movement of recreational or small craft. The unlikelihood of natural long distance dispersal due to the very brief larval phase means that management of human-mediated transport vectors could effectively control future spread of these tunicates.

The Department of Fisheries & Oceans Canada conducted a national risk assessment, including a peer-review workshop, to determine the potential risk posed by non-indigenous tunicates in Canada. This assessment included evaluating the risk of arrival, survival, reproduction and spread of each species, as well as their pathogens, parasites or fellow travelers (e.g. other invasive species) should they be introduced as well. These components were assessed in an expert workshop using best available information on their biology, potential vectors of introduction, and impacts in both native and introduced ranges. The assessment concluded these non-indigenous tunicates posed a high ecological risk on both coasts with the exception of C. intestinalis where the ecological risk on the Pacific coast was deemed moderate. The genetic risk was moderate for each species on both coasts. For pathogens, parasites or fellow travelers the ecological and genetic risks were low for each of the colonial species. As C. intestinalis can harbor an amoebic parasite that could impact native and cultured salmon stocks the ecological risk was deemed moderate. The ecological risk posed by hitchhikers of S. clava was high as they include the colonial tunicates. However, as little is known about many potential pathogens, parasites and fellow travelers of these tunicate species there was considerable uncertainty.

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