Abstract
Despite sporadic observations of the vase tunicate, Ciona intestinalis, on boats and mooring structures in Charlottetown Harbour, Prince Edward Island, the species has not established a population in the harbour nor dispersed the ~12 km downstream to Hillsborough Bay, an important source of blue mussel (Mytilus edulis) spat for the PEI aquaculture industry. A population matrix model used in conjunction with an oceanographic model suggests that advection of larvae from the harbour to the spat production area requires more than one or two generations, and the use of intermediate settlement nodes, such as navigational aids and aquaculture sites located in the upper part of Hillsborough Bay, as ‘stepping stones’. Maintaining potential settlement nodes in a tunicate-free condition could delay oceanographic dispersal of C. intestinalis within the estuary. According to observations of colonial tunicate dispersal in 2010, most likely originating from colonies established in the same locations where C. intestinalis inoculations have been detected, dispersal was occurring in the vicinity of one of the nodes identified by the model as priorities to be monitored for early detection of tunicate settlement, but had not yet reached the other node. A major finding is that the dispersal of solitary tunicates by oceanographic processes, often considered uncontrollable, is evidently amenable to management through monitoring and cleaning of the intermediate settlement nodes.
Highlights
The blue mussel (Mytilus edulis Linnaeus, 1758) aquaculture industry in Prince Edward Island (PEI), Canada, supplies 80% of the blue mussels in the North American market (DFO 2006), and is adversely affected by invasive tunicates, especially the vase tunicate Ciona intestinalis (Linnaeus, 1767)
It has sometimes been suggested that, if oceanographic vectors are expected to transport a species to a given area, attempting to control dispersal by other vectors to that area is at a best a delaying tactic and at worst a wasted effort (Locke, pers. obs.). Such an assumption serves to discourage the development of management strategies for potentially controllable anthropogenic vectors after all, why bother to invest the time and resources to protect an uninfested area if the species will be carried there by natural processes anyway
The model results indicated that the oceanographic vector would be unlikely to disperse Ciona intestinalis directly from Charlottetown Harbour to the aquaculture leases at Nine Mile Creek during a single generation (72 hours of planktonic larval drift) (Figure 3)
Summary
The blue mussel (Mytilus edulis Linnaeus, 1758) aquaculture industry in Prince Edward Island (PEI), Canada, supplies 80% of the blue mussels in the North American market (DFO 2006), and is adversely affected by invasive tunicates, especially the vase tunicate Ciona intestinalis (Linnaeus, 1767). Tunicate management in PEI, through the Introductions and Transfers permitting process, has been successful in limiting or at least slowing the spread of solitary tunicates between estuaries, but does not address the issue of dispersal of tunicates within an estuary (Locke et al 2007). Nor does the permitting process address vectors outside of the aquaculture industry, whether anthropogenic (e.g., boating, shipping and fishing), or natural (e.g., advection by oceanographic currents). Such an assumption serves to discourage the development of management strategies for potentially controllable anthropogenic vectors after all, why bother to invest the time and resources to protect an uninfested area if the species will be carried there by natural processes anyway
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