Science Advisory Report  2015/016

Evaluating transfers of harvested shellfish products, from the west to the east coast of Vancouver Island, as a potential vector for European Green Crabs (Carcinus maenas) and other non-indigenous invertebrate species

Summary

• Fisheries and Oceans Canada (DFO) Aquaculture Management Division (AMD) requested science advice on the shellfish aquaculture licence condition requiring that all shellfish cultured on the west coast of Vancouver Island  be rinsed and visually inspected for European Green Crabs (Carcinus maenas) before transport to processing plants on the east coast of Vancouver Island and the lower mainland. Specifically, AMD wanted to know if shellfish can be a vector for Green Crab movements and, if so, what mitigation measures could be used to reduce the risk of their transport (the ultimate aim being to prevent C. maenas from being transported from the west coast of Vancouver Island, where populations currently exist, to the Strait of Georgia where the species has not yet been found).
• DFO Science conducted a field study to assess three species of shellfish (Pacific Oysters, Crassostrea gigas; Manila Clams, Venerupis philippinarum; and California Mussels, Mytilus californianus) as potential vectors for Green Crabs and other NIS. In addition, oysters and clams collected directly from growers and processors were examined for C. maenas and other NIS. A literature review was conducted to assess possible mitigation measures for NIS transport on shellfish.
• The transfer of shellfish (clams, oysters, and mussels) from NIS-infested areas was confirmed as a vector for both mobile (e.g. Green Crabs) and sessile (e.g. tunicates/ bryozoans) NIS.
• The potential propagule pressure due to the shellfish movement vector may be sufficient to overcome invasion barriers.
• Based on a literature review, no mitigation measures to remove or destroy NIS on cultured shellfish are 100% effective; suggesting any transfer of shellfish poses some level of invasion risk.
• The only mechanism to ensure NIS are not inadvertently moved from infested to non-infested waters via this vector is to restrict all movement of shellfish (i.e. strict quarantine).
• The results of the experimental and grower/processor studies and the literature review suggest that the present conditions of licence do not eliminate NIS propagule pressure. There are gaps in our knowledge surrounding various stages of the transport process (e.g. efficacy of rinsing in removing small individuals, survivability of individuals during transport, escape of live NIS from processing facilities, survivability in the receiving ecosystem). Due to these gaps it is probable that the intended reduction in propagule pressure is not realized.
• A conceptual framework was developed to identify control points where management intervention, such as application of licence conditions, could lower propagule pressure and hence invasion risk. A full assessment of the relative effectiveness of each control point in the framework was beyond the scope of this project.

This Science Advisory Report is from the December 3-4, 2013 Evaluating transfers of harvested shellfish products, from the west to the east coast of Vancouver Island, as a potential vector for European green crabs (Carcinus maenas) and other non-native invertebrate species. Additional publications from this meeting will be posted on the Fisheries and Oceans Canada (DFO) Science Advisory Schedule as they become available.

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