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Research Document - 2007/056

Application of QBRAT for a Risk Assessment of the Invasive Tunicate Didemnum sp. in British Columbia

By Herborg, L.-M. and T.W. Therriault


The invasive tunicate Didemnum sp. has been reported in Canadian waters and has the potential to negatively impact native flora and fauna. In British Columbia, limited surveys have found Didemnum sp. in the Strait of Georgia and at sites along the west coast of Vancouver Island. This tunicate possesses several traits that likely enhance its invasion success including its ability to grow and sexually reproduce quickly, smother competing or co-occurring organisms, and its tolerance of a wide variety of environmental conditions. Didemnum sp. has the potential to negatively impact water quality, macrophytes, invertebrates, fishes, and aquaculture facilities. This report summarizes available information on Didemnum sp. including a taxonomic description, biological characteristics, its distribution (native and non-native ranges), and potential impacts of its invasion.

We also present a biological synopsis and a risk assessment for the British Columbia coastline. The risk assessment is based on estimates of propagule pressure, which combine the spatial distribution of aquaculture and boating facilities. Additionally, it incorporates measures of environmental suitability from a environmental niche model. Impact is measured based on the sensitive habitat and shellfish aquaculture facilities. Overall, the Strait of Georgia (the current location of most Didemnum sp sightings) has the highest total biological risk as calculated by QBRAT. The next highest risks are predicted for Johnstone Straight and three different areas along the west coast of Vancouver Island.

A sensitivity analysis of different methods of determining impact, discovered a very strong effect on the risk ranking for the different study areas. This highlights the great importance of developing clearer guidelines on quantifying impact for comparable results.

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