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Research Document - 2007/077

Using the Quantitative Biological Risk Assessment Tool (QBRAT) to predict effects of the European green crab, Carcinus maenas, in Atlantic Canada

By Locke, A. and G.J. Klassen


The European green crab or shore crab, Carcinus maenas (Linnaeus) is ranked among the “100 worst alien invasive species” in the world. It initially invaded Atlantic Canadian waters in 1951 via Passamaquoddy Bay, where the subsequent failure of fisheries for infaunal bivalves was attributed to predation by the green crab. In the 1990s, green crabs spread into the Gulf of St. Lawrence and British Columbia. There has been considerable interest in forecasting their eventual effects on these ecosystems and their fisheries. Green crab was therefore selected as a candidate for a case study for the National Workshop on the Evaluation of a Quantitative Biological Risk Assessment Tool (QBRAT) Through Various Case Studies, 29-30 November 2006. The objective was to evaluate QBRAT v. 2 by assessing the risk of green crab invasion to Atlantic Canada, over a temporal scale of 30 yr, and by estimating environmental impacts (on a probabilistic scale between 0 and 1) and economic impacts (as dollar values according to scenarios we devised as approximations of the cost over 30 yr).

QBRAT was a useful device for structuring and quantifying assumptions about each step in the invasion process. It was also valuable in identifying where more work was required in order to provide an adequate estimate of a variable. Many of the values we assigned to steps in the invasion were estimated with a high level of uncertainty. We did establish with reasonable certainty that there was a high risk of continued establishment and spread of the species throughout Atlantic Canada. The calculated economic risk, based on hypothetical scenarios of commercial fishery and aquaculture losses in bivalve harvests in the Gulf of St. Lawrence, averaged just over $1 million annually. We stress, however, that this value should not be quoted as a definitive estimate of economic impacts of green crabs in Atlantic Canada. Indeed, we consider that no valid estimates of economic impacts of this species exist for these waters.

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