Photo: Invasive crabs: Green isn't always good

Scott Wallace, senior research scientist at the David Suzuki Foundation, holds a green crab specimen. (credit: Scott Wallace)

by Scott Wallace, senior research scientist

Green is usually a good thing when talking about the environment, but not when talking about the invasive European green crab, which is becoming more common in B.C.'s intertidal zones. In mid-August, I was kayaking in one of the most remote areas of Vancouver Island when I observed my first green crab up a narrow inlet and above a traditional First Nations fish weir. I've been casually looking since 1999 when I first heard of their presence on the coast. Now, without really looking, I came across two live crabs and a recent moult (they shed their exoskeletons when growing) within a few minutes.

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Of all the things people do to the ocean, perhaps surprisingly, introducing invasive species can have the greatest impact. Overfishing can be reversed or managed, and pollution can be controlled, but invasive species are almost always impossible to get rid of and can become a dominant part of the marine ecosystem with cascading and unforeseen effects on wildlife and fisheries.

The European green crab has invaded enormous stretches of the world's coastlines in South Africa, Australia, South America, Canada's East Coast and most recently, Canada's Pacific Coast. They have not yet been observed in Alaska. They can live with wide temperature fluctuations, changes in salinity and low oxygen levels, and there is little that can be done to control them once they are established.

Larval green crabs first arrived in the U.S. in San Francisco in the late 1980s when they were dumped from international ships in ballast water. Green crabs are thought to have arrived on the B.C. coast via an El Niño oceanographic event that spread them further north from Oregon.

Commercial fisheries and shellfish aquaculture in Atlantic Canada have paid the price, with estimates of losses ranging from tens to hundreds of millions of dollars. We don't yet know what their impact will be in Pacific waters where they haven't yet firmly established. There have been reports from around the world of these crabs devouring several hundred types of prey, with a particular fondness for clams, other crustaceans and snails. Some of our "greenest" most sustainable seafood, such as farmed clams and Dungeness crab, are most susceptible to this invasive species.

Fisheries and Oceans Canada launched a webpage devoted to aquatic invasive species where you can find out more or be a citizen scientist and contribute your own findings of invasive species. Reporting your findings can help officials know where to focus to try to stop green crabs from setting up home permanently. Or, join people who eat invasive species and try this green crab recipe.

Additional links:
Guide for identifying native Pacific species vs invasive crabs (PDF)
Biological synopsis of the European green crab (PDF)

August 27, 2014

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