Keeping The Green Crab At Bay – A Proactive Approach

A penny-sized juvenile green crab.

A penny-sized juvenile green crab.

Aquatic invasive species are a scourge the world over.  They can damage aquatic ecosystems and threaten native species through predation, disease or by outcompeting them for food or space.  Not only can they wreak havoc on the environment, they can damage local economies and fisheries.

The European green crab is one of the ten "most unwanted" invasives in the world.  They can reduce invertebrate and fish diversity, impact wild shellfish populations, damage eelgrass beds and pose grave risks to shellfish aquaculture in particular.  As a predator species, it is a specific threat to bivalves such as clams, oysters and mussels.  European green crabs were introduced to the Atlantic coast of North America in 1817 and subsequently became firmly established in the waters off Canada's east coast.  It wasn't until 1999, however, that they were first detected in Canadian Pacific waters along the west coast of Vancouver Island.  Although the species doesn't yet appear to have caused any significant damage to wild or cultured shellfish on Canada's Pacific coast, they are certainly on the radar of research scientists such as Chris Pearce of Fisheries and Oceans Canada who works out of the Pacific Biological Station in Nanaimo, British Columbia (B.C.).  Dr. Pearce concentrates specifically on shellfish science issues.

Most aquatic invasive species are imported to our shores in ballast tanks of ships or attached to boat hulls.  In the case of the green crab, however, it most likely arrived in B.C. as a result of larval movement – drifting north on currents flowing from south of our border and moving into the waters off the west coast of Vancouver Island.  Presently, B.C.'s green crab population is confined to the west side of Vancouver Island and the Central and North Coasts and it does not appear that they have moved beyond these areas.  No populations have been reported, as of yet, from the east coast of Vancouver Island, including Queen Charlotte Strait, Johnstone Strait, and Strait of Georgia.  While green crab populations overlap with shellfish culture operations on the west coast of Vancouver Island, the aquaculture sector has not yet reported a particular problem with them. 

But of course, in line with the Department's precautionary approach to keep aquatic resources safe from harm, Dr. Pearce and his colleagues, as well as the Department as a whole, are very keen to inhibit the spread of green crabs and other aquatic invasive species.  A major recent focus has been on making sure that human activity doesn't inadvertently transfer green crabs from the waters off the west coast of Vancouver Island to the eastern side of the island where they are not presently known to exist. 

One of the concerns has been whether the very process of transferring cultured and wild shellfish from one side of Vancouver Island to the other could act as a vector for the spread of green crabs to heretofore uninfected waters.  This is an important line of inquiry as there are no shellfish processing plants on the west coast of Vancouver Island and all harvested shellfish must be transferred to the eastern side of the Island or to the lower mainland for processing.  And what everyone wants to avoid, at all costs, is the inadvertent transport of green crabs to the Strait of Georgia. 

To that end, in 2010, the year the Department assumed regulatory authority over the shellfish aquaculture industry in B.C., the Aquaculture Management Division of Fisheries and Oceans Canada put several stipulations on the transfer of shellfish aquaculture products from the west coast of Vancouver Island to the east coast, as conditions of license.  Among other things, shellfish culture license holders on the west coast of Vancouver Island have been required to closely examine harvested shellfish (oysters, clams, scallops and mussels) for signs of green crabs and rinse the shellfish thoroughly prior to them being transferred to the eastern coast of Vancouver Island – similar stipulations apply to culture gear such as trays and lines.  In addition, shellfish harvested on the west coast of Vancouver Island cannot be wet stored in the Island's eastern coastal intertidal zone and must be held in tanks within licensed processing facilities.  Further, shucked oyster shells from the western side of the Island cannot be placed in or adjacent to the intertidal zone on the eastern side, where they may be washed by the tide or where any entrapped crabs could travel to the shore, until the shell refuse is sufficiently dried out to kill any crabs that may have accompanied the shipment.  As well, transport containers must be rinsed in such a manner that the water does not run back to the shore or enter marine waters.

Then in 2011, the Department's science sector was asked to provide advice on these license conditions.  Specifically, the Department's aquaculture managers wanted to know more – from a science-based perspective – about the potential for transporting non-indigenous species, especially European green crabs, on cultured shellfish product, whether from culture, fisheries or other programs.  And if, and what, mitigation measures could be used to reduce the risk of such transfers. 

DFO Research Biologist Lyanne Curtis and DFO Post-doctoral Fellow Dan Curtis measuring green crabs at one of the project's study sites on the west coast of Vancouver Island.

DFO Research Biologist Lyanne Curtis and DFO Post-doctoral Fellow Dan Curtis measuring green crabs at one of the project's study sites on the west coast of Vancouver Island.

Consequently, Dr. Pearce and his colleagues conducted a research project from July 2011 to December 2012, under the auspices of the Department's Program for Aquaculture Regulatory Research.  The main objectives of the study were to:

  • Determine if various shellfish species from the west coast of Vancouver Island could, indeed, provide a mechanism for the transfer of aquatic invasive species to un-invaded ecosystems on the east coast of Vancouver Island using present aquaculture techniques;
  • Describe the attributes of European green crabs that could influence their ability to survive transfer on shellfish products and to establish populations in a receiving ecosystem; and
  • Provide advice on which potential mitigation measures may best be utilized to reduce the potential for transfer of non-indigenous species. 

The study has recently been completed and includes the results of experimental studies, historical data analyses and observations to assess the potential for transferring non-indigenous species on shellfish products.  One of the main findings was that green crabs were found on samples of three shellfish species which are regularly transferred from the west to the east coast of Vancouver Island for processing.  The sampling, thus, confirmed that shellfish transfers are a potential vector for movement of green crabs.  Various mitigation measures were recommended for reducing the risk of transferring green crabs on shellfish and having them become established in waters along the eastern shore of Vancouver Island.  The study also found that the transfer of shellfish products is a potential vector for the movement of other aquatic invasive species such as various tunicates and bryozoans. 

One of the great challenges for fisheries scientists, managers and regulators is to come up with a precautionary regime that will protect aquatic habitats, but not make that regime so onerous that it significantly impacts local community economies that depend on fisheries and aquaculture for their livelihood.  But, of course, pre-emptive preventative measures must take precedence because, once established, aquatic invaders can be very difficult or virtually impossible to eradicate.  As Dr. Pearce confirms, "If green crabs make it into the Strait of Georgia through this transport vector – or any other vector such as commercial or recreational fishing, commercial shipping or pleasure boating – they could then have a significant and damaging impact on the local ecosystem and the shellfish culture industry.  And there would be no turning back."

That is why initiatives such as the Department's Program for Aquaculture Regulatory Research are so important to ensuring sustainable aquaculture in Canada, and in the case of the European green crab on the Pacific coast, this project is a prime example of how science supports regulatory decision-making.

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