Invasive Species

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  5. European Green Crab

Early detection combined with a coordinated response, will help to prevent invasive green crabs from establishing and spreading further and damaging ecologically, economically, or culturally important sites.

Together, with Coastal Restoration Society, we are working with communities to support early detection monitoring for European green crabs in BC waters. The project, entitled 'Pacific Region European Green Crab Mitigation and Capacity Development Project', is funded by the Department of Fisheries and Oceans Canada through their Aquatic Invasive Species Program. We have also worked with DFO for a number of years to raise awareness and track the spread of European green crab sightings through signage, an informational StoryMap, and mapping efforts by the Strait of Georgia Data Centre.

European green crab, by Anisha Parekh

About European Green Crabs

Background

European green crabs are native to Europe and North Africa but became established on North America’s west coast in San Francisco Harbour in 1989. Since then, they have thrived and expanded their range northward, landing in British Columbia in 1998. Populations are now established on the west coast of Vancouver Island and there are concerns they will establish in the Salish Sea and further north along Haida Gwaii and BC's central coast. The evolving conditions associated with climate change, such as warmer water and shifted currents, may facilitate range expansion.

Map showing native and invasive range of European green crab. Credit: Brett Howard.

European green crabs are intertidal shore crabs and are often found in sheltered areas, estuarine habitat, and where there is freshwater influence. They are able to survive a wide range of environmental conditions – from high tide levels to depths of 5-6 meters, temperatures of 0oC to 35oC and varying salinity. Green crabs have a life span of 4-7 years, can grow up to 10 cm in length, and release approximately 185,000 larvae once or twice a year. Those larvae live up to 90 days in the currents, which facilitates their spread to new destinations. This resilience and fecundity, unfortunately, makes them excellent invaders.

European green crabs are not necessarily green, they come in a range of colours. Photo credit: Allie Simpson.

Impacts

Nearshore environments can be significantly disrupted and damaged by invasive green crabs. They compete with native species for food and shelter and they prey upon native species such as oysters, mussels, clams and other juvenile crabs. Eelgrass and saltmarsh meadows, which are important rearing areas for native species, can be uprooted by green crabs digging for food and burrowing. As a result, there are concerns that green crab activities also increase bank erosion. These impacts flow through the marine food web to other species like Pacific salmon who rely on healthy eelgrass meadows and saltmarsh habitat. All of us, especially First Nations communities that depend on healthy, balanced ecosystems, are affected by these impacts.

Identification

While there are several native shore crabs that are green or have a similar appearance, you can identify European green crab is by their 5 distinct marginal teeth (points) on either side of the eye. It is also worth noting that not all European green crabs are actually green in colour, their colouration varies from orange to brown.

See the photo on the left for the identifying features and check out the Crab Field ID Guide created by the Washington Sea Grant.

Identifying features of European green crab, courtesy of Washington Department of Fish and Wildlife.

Invasion Tendencies

In early stages of invasion at a new location, European green crabs are often found higher up in the intertidal – even as high as the tidal reach in a river where the salinity is quite low. In addition to a wide range of salinity and freshwater influences, they tend to be found in locations where prey species are present (native shore crabs, shellfish and snails) and where they have less competition from other native crabs. Green crabs prefer low wave action, softer substrates, beaches that have shallow slopes, isolated lagoons, meandering channels with undercut banks, tidal flats with pools that don’t dewater at low tide, and cover (e.g., log debris, marsh vegetation, or seaweed). If European green crabs become established, they then typically move into deeper habitats.

Outreach and Capacity Development

Controlling and managing the spread of European green crabs is a high priority within the Pacific Region.

With Coastal Restoration Society, we are delivering outreach and training to build capacity with Indigenous and non-Indigenous partners in coastal communities within the Salish Sea, northern portions of the South Coast (i.e., Johnstone Strait, Discovery Islands, Broughton Archipelago, etc.), and the Central and North Coasts of British Columbia. The aim is to provide the methods and materials needed to empower local communities to begin a standardized and organized surveying effort for green crabs. This will help identify areas where European green crabs have not been found previously, either because there has never been a survey in that location or because there is a new population potentially becoming established. Then, on-the-ground actions can be initiated to mitigate the effects of green crabs on the marine ecosystem. For example, ongoing monitoring or mass trapping efforts can be made to contain the population and protect adjacent areas.

Tools and Resources

Site Selection Tool

To help communities plan their surveys, the Strait of Georgia Data Centre created a site selection tool. The tool is a map interface with spatial data layers of shoreline features, habitats, and prey availability that match conditions of where European green crab tend to establish. Local areas of interest along the coast can be zoomed into with the various layers of data switched on and off to prioritize locations for scouting.

The site selection tool is a handy desktop method for planning, however, it does not replace in person examination for suitability.

Methods and Protocols

Below are the documents and protocols for the green crab surveillance monitoring program that are provided to communities involved in the project. If you have questions, please reach out to mcatanzaro@psf.ca.

Methods

Data collection

DFO requirements

European green crab signage

For more information and updates, see this newsletter.

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