(Disponible en français : Le crabe vert européen : un virage vert à éviter)
Classified as “one of the world’s most successful aquatic invaders” by the Global Invasive Species Database, the European Green Crab (Carcinus maenas) is native to Europe and northern Africa. Commonly referred to as green crab, the species is now also found in Australia, the North and South Americas, and South Africa.
Fisheries and Oceans Canada describes the green crab as an aggressive and “dominant predator, feeding upon many shellfish species such as clams, mussels, oysters, smaller crab and other crustaceans and even small fish.” The presence of green crab is problematic for those who commercially fish or farm the prey species mentioned above.
Green crab, which can also be yellow or red, was first identified in Atlantic Canada in 1951 in the waters of Southwest New Brunswick. On Canada’s Pacific coast, it was first spotted sometime in 1998 or 1999. The release of ballast water – the water pumped into the ballast tanks of ships to control, among other things, the stability of the ship – is the most likely means of introduction on both coasts.
Maps prepared by Library of Parliament, Ottawa, 2019, using data from Natural Resources Canada (NRCan), Administrative boundaries in Canada – CanVec Series, “Administrative features”, 1:5M, Ottawa, NRCan, 2018; and Natural Earth, 1.10m Physical Vectors, version 4.1.0. Data on green crab observations compiled by the regional offices of Fisheries and Oceans Canada. Contains information licensed under Open Government Licence – Canada. The World Oceans Basemap layer is the intellectual property of Esri and is used under licence. Copyright ©2014 Esri and its licensors. All rights reserved. The following software was used: Esri, ArcGIS Pro, version 2.1.0.
As shown in the maps above, the dispersion of this invasive species has been swift and widespread. On the east coast, the species has afflicted New Brunswick, Nova Scotia, Prince Edward Island, Newfoundland and Labrador, and the Magdalen Islands of Quebec. On the west coast, green crab has been observed north of Bella Bella and as far south as the southern tip of Vancouver Island.
This omnivorous species is said to have a wide tolerance for variations in water salinity and temperature. This explains how it survives in so many diverse ecosystems around the world. In Canada, Fisheries and Oceans Canada notes that green crab can be found in shallow waters with various types of substrates – the material found at the bottom of marine habitat – such as mud, sand, pebbles, and vegetation.
Socio-economic and Environmental Impacts
This voracious predator has been blamed for the “collapse of the soft-shell clam industries, in both New England [United States] and Nova Scotia” and is said to be able to outcompete various native crab species for food and to shift ecosystem dynamics. Fisheries and Oceans Canada also attributes the destruction of many eelgrass beds, which are a key habitat for species such as Atlantic cod, to this invasive species.
According to Fisheries and Oceans Canada, “once green crabs have started to invade an area though, it is practically impossible to eradicate them, but it is feasible to limit population spread and hence the damage caused by this species.” So, what can be done about this nuisance species in Canada?
Fisheries and Oceans Canada has, in collaboration with partners (fishers, academics from Memorial University and the government of Newfoundland and Labrador), found that sustained removal of the species has resulted in native crab species regaining their territory.
In 2016, Kejimkujik National Park near Caledonia, Nova Scotia, made an eco-adventure of the removal of green crab from the estuary located in the park. Park visitors can help pull out crab traps and remove this invasive species from the ecosystem after enjoying a side-by-side ATV ride to the site.
Reduce the Risk of Spread
In 2010, Fisheries and Oceans Canada implemented measures on the west coast to help reduce the risk of green crab spreading to unaffected waters, particularly the Strait of Georgia. More specifically, shellfish aquaculture was identified as a potential vector for transfer with green crabs “hitchhiking” on cultured shellfish or gear transferred from the west coast of Vancouver Island to the east coast of the island for processing. Preventative measures were included as conditions of aquaculture licences to ensure compliance.
Create a New Commercial Fishery
Many fishers and fishing associations have shown interest in commercially fishing green crab and noted that although there is not yet an important domestic market for the species, international markets exist. Green crab is commercially fished and considered a delicacy in certain European countries, such as Italy.
Fisheries and Oceans Canada conducted a pilot project from 2015 to 2017 in the Gulf Region by exchanging eel fishing licences for green crab fishing licences. However, only 15 licences were issued in 2017 and the pilot project was terminated by 2018. The department also tried a similar pilot project in the Maritimes Region, where fishers were permitted to sell their catch as bait; 69 commercial licences were issued in 2018 by the department.
Fisheries and Oceans Canada also confirmed that off the south and west coasts of Newfoundland, 18 and 14 experimental fisheries licences were issued for European green crab, in 2017 and 2018 respectively. However, as part of the conditions of licence for these fisheries, fishers were not permitted to sell their catch because licence conditions required that the catch be destroyed immediately. Although this commercial fishery was not a profitable one, by participating, fishers contributed to the sustained removal of this invasive species.
In 2018, a Saltonstall-Kennedy Grant worth more than $267,000, awarded through the U.S. National Oceanic and Atmospheric Administration, was allocated to a not-for-profit organization to help develop a green crab fishery in the New England area. The project aimed to promote, develop, and market green crab in that region.
As noted by Fisheries and Oceans Canada, the European green crab seems to be here to stay. However, its continued presence will certainly lead to undesirable social, economic and environmental consequences including the destruction of habitat, negative impacts on ecosystems, and the endangerment of commercially fished and cultured crustaceans.
Therefore, there is a need for continued effort towards the systematic removal of the species to limit its spread. Whether that occurs through a commercial fishery, a program in a national park, or a new way that has yet to be tried, in this case, it is important to make it difficult to be green.
Author: Daniele Lafrance, Library of Parliament