Look Out for Invasive Crab!
The green crab is invading from the west coast and making its way to Alaska.
What is a green crab?
The green crab is considered one of the most invasive species in the marine environment. It has few predators, aggressively hunts and eats its prey, destroys seagrass, and outcompetes local species for food and habitat. It has been documented that green crab devour juvenile king crab as well as juvenile salmon. They also destroy eelgrass habitat that larval fish use to hide from predators, and outcompete Dungeness crabs for food and habitat. Green crab could potentially damage Alaska’s multi-billion dollar fisheries industries, especially for salmon, crab, and mariculture operations. Resource managers in Alaska have been keeping an eye on the invasive crab’s northward movement for years.
How did they get here?
Green crabs were first introduced to North America in the 1800s, likely hitching a ride in the ballast water of merchant ships from Europe. Experts believe the invasive crab was transported to the West Coast in ballast water as well. They may also be transported with shellfish, equipment, or packing materials in aquaculture operations. Larval green crabs can also spread from one invaded area to another in ocean currents. Scientists and resource managers generally agree that with climate change warming Alaska waters, it is no longer a matter of ‘if’ but ‘where and when’ green crabs will arrive in Alaska. This map shows the range of green crab throughout the world with their native range in blue and all other colors indicating areas they invaded or have the potential to spread.
What can I do to help?
Where should I look?
How do I identify a green crab?
Green crabs are not always green! The top of the shell may be mottled dark brown to dark green, with small yellow patches. The bottom may be orange or red during molting. Green crabs can be identified by their unique shell shape. Adult shells can be up to 4 inches across.
What should I do if I find a green crab?
More Resources for Citizen-Based Invasive Species Monitoring
If you are interested in learning more about citizen-based invasive species monitoring, call 1-877-INVASIV or one of the other agencies listed here.
- Alaska Department of Fish & Game, 1-877-INVASIV (1-877-468-2748) Toll-free statewide in Alaska
- U.S. Fish & Wildlife Service, 907-786-3813, Anchorage, Alaska
- Kachemak Bay National Estuarine Research Reserve, 907-235-6377, Homer, Alaska
- Alaska Invasive Species Partnership
Using eDNA to Monitor Alaskan Waters for Invasive European Green Crabs. Voracious crustacean known to gobble juvenile salmon and outcompete Dungeness crab.
Shorezone Habitat Capability Modeling: A study of potential suitable habitat for the invasive European green crab (Carcinus maenas) in Southeast Alaska, British Columbia, and Washington State, August 2007. (PDF, 75 pages)