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Invasive Green Crab Are Here: What Does This Mean?

European green crab have been the scourge of American fisheries for centuries, but they've only now arrived in Alaska. What can you do?

by | February 10, 2023

European green crab.
Alaska Department of Fish and Game

The invaders have landed. Last summer, members of the Metlakatla Indian Community in Southeast Alaska discovered three green crab shells (carapaces) during a survey of the Annette Islands Reserve. After setting traps in the area to look for others, they soon found hundreds of live green crabs, including molted shells, and carcasses.

Green crabs are the latest unwelcome visitor to Alaska coastal waters. This invasive species is like a bulldozer, wreaking havoc on marine habitats. Green crabs have few natural predators. They are aggressive hunters, loving to feast on the same food resources preferred by juvenile king crabs and salmon. In their search for food, they also destroy the eelgrass used by larval fish to avoid predators. They can eat upwards of 20 clams per day. 

A closer look at invasive European green crab

The common name of this unwelcome crustacean is the European green crab (Carcinus maenas). They originate from the North Atlantic and Baltic Sea, and they’ve been carried around the world in the ballast tanks of merchant ships. Green crabs have been established in eastern U.S. waters since the 1880s and appeared on the west coast of California in 1989. Green crab didn’t show up in Alaska until 2022, but they have the potential to spread throughout Southeast and Southcentral Alaska coastlines as marine waters trend warmer.

Adult green crab are about four inches across, but juveniles can be much smaller, for example, as small as your pinkie fingernail. In spite of their common name, you might find these invasive crabs with dark green shells, but they can also be dark brown, red, or orange and they may have yellow spots. The best way to identify them is by counting five pointy spines on the shell’s outer edge, located behind their eyes.

Surveys spotted these invaders

Tammy Davis, the invasive species program coordinator for the Alaska Department of Fish and Game, predicted that green crab would appear in Alaska after running into them on a trip to British Columbia and noting that ocean temperatures are becoming suitable for green crab survival. 

The Metlakatla Indian Community, Department of Fish and Wildlife has been funded by the National Oceanic and Atmospheric Administration (NOAA) to monitor the waters around the Annette Islands Reserve since 2020. Together, they discovered evidence of green crab while walking along one of the islands’ beaches on a molt survey. A Sealaska intern at NOAA spotted an empty small, brown carapace in the beach grass among much larger Dungeness shells.

The Metlakatla community trapped the invaders last fall, catching more than 700 live crabs.

European green crab closeup
To identify a European green crab, look for the five spines on each side of its carapace. ADF&G

How to spot green crab

Davis encourages coastal residents from Ketchikan to the Aleutians to look for European green crab carapaces and live crabs when they visit local beaches. Look for the five spines behind the eyes and three small bumps or ridges between the eyes. Those interested in joining a community-based European green crab early detection monitoring network can contact ADF&G for equipment and training. 

If you see what you think is an invasive green crab, Davis encourages you to take a photo next to a car key, credit card, or dollar bill to provide scale. Try to show the spines on the outer edge of the crab shell.  Keep in mind that European green crab are a banned invasive species which means it is against the law to collect or transport them without a valid permit.

If you see one, say something

If you see a European green crab in your catch, report it immediately to the Alaska Department of Fish and Game’s invasive species hotline. Call 1-877-INVASIV or use the form on the ADF&G invasive species reporting webpage.

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