Non-native Crab Species
Crabs are prolific breeders and predators, so any non-native crabs compete with native crabs for food and habitat, and increase the pressure on other prey organisms. In large numbers, they can cause ecologic and economic problems, particularly for shellfish. Two species of crabs are of special concern in the Pacific Northwest: the European Green Crab (Carcinus maenus), and the Chinese Mitten crab (Eriochir sinensis).
A captured European Green Crab, picture courtesy of Pacific States Marine Fisheries Commission.
Chinese mitten crabs are easily identified because their claws are hairy. The juveniles spend up to two years in freshwater before migrating downstream to spawn in the saline estuaries. Since they live in both salt and freshwater during various life stages, they can potentially impact both habitats. As juveniles, mitten crabs eat mainly vegetation, but adults will also consume invertebrates, particularly worms and clams. Crabs also eat salmon eggs, which could pose a problem for successful spawning, and they burrow into dikes and riverbanks, causing instability and erosion. The crab is also a potential human health concern, because it is a host for the Oriental lung fluke. Mitten crabs are at home on land, and if blocked by barriers during migration, they will readily move out of the water to bypass them. Females can carry up to a million eggs, and populations can expand quickly, impacting both recreational and commercial fisheries. Mitten crabs were first noticed on the west coast by San Francisco Bay shrimp trawlers in 1992, and their range is expanding. The west coast introduction pathway is unclear, but a deliberate introduction to start a fishery is suspected.
The green crab is an adaptable voracious predator. It can colonize a wide variety of habitats, and has the potential to alter any ecosystem it invades. A small shore crab of varied, but usually mottled, green color, it is one of the most successful estuarine invaders, and has established populations on five continents. The green crab eats a wide range of foods, including plants, insects, clams, oysters, worms, snails, mussels, chitons, urchins, sea stars, other crabs, and fish. It preys heavily on bivalves, and it is able to open them more efficiently than most other crabs. It is blamed for collapse of the Maine soft shell clam industry, and it potentially presents a significant threat to Pacific Northwest shellfish culture. First discovered on the west coast in San Francisco Bay, in 1989, it has since spread into Oregon (1997), Washington (1998) and British Columbia (1999). This coast-wide colonization has been correlated with an El Nino event of strong northward currents in 1997-98, but there are least three possible mechanisms of Pacific Northwest of introduction: planktonic larvae from populations in California, transplant of oysters and other shellfish, and ballast water.
Status in the Pacific Northwest:
Under the Lacey Act, it is illegal to import eggs or live green or mitten crabs into the United States, and it is also illegal to transport or possess them in Washington and Oregon. As of 2008, only 11 dozen mitten crabs have been found on the east coast, but they have been caught by the thousands in California, where nearby rivers and fisheries have been impacted by clogged fish screens, eroded banks and levees, and damaged fish nets. One was also found in the Columbia River in 1997, but no established population has been found.
Green crabs are present along the Washington and Oregon coast, but not in the numbers that have caused such problems elsewhere. In 1998, Washington established a Green Crab Task Force, started a volunteer monitoring program, and set up traps in Willapa Bay and Grays Harbor to keep the population in check. Green crabs are abundant on the west coast of Vancouver Island, and every embayment on the west side of the island appears to be infested. There is concern that if they spread further north and around the island, they may be able to enter Puget Sound via the Strait of Georgia. There are also concerns that the crabs will invade Alaska. EPA has developed a bioeconomic tool for estimating economic impacts of aquatic invasive species, in this case green crab, that should be available in April, 2008. The tool estimates both market and non-market economic impacts, and the case study shows different impacts on U.S. east and west coasts.
What can you do?
If you find a green or mitten crab, catch it or photograph it if possible, note the location and date, and contact the appropriate state agency:
In Washington: 1-360-902-2700 (ANS Coordinator)
In Oregon: 1-866 INVADER (hotline)
In Alaska: 1-877 INVASIVE (hotline)
The links provided may be outside the EPA.gov domain.
ID guide comparing Green and Mitten crabs with native species
EPA study: Estimating Economics of European green crab:
(study to be released April, 2008. Contact Marilyn Katz for more information)
Mitten Crab Fact sheets
Green Crab Fact Sheets and profiles
Green Crab Identification
Green Research links