By Monaca Noble and Linda McCann
Published March 2013

European Green Crabs are eating and marching their way up the west coast. One of nine marine invertebrates to make the list of "The World's 100 worst invasive species", they’ve had major economic impacts on shellfisheries in New England including blue mussels, the Virginia Oyster and Bay Scallops. Impacts are mounting on the West Coast too, where losses to bivalve fisheries (Pacific Littleneck, Japanese Littleneck, Softshell Clam, Blue Mussel) are projected to reach $20,000-60,000 per year (Grosholz et al. 2011). Ecologically, their impact has been no less severe, as they prey on and compete with other crabs, bivalves, and gastropods (snails and slugs), and many other invertebrates.

Green crabs are exceptional world travelers, making it from their native range along the European Coast to six major regions of the world, including the Northwest Atlantic (from Maryland-Newfoundland), the Southwest Atlantic (Patagonia), the Southeast Atlantic (South Africa), the Northeast Pacific (from California-British Columbia), the Northwest Pacific (Japan) and the Southwest Pacific (Australia). Their mode of transport may vary regionally, but evidence suggests they’ve been transported with the live-bait trade and in ship’s ballast water. 

Looking at the history of the North American invasion on NEMESIS (National Exotic Marine and Estuarine Species Information System) we see that green crabs have been on the East Coast of the US for about 200 years, making their first appearance near New Jersey in 1817. From there they moved north, reaching the Bay of Fundy, Nova Scotia in 1953, the Gulf of St. Lawrence by 1994, and finally, Placentia Bay, Newfoundland in 2007. Their southward expansion stopped at the Chesapeake Bay; possibly they couldn’t compete with the Blue Crab (Callinectes sapidus). Green crabs were first recorded on the West Coast in 1989 near Bodega Harbor, California and appeared in San Francisco Bay a year later. Their northward march took them to the north end of Vancouver Island, British Columbia, Canada by 2007, and by 2011 they were 200 miles from the Alaskan border, the known current extent of their range. Similar to their movement on the East Coast, southward dispersal has been much slower. They reached Morro Bay, the current southern limit, about 200 miles south of San Francisco Bay by 1998.

Concerned about the impact of the green crab invasion on commercial shellfisheries in New England, scientists at the Smithsonian Environmental Research Center (SERC) began a series of lab experiments in 2002 aimed at predicting the future range expansion of the crabs on the West Coast. These experiments tested the ability of the larval crabs (the most fragile life stage) to grow and develop at various temperature and salinity combinations, the idea being that locations on the west coast within the crab’s tolerance limits might be susceptible to an invasion. These experiments suggested that most bays in Southeast Alaska are suitable habitat for green crabs and therefore are at risk of invasion. 

Armed with this information and the knowledge that green crabs are advancing northward rapidly, researchers at SERC began training volunteers to monitor for green crabs in Alaska. The first volunteer-based monitoring effort began in 2004 at the Kachemak Bay Research Reserve and soon after, sites around Prince William Sound and Southeast, Alaska were added. Monitors include summer cabin residents near the Kachemak Bay Research Reserve, tribal environmental technicians, retired biologists and teachers. In Kachemak Bay, teachers incorporated the monitoring effort into their classrooms, with kids as young as 10 participating in the trapping. One young monitor’s mother put it this way: 

Monitors trap throughout the summer months recording their catch for inclusion in a statewide database and serving as an early warning system for green crabs. In an effort to recruit additional volunteer monitors in other parts of Alaska and help existing monitors find information and upload their monitoring data, SERC created the Green Crab Watch website. So far no green crabs have been found, but the traps have held many other wonders. Monitors are learning about native Alaskan crabs and fish, which are largely unaffected by invasive species and with any luck, their efforts will keep it that way. 

Grosholz, Edwin (2011) Crabs, University of California Press, Berkeley and Los Angeles. Pp. 125-128.