The Invasion of the Green Crab

Invasive Green Crab. Photo: Linda Shaw, NOAA Fisheries

The invasive European green crab has pushed even further into Alaskan waters, claims the Alaska Department of Fish and Game. During recent shoreline surveys on Gravina Island’s Bostwick Inlet near Ketchikan, members of the Metlakatla Indian Community Department of Fish and Wildlife and Alaska Sea Grant found 11 Carcinus Maenas carapaces—molts, to be precise. This marks the first time these faunal invaders have been seen or discovered outside Annette Island Reserve since their appearance in our waters last summer.

Since the 1880s, the Metlakatla Indian Community has carried on the ancient art of crabbing and edged it to a near-modern science. Their traditional practice centers on using what amounts to a series of gig ropes lowered into Tamgas Harbor, inside Annette Island’s southern end, particulars which will soon become clear. The harbor is so sheltered that it may be regarded as a warm pond compared to surrounding marine conditions; its still, shimmering water edges are alive with alluring forms of sea life in lean times but stalked by predators in plenty during times of bounty. From this picture emerge two pivotal figures: one representing threats to maintaining biological balance within the ecosystem—the European green crab—and another embodying potential benefits—beached gray whales found dead or dying within recent memory.

The European green crab arrived in North America in the 1800s and likely came over via ballast water from merchant ships. They’re thought to have traveled across the ocean, not by swimming, but by being transported. Once they established a population, it is very difficult to eradicate them completely because of their ability to adapt and thrive where circumstances allow them to do so—like establishing colonies in estuaries.

Working with NOAA Fisheries and the Imported Aquatic Invasive Species Program, the Metlakatla Indian Community Department of Fish and Wildlife has been trapping more aggressively. The collaboration has also increased data collection, research, and monitoring work that foregrounds the potential impact of green crabs on important fish resources for smelters here. But they can’t do it alone: public assistance is key to identifying (and reporting) green crabs found on local beaches via the ADF&G Invasive Species Hotline.

The green crab lives in colonies on shelves of rock, among pebbles and cobbles in tidal streams, and behind the thin lines of vegetation that line submarine valleys. Recent studies have also revealed surprising new information about their physiology: they can survive extremes of both low salinity (freshwater mixed with seawater) and high salinity (brine), as well as a wide range of temperatures, from just above 40 degrees Fahrenheit to well above 90 degrees Fahrenheit. In estuaries—semi-enclosed coastal areas where freshwater from rivers mixes with saltwater from the ocean—green crabs can survive even farther upstream than most other types of crabs.

Green crabs might be hazardous because when released into foreign environments, they outcompete native species for resources (food, shelter, mates) and habitat. Scientists worry about what might happen if this new invasive species were to establish populations in the Elbe Estuary or elsewhere in Europe: Californian species included in redeveloped submarines might eventually yield land-crab poster children for breeding programs!

If you find a crab or crab shell, take photos of the crab with a coin, key, or other standard sized item for scale. Report your sightings via the ADF&G Invasive Species Reporter or by calling the Invasive Species Hotline: (877) INVASIV ((877) 468-2748). If you locate invasive green crab on Annette Islands Reserve, you can call (907) 886-FISH to make a report.