They thrive in shallow water and soft sediment, which Washington’s estuaries provide.
“Green crabs in Willapa Bay and Grays Harbor are appearing at densities easily five to 10 times greater than they were ever observed in the late 1990s.
She says this sudden proliferation in coastal areas is somewhat surprising because they did not spread rapidly after those initial sightings two decades ago.
Some sites have seen declines, notably Westcott Bay on San Juan Island – site of the first inland trapping five years ago – and Dungeness Spit, where only three were found last year after finds of 50-100 in years prior.
By contrast, the coastal locations with good estuary that they have started monitoring have recently seen big increases.
Some shellfish growers in Washington have seen recent declines in some products, such as wild-seed manila clams, Grason says.
Arianne Akmajian, a marine ecologist with the Makah Tribe, says a member of the public found and reported their first green crab in August 2017.
The area around the reservation has two rivers that mix with saltwater, providing ideal habitat for the invasive species, and their catch rates have steadily gone up.
“What concerns me with this season is that we are seeing a lot of small crabs and small molts, which to me tells me there’s a really good crop of crabs this year – young-of-year crabs that are going to grow.
The state says the public can play a key role in helping contain green crabs by learning what they look like – they’re not necessarily green; the best identifier is their shape, with five spines on each side of their eyes – and snapping photos and uploading them for state biologists to review.
On the East Coast of the U.S., they have been blamed for damaging shellfish harvests and decimating sea grass beds, which are critical habitat for juvenile salmon.
This week, scientists are scouring shoreline habitat near Westcott Bay on San Juan Island, hunting for green crabs.