Green menace: Scientists hopeful tracking study will reveal green crab secrets

WILLAPA BAY — WDFW coastal shellfish biologist Zach Forster used a knife to scrape away the barnacles before reading the receiver serial number to NOAA Fisheries research biologist Dr. Curtis Roegner.

After roughly four months submerged in Willapa Bay, the Pringles-can sized plastic cylinder had accumulated a thin crust of arthropods, but sealed safely inside the container scientists are hopeful to find answers about the mysterious movements and habits of the small, invasive European green crab responsible for increasing destruction on native plant and shellfish species within the bay.

Last Wednesday, March 1, scientists and biologists from NOAA and the Washington Department of Fish and Wildlife departed from the Port of Peninsula in Nahcotta to collect eight of the underwater acoustic receivers used to track the habits and migration of the crabs, dozens of which had coded wire tags attached to their shells before being released last October.

The invasive crab were first documented in Willapa Bay in the 1990s. After an initial monitoring effort was axed by the state during a budget crisis, the population has exploded in recent years aided by environmental changes, causing problems for shellfish farmers and impacting native habitat.

The goal of the project is to help maximize trapping efforts by better understanding where to find the highest concentration of the crab, which have devastated clam populations on the U.S. Eastern Seaboard and elsewhere around the world.

Analyzing the data

Roughly 20 inter-tidal and sub-tidal acoustic receivers, part of an array eventually spanning Willapa Bay, have read and tracked the tagged crabs while “talking” to other receivers in bay since October.

“When a crab travels through one of these, it gets pinged. If it gets pinged by three receivers, we’ll be able to follow the real-time movement of the crab through this array,” Forster said.

“We decided on inter-tidal and sub-tidal receivers because one of the theories is in the fall and winter — which is when we had this deployed through — that these crab maybe migrate into deeper [sub-tidal] water as a refuge.”

The receivers ranged in depth and location, from along the muddy tidal banks to more than 25-feet deep “in the middle of the Nahcotta channel.”

Now that the receivers have been collected, the data will be downloaded and analyzed, giving scientists a first-hand glimpse of where the crab go and what they do, particularly during the fall and winter. The data will be analyzed over the coming months.

“All the data is coded in the receivers and we’ll download that,” Roegner said.

“Each receiver has a timestamp or tag, so we’ll know which crab was within the radius of the receiver,” he continued. “The receivers are part of a big array that are talking to one another, with overlapping reception ranges. That means we can do positions on each of the crab. That’s the ultimate goal, to see exactly where the crabs are relative to things like the oysters or grass beds on the other side. We’ll be able to look at their habitat use in these different areas. The sub-tidal and inter-tidal link is very important because we’ll be able to see if they’re moving up or down with the tide, and if they are, where they’re doing that, because there might be some special places like in the channels, as other crabs do that. There’s a lot we’re going to find out that we don’t know about their behavior.”

Early indications reveal green crab behave differently compared to native Dungeness.

“These are only single arrays. There are also bigger ones that will show us some of their migration patterns, if they’re doing that. The Dungeness that we tagged here did do that — they all went away from the site we tagged them. Whereas, so far, there’s a lot more green crabs that stayed right around where we released them. There’s a big difference between the species already that we know,” Roegner said.

Roegner said preliminary data will be available in the coming days, but analyzing the extensive tracks could take a month or longer.

The biggest question Roegner is hopeful about answering regards the habitat use of the crabs, and how much time they’re in the shallower inter-tidal zone versus the deeper sub-tidal.

“Is the inter-tidal habitat their main focus? Or do they go back and forth? How long do they stay there? Those are the crucial things,” he said.

‘Now or never’

Information learned from the tracking study will be used to improve green crab trapping efforts.

“We’ll be able to see how they respond to heavily-trapped areas, which will help inform removal efforts,” Forster said.

“We’re hoping that by doing it this time of the year, we can see why catch rates for removal efforts tend to drop off. Are these crab going sub-tidal? Are they burying up in the inter-tidal areas? What’s going on? Why are people having a hard time catching them to remove them in the winter months?”

In 2022, nearly 270,000 invasive green crab were removed from Washington waters, according to WDFW — more than half of those in Pacific County.

In February, the Facebook group Willapa Green Crab Disaster announced a grim milestone, when they detailed that their trapping efforts over the past year removed 66,662 green crab from Willapa Bay among 55 traps. The group, comprised of local shellfish growers concerned about the growing impact of the invasive crabs on Willapa Bay, declared the situation an “environmental emergency.”

“We are on the verge of losing Willapa Bay forever … It’s now or never,” declared the Feb. 10 post.