European green crab

Tag: European green crab

As in a pandemic, the battle against invasive species may well depend on early actions

As Americans, we have become all too familiar with the spread of a deadly virus and the terrible consequences of a delayed response to an outbreak. As a result of our experience, I’m wondering if some of us might have a more visceral sense about the need to control invasive species.
I’m not saying that the ecological, economic and cultural disturbances wrought by invasive species are on par with a massive loss of human life. But the common denominator is a biological perturbation that occurs suddenly and threatens to expand rapidly out of control, leaving permanent damage. The ultimate outcome depends on reaction to the threat.
“In the invasion world, early detection and rapid response is the best way to manage invasive populations,” said Emily Grason, a marine ecologist with Washington Sea Grant. “That’s similar to a pandemic. You want to find outbreaks when they are small and then react very quickly.”

Trapping and detection sites along Washington’s inland shorelines in 2020. (Click to enlarge.) The filled yellow circles show where European green crabs were captured in 2020, with the size of circle representing average capture rate. The open yellow circles show where green crabs were trapped in previous years but not in 2020. Open black circles are sites where no crabs have ever been caught. Graphic: Washington Sea Grant

Emily manages Sea Grant’s Crab Team, a group of more than 200 volunteers and professionals who systematically set crab traps in bays and estuaries to locate early infestations of the European green crab. This small crab, considered one of the world’s most damaging invasive species, has taken over bays on the East Coast, disrupting natural conditions. When a green crab is caught in Puget Sound, the first effort is to rapidly deploy many more traps to assess and reduce the crab population before it grows out of control.
Emily describes several ways that reactions to an invasion of nonnative species may resemble a pandemic — such as communicating the risks to the public and political leaders, determining priorities for action with limited resources, and enlisting the help of enough people to stay ahead of the threats.
“We don’t want to be flippant about this comparison,” Emily told me. “Human lives are at stake in a pandemic, and there are real differences in the immediate and long-term consequences.”
While not a direct threat to human health, European green crabs could take a heavy toll on native shellfish, destroy eelgrass beds important to salmon and forage fish, and consume commercial clams and oysters with financial losses to the shellfish industry. The Legislature has awakened to the threat with increased funding in recent years, as lawmakers also invest in battles against other invaders — from zebra mussels that could clog up critical water pipelines to Asian giant hornets that threaten to wipe out honey bee colonies.
Meanwhile, there is no letup in the 47-year effort to hunt down and destroy gypsy moths, which could defoliate trees by the thousands if the invasive moths ever gain a foothold in this state. The gypsy moth program is run by the Washington State Department of Agriculture. See also “Priority Species,” Washington Invasive Species Council.
The volunteer search for European green crabs in Puget Sound actually started with limited surveys more than 20 years before the first green crab was spotted on San Juan Island in August of 2016. Washington Sea Grant had taken over formal management of the program the year before — in 2015 — after green crabs had established a breeding population in Sooke Basin, west of Victoria, B.C. That Canadian site is just across the Strait of Juan de Fuca from Washington’s Dungeness Spit.
In August, it will be five years since the beginning of the Puget Sound invasion, and I asked Emily what she has learned along the way. One thing, she said, is the “value of doing your homework,” such as checking out all your assumptions.
For example, it was once assumed that the Puget Sound invasion began with crab larvae drifting across the Canadian border from the infestation in Sooke. But genetic studies revealed that the population on the Dungeness Spit was far more similar to green crabs found along the Washington Coast. With that in mind, researchers were able to connect that invasion to occasional oceanic currents that flow inland along the south side of the Strait of Juan de Fuca, opposite to the normal outflow. Such “reversals” are capable of bringing crab larvae into Puget Sound, although Canadian crab populations appear to be the source for some invasions on this side of the border.
“How green crabs were getting into the Salish Sea was not expected,” Emily said. “That reminds me that it is always worth checking our own assumptions to make sure that we are doing the right thing.
“When people talk about invasions, the only surprising thing is when there are no surprises,” she added. “Surprises are the rule, not the exception.”
Emily says she has also learned the value of meaningful collaborations. With respect to green crabs, many experts from a variety of agencies as well as community members have contributed knowledge, innovations and new ideas to the effort of protecting the Puget Sound ecosystem.
“No one person is better situated than another,” she said. “If you’re not talking and working together in good faith, you risk missing something and not seeing the full picture.”
Top graph shows average annual catch rate of European green crabs per 100 trap sets since 2015. Bottom graph shows percentages of the total crabs trapped each year at selected sites. Graphic: Washington Sea Grant

While the pandemic blunted the Crab Team’s efforts and put volunteer recruitment on hold last summer, more than 90 percent of the identified trapping areas were covered through an intensified effort of team members following strict pandemic protocols (Salish Sea Currents, May 21, 2020). More than 11,000 traps were deployed, as team members also confronted a relatively new invasion discovered during the fall of 2019 in Drayton Harbor, a mile south of the Canadian border.
A new management team, led by Chelsey Buffington of the Washington Department of Fish and Wildlife, captured 255 crabs in Drayton Harbor last year. That’s more green crabs than the total captured at all the inland sites the previous year.
“All of the mud slogging and crab wrangling was done almost exclusively by the three dedicated crew members: April Fleming and Lindsey Parker, seasonal technicians with Washington Department of Fish and Wildlife, and Allie Simpson, the local coordinator of the effort working with Northwest Straits Commission,” Emily wrote in her extensive Crab Team Blog post at the end of the year.
A key finding was that green crabs in Drayton Harbor were concentrated in a few locations, rather than being dispersed throughout the harbor. Now the hope for Drayton is that it will follow a pattern that has apparently created a favorable trend for Dungeness Spit. There, a heavy trapping effort over three years reduced a large number of crabs down to just three caught last year in 1,883 trap sets.
An even larger infestation last year was seen in Lummi Bay near Bellingham (south of Drayton Harbor), where crabs were first discovered in 2019. The population growth in Lummi Bay was so rapid that a trapping effort, led by the Lummi tribal biologists, caught more than 2,500 green crabs last year. That includes a large number of younger crabs that showed up in August and September. Most of the crabs were caught within a 750-acre sea pond, an artificial water-impoundment used to support nearby salmon and shellfish hatcheries.
“The sea pond appears to be, in some ways, a nearly perfect incubator for green crabs,” Emily said in a story published in Northwest Treaty Tribes magazine. “The enclosure protects the site in such a way that it creates conditions we don’t really see on that large of a scale anywhere else. It’s quite shallow but retains water all of the time, and it’s very protected from a lot of other conditions that might make it hard for green crabs to survive.”
Volunteer training resumed this year, with the trapping effort underway in early April. So far, a relatively few crabs have been caught in Drayton Harbor, Dungeness Spit and Samish Bay.
Related stories:

Hornets, crabs and rodents: setting traps to locate and contain invasive species

While media reports were raising alarms about an invasion of dangerous “murder hornets,” Washington state entomologists were quietly planning a trapping program, which will mark the beginning of a search-and-destroy mission against the Asian giant hornets.
Trapping has become a primary tool in the early detection of invasive species. Traps are often used to control or reduce breeding populations of destructive pests — from insects such as the gypsy moth to rodents such as nutria to aquatic organisms such as the European green crab.
Officials with the Washington Department of Agriculture say they are hoping the Asian giant hornets can be eradicated from Washington state or at least kept under control, as we’ve seen for gypsy moths and green crabs.

Asian giant hornet // Photo: Washington State Department of Agriculture

Sven Spichiger, managing entomologist for WSDA’s Pest Program, said it was disappointing to learn that this new species of hornet was able to survive the winter in Western Washington. But that doesn’t mean that we must tolerate this dangerous insect, which can literally kill every honey bee in a hive.
“It is my belief that it is still very early on for any sort of infestation, which gives us an excellent opportunity to use everybody’s eyes and ears, find out where it is and wipe it out where we find it,” Sven during a press briefing recorded on YouTube.
In the first trapping program for Asian giant hornets, WSDA trappers will install about 300 traps in Whatcom County near the Canadian border. It’s where the first-ever queen was found in Washington state on May 27, not far from where a worker hornet was found in December.
Where Asian giant hornets are picked up in traps, experts will search for active underground nests and carefully destroy them. If they can catch hornets alive, they could try out a miniature tracking device to see if the hornets can lead them to a nest, Sven said. The trapping effort will begin in July before the worker hornets are expected to emerge from the nests.
Although these hornets really are quite large and bear over-sized stingers, officials stress that they do not normally attack humans or pets unless they are threatened. Information about Asian giant hornets, including how to identify and report them, can be found on the WSDA hornet page.
Meanwhile, trappers will continue to put out gypsy moth traps — some 20,000 this year — throughout Washington state. Some will be placed at high density to reduce the number of moths in some areas, while others will be placed at low density to locate new infestations. Two areas of Snohomish County are designated for eradication by using aerial spray. See “2020 eradication information.”
European gypsy moth // Photo: John H. Ghent, USDA Forest Service

The effort this year involves both types of invasive gypsy moths: 1) the European gypsy moth, which often comes to the Puget Sound region from established areas in the Eastern United States, and 2) the Asian gypsy moth, which can devour our Northwest evergreen trees.
Infestations of Asian moths are more worrisome, because both males and females can fly, making their spread faster and less predictable. Females of the European variety generally don’t move very far. See “Gypsy moth in Washington” (PDF 2 mb).
Trapping for other invasive insects varies by year. They can include other tree pests, such as emerald ash borer and sirex woodwasp; fruit pests, such as apple maggot and European grapevine moth; and wide-ranging destroyers of a variety of plants, such as Japanese beetle and light brown apple moth. See “Invasive Insect Detection” (PDF 1.2 mb).
The COVID-19 pandemic has caused some changes in the trapping protocols this year. Before placing a trap, officials with the WSDA typically check with the land owner, even though state law gives them authority to place a trap on private property. This year, due to concerns about spreading the coronavirus, trappers — who will wear a safety vests with “WSDA” on the back — will follow a “no knock” policy. Property owners who have questions or object to the traps may call (800) 443-6684.
Among the aquatic invaders in the Puget Sound region is the European green crab, which showed up in Puget Sound in 2016. The trapping program, which started before the first invasive crab arrived, has been credited with locating small populations of the destructive crabs before they could gain a permanent foothold. Where green crabs are found, trappers put out many more traps to curb the population.
A small male European green crab found at Dungeness Spit.
Photo: Allen Pleus, WDFW

Washington Sea Grant’s Crab Team, a major volunteer effort that conducts trapping and identification, was recently honored with the SeaDoc Society’s 2020 Salish Sea Science Prize, as shown in a video on YouTube.
Eric Wagner recently wrote an article for the Encyclopedia of Puget Sound explaining how the green crab trapping program has continued despite COVID-19. See “Search for invasive green crab continues during pandemic.” My coverage of green crabs goes back to just before the first invader was found in 2016, when I examined a variety of invasive species threats. See “Invasive stowaways threaten Puget Sound ecosystem.”
A lesser-known aquatic invasive species that has placed officials on high alert is the African clawed frog, a species found in two groups of stormwater ponds, one near Lacey and the other near Bothell. The ponds discharge to natural water bodies, and both areas have been actively trapped to control the frog populations.
Native to sub-Saharan Africa, African clawed frogs are considered a severe threat for their ability to out-compete and even consume native frogs. Similar to bullfrogs, these clawed frogs can eat just about anything that fits into their mouths, including small fish and birds. They also are known to carry diseases that can kill other amphibians as well as fish. These imported frogs were commonly used in laboratories and classrooms.
Also found in ponds and wetlands, beavers historically were trapped for their fur, but today there is a growing recognition that they provide important aquatic habitat to help certain species of salmon. Such cannot be said for the distant relative of the beaver: an invader called nutria, a semi-aquatic rodent that burrows into embankments, destroys wetland vegetation and displaces established beaver populations.
Nutria // Photo: Washington Invasive Species Council

Nutria were first brought to the Northwest from South America to be raised for their fur on local farms. In the 1940s, the demand for fur declined and most farms went out of business. Some of these animals have survived in the wild, with large numbers reported in some areas, including Lake Washington. Organized trapping efforts have been attempted but never sustained.
Washington residents can get a better understanding of invasive plants and animals with with a smart phone app called “WA Invasives,” available from the Apple App Store or Google Play. The app includes pictures of the dozens of invasive species with a process to report the location of sightings. See also “Report a Sighting” on the website of the Washington Invasive Species Council.

View the European green crab story map at

Story map details efforts to stop green crabs in Puget Sound

Researchers have been on high alert since the 2016 discovery of the invasive European green crab in Puget Sound. So far, monitoring teams have found relatively few crabs, but experts worry that a population could grow rapidly and damage the native ecosystem. Now a collaboration of volunteers, agencies and tribes is working to stop the crab’s spread. They say intensive, early action could make the difference. A new story map from the Puget Sound Institute and the Washington Sea Grant Crab Team documents the effort.
View the story map on the Encyclopedia of Puget Sound.