It was a cool, pleasant morning on San Juan Island, Aug. 30, 2016. A few “citizen scientists” were out checking their crab traps in Westcott Bay in the northwest corner of the island when they noticed that one trap had captured a creature that did not belong in Puget Sound.

“Sylvia, I think we caught one,” yelled Craig Staude after he and other surprised members of the group had emptied the trap, bringing them face to face with a European green crab, one of the most dreaded invasive species in the world. 

It was the first green crab ever observed anywhere in Puget Sound. 

“We had really good training and knew what to look for,” recalled Staude, speaking of the Washington Sea Grant Crab Team, formed a year earlier to engage average people in a search for the first signs of an invasion into Puget Sound. 

“We had been trapping for months without seeing any green crabs,” Staude continued. “But this time, when we shook out the trap, we saw this crab. It was unusual, and it was big.”

The five distinctive spines on each side of the crab’s back, or carapace, revealed it to be a European green crab. 

Sylvia Yamada, a longtime green crab researcher from Oregon State University, just happened to be working some of her own crab traps nearby. She came over to the Crab Team location, confirmed the crab’s identity and informed the volunteer scientists that they had just made history.

Since that first discovery, nearly nine years ago, green crabs have spread to more than 30 trapping sites throughout the northern half of Puget Sound and Hood Canal. The crabs typically arrive as free-floating larvae — their early life form — before taking on the familiar crab shape. So far, the total number of crabs trapped in Puget Sound has reached nearly 200,000, though most of those came from a unique “sea pond” in Lummi Bay. 

Intense trapping remains the primary method of maintaining some control over the population of invasive crabs, thus limiting damage to native species, sensitive habitats and commercial shellfish operations.

Emily Grason, a marine ecologist and primary leader of the Crab Team, said she believes that without the ongoing trapping effort, the green crab population would be far, far greater than it is today and would be causing much more damage.

“As our numbers tick up in Puget Sound, so do the local sources of larvae within Puget Sound,” she said. “At the same time, we are also getting more larvae from growing coastal populations. 

“When a location is trapped heavily, we see a reduced adult population the following year. Conversely, when we are not able to get an intensive trapping effort going at a new detection site within the first year or two, we observe that green crab numbers can grow there much more quickly. It's so much harder to trap down a population than it is to keep it from growing.”

Since the Crab Team started 10 years ago, scientists have made important advances worthy of recognition during this anniversary year. For example, new techniques for tracking the green crab invasion have been employed, as researchers gain insights into what the crabs are eating and why they are behaving as they do. 

Geneticists are beginning to grasp what makes green crabs extraordinarily resilient, explaining why their invasions have been so successful worldwide. And most experts are coming to believe that the total eradication of green crabs from Puget Sound — once a primary goal — can never be accomplished. So new strategies are being devised to keep their populations in check. 

Long-term invasion

Before attention became focused on green crabs in Puget Sound, this invasive population, originally from Europe, had been working its way up the West Coast from San Francisco, where the first crab was discovered in 1989. Researchers suspect that young crabs came to California directly from the East Coast, where the invasion was raging out of control, costing an estimated $23 million per year in damages to commercial clam, mussel and scallop production, according to a 2019 economic impact study (PDF). The new arrivals to California likely hitchhiked on moist clumps of seaweed commonly used as packing material for Maine lobsters, bait worms and other live organisms, according to baitworm transportation studies (PDF).

From California, the crab larvae moved north with the warm El Nino currents of 1997 and ’98, eventually reaching the outer coast of Washington. There, they became an ongoing threat to commercial shellfish operations and native habitats in Grays Harbor, Willapa Bay and other coastal areas. In 2012, an isolated population of green crabs was discovered in the Canadian portion of the Salish Sea, in Sooke, BC. 

Concerned that green crabs could invade Puget Sound, the Crab Team was formed in 2015 under the auspices of Washington Sea Grant with strong support from the Washington Department of Fish and Wildlife. The Sea Grant program itself is a partnership between the National Oceanic and Atmospheric Administration and the University of Washington. 

A first-year pilot program in August and September of 2015 involved volunteers at seven sites. The following year, 116 volunteers from throughout Puget Sound joined the Crab Team, immersing themselves in training to identify the various types of crabs they were likely to encounter. They learned to bait and deploy surveillance traps in hopes of detecting early signs of an invasion. Twenty-six sites were chosen for monitoring, primarily lagoons and tidal sloughs, which seemed to be preferred habitat for the crabs.

It was during the Crab Team’s second year in 2016 that the four-member group assigned to San Juan Island captured the first crab in Puget Sound — a fully grown adult — in Westcott Bay. 

Sean McDonald, a University of Washington researcher and one of the organizers of the Crab Team, said the involvement of volunteers has been critical to the surveillance program. Monitoring efforts need to be maintained over time to be meaningful, he noted, and ongoing government funding for more costly professional field work cannot be assured. 

McDonald’s own research, which began as green crabs were reaching Washington’s outer coast, involved studying the relationship between green crabs and the Northwest’s native crabs. 

“In laboratory experiments, we found that little Dungeness crabs (early in life) are no match for green crabs,” McDonald said, adding that a later study revealed green crabs consumed a diverse diet, preying upon native shore crabs, clams and a wide variety of aquatic creatures. The ability to find food in a complex ecosystem is just one of the green crab’s key advantages as it moves into new habitats and expands its range.

McDonald recalls that that nobody was expecting to find green crabs in Puget Sound so soon after the Crab Team was formed. After all, limited efforts to locate these invaders at strategic locations in Puget Sound had been going on for 19 years — ever since the crabs first appeared along the Washington coast.

 “We thought we could go on for years,” McDonald said of the Crab Team. “When we got that first detection, you could have knocked me over with a feather. I was shocked, and I think there was a little swearing.”

Aware of the costly East Coast experience with the invasive crabs, officials with the Washington Department of Fish and Wildlife launched an intensive trapping effort using nearly 200 traps set in and around Westcott Bay. Not a single green crab was captured, although someone did find a molt — a discarded exoskeleton, which probably came from another crab in the area.

Three weeks later, another green crab was found about 30 miles to the east within Padilla Bay National Estuarine Research Reserve. A member of the reserve’s education staff found the crab while turning over rocks on the shore. Subsequent intensive trapping caught three more young crabs in the Padilla Bay that year. These findings confronted ecologists with the prospect that a green crab invasion of Puget Sound was underway.

The right conditions

What brought these first green crab invaders to the inland waterway, and what allowed them to gain a foothold? The answers, developed through much research, shed light on the reasons for listing green crabs among the 100 worst invasive species in the world (PDF). The list, produced for the International Union for Conservation of Nature, places the green crab among the 20 most formidable alien aquatic animals. 

At first, experts suspected that the first green crabs in Puget Sound probably drifted as young larvae across the Strait of Juan de Fuca from Canada’s Sooke Basin — the nearest established population of green crabs. Eventually, however, genetic testing revealed that they were more closely akin to the populations established along the West Coast. 

It appears that the first crabs in Puget Sound may have ridden into the waterway during somewhat unusual conditions. Normally, surface waters flow out of Puget Sound, through the Strait of Juan de Fuca and into the Pacific Ocean. This outward “estuarine flow,” which is typical of inland waterways, seems to prevent the movement of crab larvae into Puget Sound most of the time. But, occasionally, the outward flow reverses during stormy periods when strong winds blow the seas inward toward Puget Sound.

Temperature is the second key factor. Colder ocean temperatures in Washington, compared to California, tend to reduce the growth rate and lessen the survival of green crab larvae. But warm waters favor survival. Based on years of study, researchers have come to expect a greater abundance of green crabs — and more extensive spreading to new areas — following a winter in which waters are abnormally warm.

It appears that the first crabs in Puget Sound arrived as free-floating larvae during a remarkable period from 2013 to 2016 when warm waters amassed into a huge patch in the North Pacific. This patch of warm water was nicknamed “the blob.” A strong El Niña became an additional warming factor in 2015 and ’16. As warm waters reached the Washington coast, they likely picked up large numbers of green crab larvae. Thanks to the warm conditions, a fair number of the larvae were probably still alive when storms reversed the estuarine flow and carried the tiny invaders right into the accommodating bays of North Puget Sound, according to Yamada, the OSU researcher.  A recent study of larval dispersal in the Salish Sea supports this concept. 

Based on the size of the captured crabs, scientists generally agree that the one found in Westcott Bay came in as a larva in 2015 and those in Padilla Bay arrived in 2016. 

Interestingly, no crabs were caught in Westcott Bay the next year, 2017, while four were caught in 2018 and one in 2019. Since then, no crabs have been found in the bay, located near the northwest tip of San Juan Island. This year, for the first time, the presence of green crabs was detected in False Bay near the southern end of the island. Students in a UW ecology class spotted a green crab molt in June while surveying the beach during a research project. 

Meanwhile, the numbers in Padilla Bay have been increasing steadily since 2019, reaching 227 green crabs caught in the bay last year alone, and the invading population continues to expand its range into new areas. Last year, green crabs reached the southern end of Whidbey Island — the farthest south the population has come in the main portion of Puget Sound. They have also expanded to new areas in Hood Canal, such as Quilcene and Port Gamble bays as well as around Seabeck.

Green crabs are known to set up their homes in soft-sediment habitats, such as saltwater marshes and intertidal areas with limited currents. They especially like muddy areas where there is less competition from larger native crabs. Green crabs are known to dig into the sediments, destroying critical eelgrass habitats in their search for buried prey. But they also have been found in a variety of other habitats, including rocky shores. Their ability to survive in a variety of places helps them persist and relentlessly expand their range. 

“Once they were in the Salish Sea, it was game over, because there are so many suitable habitats for them,” said Yamada, who began studying green crabs in Coos Bay, Ore., in 1997, a year before the crabs were spotted on Washington’s outer coast. 

With new sources of crab larvae inside Puget Sound, the perfect alignment of warm temperatures and incoming ocean waters was no longer essential for the population to expand. Temperature and currents are still driving factors, but larval movements in Puget Sound have become less predictable, because of the tumultuous tidal currents through channels, around islands and along sinuous shorelines, as well as the ever-changing freshwater flows coming out of rivers. 

In recent years, warmer-than-average waters have sometimes increased the survival of green crab larvae by increasing the rate at which they form shells and settle out on shore. A shorter larval stage reduces their risk of predation by fish and other organisms, Yamada said. Warmer water also suits their metabolism, since green crab larvae do not do well in waters colder than 50 degrees F. Typically, Puget Sound ranges from 45 to 55 degrees, with some variation from year to year. Enclosed bays can be warmer and more favorable to larval development. 

Whether waters become warmer at times from cyclic ocean conditions, such as El Niño, or from the effects of global climate change, higher temperatures are likely to favor survival of invasive green crabs over native crabs, which are adapted to historically colder waters. 

Green crabs in Puget Sound

Throughout the history of the worldwide green crab invasion, the crabs’ ability to adapt has been the secret to their success. As their population continues to grow in Puget Sound, it seems likely that the crabs are becoming more comfortable in their new home, as they spread from one bay to another, experts say.

Young larvae in large numbers ride on the currents to new places in Puget Sound. When winter water temperatures are warm, they survive in greater numbers. Often, green crab arrivals in new bays are discovered through Crab Team trapping efforts, usually to the surprise of everyone involved.

In April 2017, green crabs were first found in Dungeness Bay, alongside Dungeness Spit, near Sequim. The discovery came just eight months after the first crab in Puget Sound was caught in a trap in Westcott Bay on San Juan Island.

Staff at the Dungeness National Wildlife Refuge had been trapping for green crabs for 17 years, but after taking a fresh look at aerial maps and consulting with Grason, the Crab Team leader, staffers moved the traps to a more suitable location along the spit. Almost immediately, three crabs were caught in six traps, according to Lorenz Sollmann, deputy project leader for the U.S. Fish and Wildlife Service.

“It was kind of eye-opening to think that now we had green crab, and we would have to deal with it,” Sollmann said. “When we are fully staffed, we are six full-time people (at the refuge). How are we going to fit this in?”

With help from the Washington Department of Fish and Wildlife, an intensive trapping program caught a total of 96 crabs that first year. To increase the trapping effort, Sollmann inquired if regular volunteers at the refuge would like to become involved in the battle against invasive species. About 30 people stepped up, went through training, and joined what was seen at the time as a full-out eradication effort. 

The number of crabs caught went from 96 the first year to 69 the second to 57 the third. “Then the numbers really started going down,” Sollmann said, noting that only three crabs were caught in 2020, then eight in ’21 and 14 in ’22. Perhaps it was time to reduce the effort along with the expense, officials thought.

“In ’23, the number really bumped up again, to 105, and we found the first gravid females,” he said. “That kind of popped our balloon. Up to then, we could say that the females we caught weren’t breeding, but then that changed.”

Sollmann’s concerns about breeding females has been realized. While the number of green crabs remained relatively low last year, with a total of 89, the local population really took off this year, reaching more than 1,300 in early August. 

The Jamestown S’Klallam Tribe took over the extensive trapping effort earlier this year and has been fine-tuning the effort, according to Hansi Hals, natural resources director for the tribe.

Even before this year’s trapping began, Sollmann said refuge managers realized they may need to change their goals.

“Our idea of eradication is a nice target, but it is not real,” said Sollmann. As long as green crab larvae are being produced in other areas, some will try to find a place on or near Dungeness Spit, he said.

While Dungeness Bay was catching 96 crabs in 2017, the year also was notable for the discovery of crabs in two other new areas: including Admiralty Inlet with two crabs and Sequim Bay with 1. The number of crabs caught in those areas has remained small, even after intensive trapping. As of the first of this year, the total number for Admiralty Inlet reached eight crabs since 2017 with Sequim Bay at 22.

Every year since then, between one and five new areas have been added to the list of locations where green crabs have been found. This year, new locations include False Bay on San Juan Island, Port Gamble and Quilcene bays in Hood Canal, and Port Ludlow on the Olympic Peninsula. Last year, the crab population reached its southernmost point in Puget Sound (aside from Hood Canal), when a large green crab was trapped at Cultus Bay on the southern end of Whidbey Island. One concern is that the Cultus Bay crab was large, meaning it had lived there several years.

More than 300 trained Crab Team members are involved in the trapping program, covering 57 monitoring sites in Puget Sound plus 11 on Washington’s outer coast. When a crab is found in an area where none have been seen before, an intensive trapping effort normally follows with the help of some 25 state employees and dozens of other permitted partners from federal agencies, tribes and other organizations. 

The Lummi experience

By far, the largest upsurge of green crabs in Puget Sound has been experienced on the Lummi Reservation near Bellingham, where the crab population exploded from 64 crabs to 81,000 in four years. Most of the crabs were captured in the Lummi Sea Pond, a 750-acre saltwater enclosure constructed 50 years ago for shellfish aquaculture by the Lummi Nation.

The sea pond turned out to be a near-perfect breeding ground for green crabs, given its slow exchange of saltwater, shallow depths, warm water and limited crab predators.

In 2019, crab trappers in nearby Drayton Harbor caught 38 green crabs, while trappers in Samish Bay caught eight green crabs. In each of those bays, the number of crabs has continually grown, a combination of crab reproduction and intensive trapping. Last year, 537 green crabs were captured in Drayton with 705 in Samish. 

With Drayton Harbor to the north and Samish Bay to the south, Lummi Nation officials “figured we should check our own backyard,” recalled Nick Jefferson, aquatic invasive species biologist for the tribe.

In October, near the end of the season, 30 traps were deployed five or six times in and around the Lummi Sea Pond and caught 64 green crabs. The following year, 2020, a more extensive trapping program brought in a distressing 2,600 of the crabs.

“We knew we had to do something,” Jefferson said, “but we didn’t understand the magnitude of it at the time. The majority of the effort went into the sea pond, because we were catching hardly anything outside the sea pond.”

In 2021, the trapping effort increased even more. Thanks to the shallow, relatively stable water level, trappers were able to deploy as many as 700 traps at one time during the peak of activity, according to Jefferson. This was as many as 1,200 trap sets in a week — more than an entire year for most trapping sites in Puget Sound. By year’s end, some 86,000 green crabs were taken into custody.

Recognizing the overall threat to natural resources and the shellfish industry, Gov. Jay Inslee declared an emergency on Jan. 19, 2022, as agencies prepared for an expanded effort to contain the invasive crabs. The governor requested an increase in the state’s endangered species account, and the Legislature raised the two-year budget from $2.3 million to $8.6 million in a supplemental appropriation, which was used to address the increasing crab populations on the coast as well as in Puget Sound. The Legislature has maintained roughly that level of about $6 million per year through the current fiscal year and into the next biennium.

Intensive efforts continued within the sea pond in 2022, when 80,000 green crabs were captured. One encouraging sign noted by managers was a much lower percentage of young crabs, suggesting a significant reproductive failure. In 2023, a relatively low 6,585 crabs were caught, again mostly adult crabs, and the problem seemed to be turning the corner. 

In 2024, the numbers dropped again to 2,700, but a cautionary note involved the significant percentage of small crabs that were caught, suggesting a successful reproductive year. That revelation raised concerns for the future, and this year has seen another large increase in the sea pond.

A question looming over leaders of the Lummi Nation and other concerned parties is what to do with the sea pond itself. Repairing the four tide gates, which would restore a more normal flow of cold water along with up-and-down tidal cycles, could cost more than $20 million, according to estimates. Decisions are pending.

Planning for the future

Many of the people involved in efforts to study and control green crabs are coming to realize that the crab’s innate resilience and pervasive nature may make eradication next to impossible for Puget Sound as well as most areas where the crabs have gained a foothold.

“We are starting to get more reflective about where we are and what we have accomplished,” said McDonald, one of the Crab Team leaders. “We are coming to terms with the fact that this is something that is not going to go away. If you could snap your fingers and eliminate all green crabs in Washington state, that would not be the end of the story, because the populations outside the state are so big and prolific.”

A concept being explored as an alternative to eradication is to trap the crabs down to a level where their detrimental effects are minimal and generally acceptable, as opposed to trying to eliminate every last crab. 

Ted Grosholz, professor emeritus in ecology at the University of California Davis, coined the term “functional eradication” as he explored the concept in relation to green crabs in California, although it could apply to any invasive species.

“What we found,” he said, “is that there is a threshold, and we can bring the numbers of green crabs down to a level where their impacts are not consequential. You don’t really need to continue much beyond that threshold. For many managers, that is doable.” 

Thresholds might be different for various locations in Puget Sound, he said. In a bay with extensive clam and oyster beds, the target population could be much less than in a place with limited natural resources or water pollution that restricts their use.

In some cases, an all-out effort to eradicate green crabs may actually backfire, Grosholz said, recalling a project in Central California’s Seadrift Lagoon. Beginning in 2009, an intense trapping effort reduced the number of crabs in the lagoon from an estimated 125,000 down to 10,000 by 2013. But the following year, 2014, the population exploded to some 300,000 — well over twice the number at the beginning of the project, according to a 2021 analysis by Grosholz and colleagues.

Their conclusion was that the trapping effort was highly effective at removing adults but ineffective at removing their smaller offspring. The question then became: Why was the resulting population in 2014 so much larger than at the beginning of the eradication project? Crabs are cannibalistic, Grossholz explained, which helped to keep the population under control as long as adult crabs could eat the small ones. Removing the adults led to a population boom in the next generation. 

Seadrift Lagoon is a relatively enclosed bay with minimal outside influence, so the lessons learned may not be universal, experts say. Still, the experiment reveals that management of green crabs is a challenging enterprise, and managers need to understand the ecological conditions and population dynamics when trying to control green crabs in a specific location. 

About a year ago, the Department of Fish and Wildlife released a long-term management plan that provides guidance for addressing the green crab problem, from early detection to rapid response, along with goals for research, education and future management. 

“We know that shellfish growers, tribes and other local leaders are intimately familiar with our state’s marine waters, estuaries, and tidelands,” said Justin Bush, the state’s incident commander for the green crab emergency. “Our goal with this plan is to shift toward long-term management that protects native species and habitats, while fostering community engagement and providing an effective framework for coordinated local control efforts" (WDFW news release).

The plan states that eradication of the green crab is not recommended unless the population is physically isolated with a low risk of repopulation from incoming green crab larvae. Functional eradication may be an effective goal for controlling green crab populations, the plan says, but more information is needed to formulate specific targets. 

Specifically, damage to natural resources must be related to the density of crabs in a given area. Potential losses in value must be considered along with the cost of removing crabs. 

According to the management plan, Washington state is the nation’s leading producer of farmed shellfish with an annual harvest worth more than $100 million. Hundreds of millions more dollars come from commercial fishing, not to mention the importance of shellfish to recreational harvesters and tribal cultures. 

“From a funding perspective, there is a real desire to keep the actions going,” said Brian Turner, an environmental scientist and lead author on the plan. “Washington by far has taken the most action on the West Coast and even globally. A lot of research is being done in California, but there’s not a lot of effort there to control the population statewide.”

The six-year management plan is intended to be a “living document,” revised as new information becomes available about green crabs and their ongoing threats to Puget Sound, Turner said. 

“It is an incredibly complicated challenge,” he added. “People realistically understand that it is a problem that we are not going to solve soon.”