Scientists and fishers battle aggressive Green crab invaders

Written By Tom Davis

A little-known invasive species has been spreading along B.C.’s coast since 1998, and for the most part the invasion has gone undetected. 

“I have seen more than 10,000 Green crabs pulled out of critical juvenile salmon habitat in the Cypre River estuary, in Ahousaht territory, in one day using only 40 traps,” said Crysta Stubbs, science department director with the Coastal Restoration Society. 

The most startling part was there was virtually no by-catch.

“Demonstrating just how intensely overrun these critical near-shore habitats are with European Green crabs and emphasizing the need for widespread control and management efforts as soon as possible,” Stubbs said. 

European Green crabs of varying colours. [Photo Department of Fisheries and Oceans]

Native to Europe and North Africa, the Green crab is much smaller and less commercially valuable than B.C.’s dominant local species, the Dungeness crab. 

Green crabs arrived on the American east coast almost 200 years ago after hitch-hiking on sailing ship ballast rock, eventually making their way north to Canada’s maritime coastline. 

“I have seen more than 10,000 Green crabs … in one day using only 40 traps.”

Crysta Stubbs

The first Green crab arrivals in Atlantic Canada came from a southern European population. They were able to survive, but not flourish in the much colder North Atlantic environment until later infestations came from northern Europe. The northern crabs prospered because they were genetically suited to cold water. The two groups then inter-bred, increasing their biomass, and accelerating their expansion to new regions. 

Andrea McQuade, a director with the Coastal Restoration Society, confirmed their rapid ability to create different genetic populations in B.C. 

“This ability to establish distinct self-sustaining populations foreshadows continued expansion of this invasive species,” said McQuade. 

The breadth of the North American continent slowed their spread to the Pacific coast, but it was only a matter of time before Green crabs appeared in B.C.’s waters. 

Map showing the where the Green crab have been recorded as of October 2023. [Map Pacific Salmon Foundation]

Like most invasive marine species, Green crabs likely arrived on the Pacific coast in ballast water taken on to add weight and stabilize cargo vessels after they’ve unloaded. When sea water is pumped into ballast tanks, tiny sea creatures come with it, including, in this case, Green crab larvae. 

When new cargo is taken on at different ports, ballast water is released from the tanks into the surrounding water. This is how Green crab larvae were unleashed onto the west coast.

They were first sighted in Barkley Sound in 1998, a decade after a population took hold in San Francisco Bay. It’s believed their larvae were transported north along the currents, establishing new populations along the U.S. coastline, before crossing over to Vancouver Island’s west coast.  

Invasive, adaptable, nearly impossible to eradicate

Green crabs are highly invasive, adaptable, and nearly impossible to eliminate, according to the federal Department of Fisheries and Oceans (DFO) and the Global Invasive Species Database calls them “one of the world’s most successful aquatic invaders.”

While small in size, these crabs pack a big environmental wallop. DFO literature ranks them as one of the top marine invasive species, while other critics refer to them as cockroaches of the sea. 

They are aggressive predators that destroy shellfish beds by preying on clams, mussels and oysters. They reproduce at high rates and out-compete native species. They also prey on small fishes and native crab juveniles, and they have few significant predators. 

In areas with heavy infestation the diversity of crab species has plummeted, with only Green crabs appearing in the traps.

Poster to track spread of the Green crab. [Image Invasive Species Specialist Group]

Worse, they can survive in variable ocean salinities, which expands their habitat from salty ocean environments to brackish marshes where salt and fresh waters mix. Marshes are difficult habitats to monitor and study, making them ideal locations for crabs to live and reproduce. 

Inhabiting water depths of less than six meters, they can also survive out of water for up to a week, which allows them to move easily from one location to another. 

‘It changes the entire ecosystem’

One of the biggest concerns is the damage done to eel grass beds, which provide cover from predators for juvenile fishes, like baby salmon, and other small marine creatures, while supporting a diverse food supply within the marine grass eco-system.

Green crabs thrive in all foreshore environments, but are particularly attracted to eel grass beds. And while they do eat eel grass, that’s not how they destroy them. 

European Green crab dig into the roots of eel grass, destroying their roots and altering aquatic ecosystems. [Image Maria Mitchell Association]

The crabs destroy the grass when they dig into sea floor sediment looking for prey. This damages the roots and disturbs sediment that holds the grass beds in place. 

“This is a huge concern across the coast,” said Coastal Restoration Society scientific director, Crysta Stubbs. “[It] changes the face of the entire ecosystem.” 

Once eel grass beds are destroyed, it takes a long time for recovery. 

Massive infestation leads to new collaborations 

Sooke’s Ryan Chamberland wears many hats. He owns Vancouver Island Lodge, charter fishes for salmon and halibut, and is the Salish Sea manager for the Coastal Restoration Society. In his capacity with Coastal Restoration he works with Vancouver Island’s T’Souke First Nation doing research on European Green crabs, which includes extensive trapping in the infested areas.

Green crab measured in Claquot Sound. [Photo Ben Hazel]

Chamberland was alerted to the Green crab problem after noticing unfamiliar crustaceans – later confirmed as Green crabs – below his lodge’s fishing docks. 

He quickly reached out to T’Sou-ke First Nations chief Gordon Planes. This led to a collaborative effort with Coastal Restoration to study best trapping strategies, while exploring commercial opportunities for Green crabs. 

Since 2021 the Sooke trapping team has removed 187,000 Green crabs from the Sooke Basin region. As of September, the organization had trapped more than 600,000 total in Sooke and Clayquot Sound.

Turning marine restoration into a business venture

Coastal Restoration was founded in 2017 with the goal of transforming aquatic and shoreline coastal restoration into an industrial scale business endeavor. This venture intends to provide new employment and career opportunities to coastal communities, with a focus on meaningful partnerships with First Nations and the appropriate government agencies.

Clayoquot Sound trapping crew, Joel Elley and Brooke Robbins. The Coastal Restoration Society has trapped more than 600,000 green crabs, mainly in Sooke Basin and Clayoquot Sound. [Photo Ben Hazel] 

To date they have partnered with a number of First Nations, most key government agencies, as well as non-profits like the Pacific Salmon Foundation, the Invasive Species Council of BC and the T. Buck Suzuki Foundation. Their projects include coastline cleanup, flood response, ghost gear removal, derelict vessel removal, vacated aquaculture site remediation and salmon monitoring and assessment.

However, the European Green crab project may be their biggest challenge, because little is known about their abundance, their rate of expansion along the coast, where the heaviest concentrations are located, and what remedial actions can be taken to control and/or eradicate this highly destructive species.

Their rapid spread continues northward. In 2022 Green crabs were discovered in Tsimshian territory at Annette Island, near the southern tip of the Alaskan panhandle.

Green crab ‘militia’ battles invaders

Actions to control these invaders are not limited to the west coast of Vancouver Island and the Salish Sea. First Nations on the Central Coast and Haida Gwaii are facing similar problems. 

In the Bella Bella area, the Heiltsuk First Nations are “hitting Green crabs hard” according to Tom Sewid, a Kwakwaka’wakw artist and advocate for pinniped, and more recently Green crab, harvest and controls.

The skipper of a Green crab trapping vessel in Haida Gwaii waters, Kwiaahwah Jones, has a passion for protecting the island’s marine environment. However, she and her crew prefer to be called “the Green crab militia”, rather than Green crabbers. The use of militia indicates they are at war with an extremely destructive alien species that could significantly alter Haida Gwaii’s pristine marine environment, she said.

With funding from the Council of Haida Nations, and support from DFO, Jones and her crew have spent the last two summers trapping Green crabs. In 2022, they removed 31,000 animals. In 2023, removals ballooned by tenfold. Jones recounted one set of 70 traps that produced 5,000 Green crabs. 

To date Jones’ crew has been working part-time, but she recommends “unleashing fishermen’s maximum capacity” in order to stop and reverse the expansion of Green crabs along the BC coast. Also a Haida artist and author, Jones is a strong advocate of expanding markets for Green crab “that would ultimately support a prosperous self-sustaining commercial Green crab fishery.” 

Potential market products include fertilizers and specialty foods for human consumption.

According to Jones, they have observed disturbing trends in the short time they’ve trapped these crabs.

In 2022, her records show females made up 33 per cent of the catch. One year later, that increased to 50 per cent females and was accompanied by a noticeable increase in average crab size. 

On the positive side, Haida Gwaii crews are learning how to effectively attack these alien invaders. 

Using raw baits, like open salmon carcasses, was very productive, Jones said, noting one trap baited with a single chum salmon contained over 400 Green crabs. No matter what, she is “in the game for the long haul,” and would gladly add another boat and crew to their crab removal operation.

Hope at the end of the tunnel

Her hope for effective crab population control, advocacy for larger eradication programs and the development of profitable commercial Green crab opportunities are shared by those working with the Coastal Restoration Society. 

Coastal Restoration has also reached out to academic institutions like Memorial University, University of BC, University of Alberta and Simon Fraser University, along with the Bamfield Marine Center, Parks Canada and US groups to initiate knowledge sharing and to obtain advice on how to control Green crabs. 

They also give credit to DFO’s Aquatic Invasive Species Department for “recognizing the dangers posed by Green crabs,” said McQuade, adding, “funds for managing this problem are limited.” 

But there may be light at the end of the tunnel. 

Sustained trapping efforts are resulting in declines in Green crabs in Clayoquot Sound and Sooke Basin, according to Brenton Twist, Coastal Restoration’s Green crab senior project manager.

Even more hopefully, “Red Rock and Graceful crab numbers are becoming more abundant in habitats where previously only Green crabs were caught,” he said.