New green crab detections around Calvert Island reveal how difficult an invasion can be to detect in its early stages and why finding the crabs before populations become established matters.
For years, seawater has been collected from a monitoring station in Kwakshua Channel, the narrow waterway separating Calvert and Hecate Islands on British Columbia's Central Coast. The samples were part of a much larger effort to understand which fish were moving through coastal waters. Nobody collected them specifically to look for European green crabs.
Then people started wondering what else might be hiding in the water.
European green crabs have been moving through coastal British Columbia, establishing themselves in bays and estuaries where they can reach extraordinary densities. Finding them early presents an obvious problem: when there are only a few crabs scattered across kilometres of coastline, actually catching one can be remarkably difficult.
Green crab traps are set in the intertidal at Goose Grass Bay during 2026 fieldwork around Calvert Island. Crews used traps across different depths and habitats to look for crabs while their numbers remained low.
Environmental DNA (eDNA) offered another way to look.
Animals continuously shed tiny traces of themselves into their surroundings. Collect some water, extract the DNA it contains, and a sufficiently sensitive genetic test can reveal that a species has been somewhere nearby, leaving behind traces of its DNA, without anyone ever seeing it.
Matt Lemay and his colleagues at the Hakai Institute applied just such a test for European green crab. The idea was to see whether water already being collected through the Hakai Institute's Integrated Coastal Observatory could double as an early-warning system for the invasive species.
They started small. This was proof-of-concept work, not a coast-wide surveillance program.
From the long archive of samples collected at a station known as KC-10 in Kwakshua Channel, they tested three samples from July 2024.
Nothing.
They tested another three from August.
Nothing again.
The samples weren't analyzed for green crab when they were collected. Matt's team tested them later, as part of the same round of proof-of-concept work, the 2024 block alongside a second batch from early 2025: three samples from January, three from February, three from March. All three January samples came back positive for European green crab DNA. One of three from February was positive. In March, two of three were.
The quantities were extremely low, and the sample was small: six bottles negative, followed by nine, six of which were positive, but only at very low levels. They represented a tiny fraction of years of archived water nobody had thought to test.
The signal had been sitting in those bottles, unnoticed until someone thought to look for it.
The assay is designed specifically for European green crab and is sensitive to extremely small quantities of its DNA. A positive result provides strong evidence that green crab DNA is in the water. It could have been shed by an adult living nearby. It could have come from larvae drifting through the channel. And the absence of a signal didn't necessarily mean the absence of crabs either, particularly when their numbers were low.
There was another complication. The water from KC-10 was collected at the surface. European green crabs live on the bottom.
In other words, the team was using samples collected for an entirely different purpose to look for traces of an animal living in an entirely different part of the water column.
Matt has seen an early reading like this turn up somewhere green crab hadn't been confirmed before, elsewhere on the coast. His team's response, he says, is to retest before saying anything: "I was concerned early on … because I know they hadn't been known to be there yet."
Even once a signal holds up, he's careful about how much credit to give the tool on its own. A positive result can mean an established population, or it can mean larvae that drifted in from somewhere else and never settled. Confirmation, when it comes, tends to arrive independently, through someone eventually finding the animal itself.
"It was nice," Matt says of one such case, "because we were independently working on it. And then that validation came through."
Matt sees eDNA as a way to identify areas that warrant closer investigation than trapping crews could otherwise cover.
He describes the Kwakshua signal as a reason to treat the channel as "a good location for a follow-up survey." When trapping later confirmed crabs there, he calls it a validating case study, evidence the tool was working the way it should. Zach Monteith, who helped run the trapping, traces this year's survey to Heiltsuk's own effort to contain the spread of green crab in their territory, and to conversations about where coverage was most needed.
Those conversations helped narrow down where to look: habitats matching the mud flats and eelgrass beds green crabs prefer, in places close enough to the facilities at Hakai to reach and check regularly. Existing long-term monitoring sites in the area weren't quite the right fit. They hadn't been surveying as high up in the intertidal zone as green crabs tend to occupy, so even where the habitat overlapped, the sites were sampling too low to catch the small numbers of crabs now present.
A faint signal turned up first in the channel. When crews went looking there this summer, they found crabs.
Traps went into bays and inlets around Calvert Island. European green crabs appeared at sites scattered across the area, including Goose Grass Bay, Pruth Bay, Kiltik Cove, Sea Otter Inlet, and Kwakume Inlet.
At several locations, the catch amounted to a single crab. Goose Grass Bay produced more animals than anywhere else the crew sampled, a cumulative total of 13 crabs across two rounds of trapping, roughly 45 trap-days of effort, but even there the numbers were nothing like the dense populations found elsewhere on the Central Coast.
A European green crab caught at Pruth Bay during the 2026 survey around Calvert Island. At several sites, crews caught only a single crab; Goose Grass Bay produced 13 across two rounds of trapping and roughly 45 trap-days of effort.
Zach and the Nearshore team think one possibility is that the Calvert Island region is seeing the front edge of the invasion as green crabs slowly expand into the area.
So far, the Nearshore team hasn't seen gravid females, large aggregations of newly settled crabs, or the dense populations that would indicate significant local reproduction. Many of the crabs appeared to the team to be roughly two or three years old, raising the possibility that a pulse of larvae arrived several years ago, carried in on currents or temperatures favourable enough to let them settle. But the age estimates are rough, and the team still doesn't know local growth rates or whether the crabs are reproducing locally.
The crabs being caught could represent the beginnings of something much larger, or conditions around this part of the coast might be slowing their establishment.
A European green crab collected at Goose Grass Bay during 2026 fieldwork. Goose Grass produced more crabs than any other site surveyed around Calvert Island, with 13 caught across two rounds of trapping and roughly 45 trap-days of effort.
Diana Chan has seen what can happen when those numbers don't stay small.
European green crabs were first found in Heiltsuk territory in 2011, around Gale Creek, a large tidal lagoon. Heiltsuk Guardians began trapping and removing them as they were able, initially catching perhaps 10 or so crabs per trap.
For six or seven years, Diana says, the population remained relatively steady before increasing dramatically.
Today Gale Creek is the most heavily invaded area in Heiltsuk territory, and the difference can be seen in the sand.
On heavily affected beaches, Diana says, it has become extremely difficult, sometimes impossible, to find juvenile clams.
Science conducted with Heiltsuk and university partners, including teams at the University of Alberta, the State University of New York College of Environmental Science and Forestry, and the University of Guelph, has found that green crabs preferentially consume clams within a particular juvenile size range. Larger adult clams remain on some of those beaches, but the younger generation that would eventually replace them is disappearing.
"There's lots of adults now," Diana says, "but once they age out naturally, we're concerned that the populations won't be able to replenish themselves unless we are able to get a handle on the green crabs."
For Heiltsuk, that isn't an abstract ecological change. Clams are food, and they carry commercial value. Diana also worries about the potential for green crabs to damage eelgrass beds, although she says Heiltsuk haven't yet observed extensive damage of that kind locally.
Catching green crabs early, Diana says, is "the most important thing." At some sites in Heiltsuk territory, Guardians found small numbers of crabs, trapped intensively, and then stopped catching them.
They aren't declaring victory. Larvae can arrive again and the sites continue to be monitored. But the apparent success suggests that removing a population while it is still small may be possible.
At one beach within Gale Creek, Heiltsuk crews spent months using multiple boats every day to drive green crab numbers down. Gale Creek contains more than 20 beaches.
"If you're able to catch them early," Diana says, "that's huge."
A green crab trap is simple enough: bait it with herring, put it in suitable habitat, and come back. But traps generally need to be checked within 24 hours, both to release by-catch and because their effectiveness declines over time.
Anika Chen deploys a green crab trap at Kiltik Cove on Hunter Island during the 2026 survey. Traps were set across the intertidal and shallow subtidal to sample different depths and habitats.
For this survey, crews used 10 traps at each site, distributed across the high intertidal, low intertidal, and shallow subtidal. Crews need boats, fuel, bait, people, and time. And even then, a trap can tell you there are no crabs inside it, not necessarily that there are no crabs nearby.
Diana has seen beaches where traps caught nothing and people later spotted green crabs walking in the intertidal.
A crew from Hakai had an even more direct demonstration.
They set traps and caught nothing.
Then a green crab crawled over someone's foot.
"We wouldn't use eDNA alone to say it's in a new area," Matt says. "But it provides … good advice on where to look."
For Diana, the appeal is practical. Trapping limits how many places a crew can investigate in a day. If eDNA could rapidly screen a larger number of sites and identify the ones most likely to contain green crabs, crews could concentrate their traps where they matter most.
"That would be a really very valuable tool to have," she says.
Heiltsuk have shared traps with the Hakai Institute along with years of practical experience about where to put them. Clam beds and eelgrass habitat are obvious places to investigate, and Diana says trapping in the intertidal has generally been most successful.
The team at Hakai, meanwhile, has a permanent presence around Calvert Island, an area that is a long boat run from Bella Bella and difficult for Heiltsuk crews to monitor as frequently as they would like.
"Having that concerted effort this summer has been really valuable," Diana says, "especially since it has resulted in those detections."
The partnership works differently with eDNA. The Hakai Institute brings expertise in developing the genetic tools, sampling resources, and laboratory analysis. Heiltsuk crews can collect samples across their territory and send them to the Hakai Institute for processing. Heiltsuk also bring local knowledge about where green crabs are appearing, where trapping should be prioritized, and connections to a broader network of science partnerships.
The aim, Diana says, is for those different streams of information to feed into an overall Heiltsuk plan for dealing with green crab across the territory.
The geography makes that coordination increasingly important. This year, new detections have appeared in places where green crabs hadn't previously been found. In the Koeye estuary, a trap placed below the narrows for the first time caught a single green crab high in the intertidal.
It isn't a single line moving steadily north. The invasion is patchy, established in some places for over a decade, barely present in others separated by only a few dozen kilometres of coastline. That's part of why Diana no longer describes Heiltsuk territory simply as the invasion's edge.
"We were that leading edge," she says. "And then now they've shot past us."
Green crabs are now causing problems as far north as Alaska.
"I think we're surrounded."
The small numbers around Calvert Island give the Hakai Institute and its partners a chance to study green crab while the animals are still difficult to find.
Krystal Bachen and Anika Chen prepare passive eDNA samplers, called metaprobes, in the Griswold Lab on Calvert Island. Here, they are placing nylon filters inside 3D-printed housings designed to let water pass through. The metaprobes were attached to green crab traps to test whether they could passively collect crab DNA for comparison with other eDNA methods.
Matt and the Nearshore team are testing several methods side by side: collecting sediment, deploying passive eDNA samplers closer to the bottom, taking subtidal water samples, and setting light traps to see which is most effective at detecting green crabs.
In a parallel, controlled experiment, the team is comparing the strength of the sediment eDNA signal across enclosures holding no, low, medium, and high densities of green crabs, fixing the crab count so they can test, cleanly, whether a stronger signal reliably lines up with a higher crab density. If it does, the tool could eventually let a crew screen dozens of beaches in a day and send traps only where the reading says to look, rather than trapping 20 sites for a week to learn what a morning of sampling might tell them just as well.
There is also an archive waiting to be explored. Monthly samples from KC-10 stretch back years, but only a small fraction have been tested for European green crab. Matt would like to process the full sequence from roughly 2019 through 2026.
Hidden in those bottles could be a genetic history of the invasion, showing when the signal first appeared and whether it strengthened, fluctuated, or followed no clear pattern at all.
The Integrated Coastal Observatory project that originally collected the samples has since wound down, but the archived samples remain and can now be compared with physical crab detections.
At Gale Creek, Heiltsuk are working to drive down a large established population and reduce the possibility that the lagoon becomes a source of larvae spreading elsewhere. At newer sites, the objective is to find green crabs quickly and remove them before they get the same opportunity. Around Calvert Island, nobody yet knows where the crabs sit along that trajectory.
For now, the crabs remain sparse enough that finding them is difficult. A trap can come back empty even when a crab is close enough to crawl across someone's boot. The genetic evidence can be so faint that it amounts to a few molecules in a bottle of seawater collected for an entirely different reason, tested only because someone eventually wondered what else might be in it.
Heiltsuk's experience suggests that faint signals are worth paying attention to.
By the time the evidence becomes obvious, the problem may be much harder to solve.
