Enchanted with Baby Crabs
It takes a community to raise a larval Dungeness crab monitoring network.
Catching crustaceans for science might sound like a childhood dream come true for countless ocean-enamored people. For a host of volunteers across the Salish Sea this past spring and summer, this aspiration was a near-daily routine—but this time around, their targets were even smaller than the familiar shore crabs found during youthful beachcombing. This group was out to catch baby Dungeness crabs.
In 2022, community groups throughout the Salish Sea answered the Hakai Institute’s call to join a new project to study where and when Dungeness crab larvae arrive during the warmer months of the year, as part of an initiative called Sentinels of Change*. The project uses illuminated traps, devices that sit just below the ocean’s surface and light up at night, to attract and trap light-loving planktonic creatures—including Dungeness megalopae, the crabs’ last larval stage before they stop swimming and settle down as juvenile crabs on the seafloor.
“People are enchanted with the Dungeness megalopae,” says Amanda Zielinski, a volunteer who led a light trap station on Hornby Island through her business, Hornby Island Diving, in collaboration with the Hornby Island Conservancy and the island’s Natural History Centre.
The Hornby Island light trap was one of 20 stations scattered across the Salish Sea north of the Canada-United States border. Community groups throughout the region adopted light traps, forming a network that extended from Surge Narrows on Read Island in the north to Saturna Island in the south, and dotted the shores of southeastern Vancouver Island, the Sunshine Coast, and the lower mainland. From April to August, each group stewarded a trap—usually at a dock or pier—and recorded their catch every couple of days to track the larval Dungeness’ arrival and abundance in nearshore waters.
“It was a big project to take on,” says Heather Earle, who spearheaded the light trap network’s first year with co-lead Matt Whalen. “There’s a big learning curve, there’s equipment that you have to figure out, and identifying things that you catch. We all learned so much together!”
Once caught, the light-loving creatures hung out in the trap—a DIY-style contraption constructed from a water jug, funnels, a light, and a battery—until a volunteer reeled it in, recorded the number and size of baby Dungeness, then released their catch back into the ocean. First photo: Heather Earle and Matt Whalen demonstrate a light trap check. Photo by Don Butts. Second photo: Heather Earle and Saturna Ecological Education Centre students examine a catch. Photo by Kelly Fretwell
At dusk on a mid-April evening, the network of traps turned on for the first time, and the volunteers diligently began checking their catch—and waited. Each collection day brought a thrill of surprise as they reeled in the trap and unscrewed its collecting bottle—what light-loving creatures would they find in the trap this time, and would there be any Dungeness?
“People loved the seasonal change of seeing different organisms as they came along and meeting plankton in a way that you don’t as a snorkeler or diver,” Zielinski says.
Finally, after weeks of catching baby fish, worms, and other crustaceans, the tiny target crabs arrived in the second week of May.
“You have to start before the season, so you’re obviously not going to see them right away,” says Zielinski, noting that even an absence of the crabs is still valuable information. “When they finally showed up, it was very exciting!”
Light-trappers who wanted to know more about the other marine critters they caught added photos to an iNaturalist project, where another community—this time one of naturalists throughout the world—continues to help identify their non-Dungeness finds. Photos by Heather Earle (sailfin sculpin), Katie Kushneryk (krill), and Mike Moore (opalescent squid)
Dungeness crabs appeared first in traps set in Winter Cove on Saturna Island, where high school students at the Saturna Ecological Education Centre stewarded the trap, and in Retreat Cove on Galiano Island, helmed by the Galiano Conservancy Association. Other stations quickly followed.
“We saw this pattern of megalopae first showing up in the Southern Gulf Islands, potentially coming in from the outer coast through the Juan de Fuca Strait,” says Earle. “Then they seemed to work their way around the Salish Sea, almost clockwise, ending in Vancouver and Horseshoe Bay.”
It’s still early days for the project, too early to draw many conclusions from this first year of tracking Dungeness megalopae. Still, Earle is intrigued by the data gathered by the network of light trap community scientists this first year and how it compares to data collected by a complementary monitoring project in Puget Sound run by partners at the Pacific Northwest Crab Research Group (PCRG).
“Horseshoe Bay got the most in a day at almost 1,600, and Hope Bay on Pender Island got the most over the season at just over 5,000,” says Earle. “It sounds like a lot, but my sense is those numbers were a bit low. We don’t know yet if that’s going to be typical for us.”
Lending weight to this sense are results from south of the border, where the PCRG traps have at times caught as many megalopae in a single trap in one night as the Sentinels network caught the whole 2022 season. But the group has assured Earle that their results can also vary a lot.
What isn’t too soon to tell, however, is the network’s enthusiasm and dedication. The thrill of seeing what the trap caught fueled some volunteers to steward a station all by themselves. Others at shared stations still didn’t miss a day’s check.
“At the end, everyone said, ‘We want to do this next year. It was really positive. We loved it. It brought us closer to the marine environment and closer as a group,’” says Earle.
“If anything, my group wanted to expand on it!” agrees Zielinski. “We appreciated that it’s part of a bigger regional and international project—that was very exciting, for people to feel part of something larger than themselves.”
The light traps have been pulled from the water and stored for winter, but come next spring these ocean stewards will be full of anticipation as they launch their traps again, eager to see what they might find.
* Sentinels of Change is an initiative driven by communities and scientists to investigate patterns of invertebrate biodiversity, change, and resilience across the coast of British Columbia.