An insidious, invasive crab is multiplying in numbers on the southern shores of Annette Island.
As of Friday, Aug. 11, Metlakatla Indian Community teams have recovered 1,622 invasive green crabs from Tamgas Harbor, a large, open bight in the southern end of the island, as well as from Muskeg Beach just outside and west of Tamgas.
The invasive green crab is a destructive predator that can change and degrade habitat and threaten native species. The crab adapts well to most ecosystems, and has boomed on the coast of Oregon, Washington and British Columbia in recent years, tearing up essential fish habitat such as eelgrass and devouring clam beds while moving ever-farther north. The crabs spread from their native Europe to North America in the 1800s by riding in the ballast water of cargo ships.
A student intern along with a team of scientists in July 2022 discovered the first evidence of invasive European green crabs in Alaska while surveying crab carapace shells on a beach near Metlakatla.
Genelle Winter, the invasive species coordinator for Metlakatla Indian Community, said a crew last week recovered a record-high 87 green crabs in just one day’s work, during a low tide cycle that allowed the crew to set traps in more areas of the shallow estuarine beach at Tamgas Harbor.
The crew has been catching about 50 or 60 crabs each day since.
Winter said the crew this summer has recovered 10 gravid females — pregnant crabs — carrying a sac of eggs that turn from bright orange to dark brown as they develop.
“We’ve been able to collect females with a full range of color of eggs,” Winter said. “It indicates that they are reproducing.”
The green crab recovery crew and other Metlakatla community members are finding “many juveniles that are really small,” Winter said. “What that potential indicates is that we have multiple generations and age classes of green crabs living here.”
Once they hatch from their eggs and become larvae, green crabs can spread to new areas by traveling through currents and settling in favorable habitat. It’s likely that invasive green crabs are already cruising shores near Ketchikan, which is just 15 miles east of Annette Island.
Emily Grason, a marine ecologist and the crab team leader for Washington Sea Grant, told the Daily News that while this invasive species has only been found in Alaska off Annette Island to date, the population probably has spread to more shores in Southeast Alaska.
Grason has studied invasive green crabs for about a decade as she works to control their numbers on the Pacific Northwest coast.
Grason said that people who spend time on the beach can be key to early detection of green crabs in new areas once they learn to identify the invasive crab by counting five spines on either side of its eyes, and three rounded lobes between its eyes. The crabs can be green, dark brown, yellow, white, red or mottled.
“Getting peoples’ eyes out on the beaches is going to be the best way to find those populations as quickly as possible before they get uncontrollably large,” Grason said.
She described how in the past, volunteers monitoring for green crabs have discovered the invasive spread to a new area. “They describe this emotional rollercoaster of finding their first green crab,” Grason said. “They’ve been training and working so hard for it so it’s exhilarating in one sense, and the other is like ‘aw, man, this is really sad, kind of heartbreaking.’ It’s in many cases their favorite place that they spend a lot of time caring for and now it’s been sort of realized that it also faces this additional threat.”
“We really don’t want the European green crabs in our waters and we’re hoping that we can work really hard to protect shorelines from them,” Grason said.
Grason said that when she visited Metlakatla with a workshop group in April, she observed at least three age classes of green crabs present at Tamgas Harbor, which is “a relatively oceanographically open area, meaning that there’s a large influx of water on every tide, in and out.”
“It is possible that green crabs arrived several years ago in the Tamgas area and started building a local population; it is also possible that those multiple age classes could have come from successive waves of larvae coming from the outside,” Grason said.
She said that finding pregnant female crabs at Tamgas indicates that the local crab population is fairly large.
“Catching gravid females is a relatively rare event in baited traps,” Grason said. “Females tend to really hunker down and not necessarily take risks required to enter a trap. It’s not impossible, it’s just rare, and what it tells us is that the population is fairly large.”
She explained, “Green crabs spend roughly 30, even up to 75, days in the plankton getting washed around, and they can go very, very far during that period.”
The crabs can begin reproducing during their first year of life. The crabs can live for about six years; mature green crabs are about four inches across their carapace or top shell.
Crab trapping data does not capture the full picture of a green crab population because trapping is geared toward catching as many crabs as possible to help quelch a local invasion. The Metlakatla Indian Community is expanding its trapping efforts, so it’s likely that the catch rate will pick up in coming months.
Dustin Winter, director of the community’s Department of Fish and Wildlife, said the effort added 160 more traps last week, with 40 more to arrive soon. The traps are baited with herring.
Trapping is “what we’re doing and what we’re going to continue to do,” Winter said.
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