Earlier this month the Alaska Department of Fish and Game announced the verified expansion of invasive European green crab distribution in Alaska waters after finding 11 molted carapaces on the shore of Bostwick Inlet, Gravina Island on June 13.
First found in Alaskan waters in July 2022, green crab are considered one of the top 100 worst invasive species globally by the International Union for Conservation and Nature due to predatory tendencies and their rampant destruction of eelgrass, a vital component of healthy subtidal and intertidal ecosystems.
According to Fish & Game Invasive Species Program Coordinator Tammy Davis, the recent findings in Bostwick Inlet are not necessarily indicative of green crab spawning in the immediate area, but the discovery does call for further action and surveying efforts.
“It’s not like you can say ‘Oh we found carapaces here, that means green crab are here’ because the currents and the tide have such an impact on what gets washed up during a high tide and flushed up onto the shoreline. It’s not indicative of exactly where the crabs are, so we’ll be setting traps again this year,” she said.
Fish & Game and its partners plan to increase monitoring of Bostwick Inlet through an expansion of their ongoing trapping in the area, with almost 50 traps planned for their next survey.
However – even with trapping efforts and early detection protocol in place all across Southeast Alaska – Davis emphasized that the carapaces in both Bostwick Inlet and Annette Island were found through simply walking the beach.
“That’s one of those techniques that we are asking our monitors to use because it’s much less labor intensive. Lots of Alaskans like to walk the beach – so if you walk a beach area, look for those cast off shells,” she said.
Despite the name, European green crab carapaces are actually more likely to be red, pink, or orange.
According to Davis, the main distinguishing characteristics are the five jagged spines on either side of their eyes and the three bumps in between.
“The five spines are very clear and distinct, and there isn’t a native crab that has those five spines. The other thing is they’re not very big. A large adult is about four inches across,” she said. “If you can’t identify the shells that you’re looking at, take a picture, send it to me, send it to your local ADF&G office, [or] send it to Kachemak Bay NERR or NOAA fisheries, and we can help you identify what those crabs are.”
Expanding green crab populations have the capacity to wreak havoc on coastal ecosystems, making public participation all the more necessary.
Green crabs prey on organisms that live in the nearshore seabed – including eelgrass beds, which Davis said are enormously important for the local marine environment.
“They provide a substrate cover of food for invertebrates and fish, and notably they provide nursery habitat – again, protection and food – for juvenile salmon and other fish, as well as native crabs like Dungeness crab,” she said. “They provide habitat for herring spawn – and then because they’re protected, they are really important for other shellfish, small mussels and other gastropods, crustaceans, and then polychaete worms, which live in the soft sediment of eelgrass beds.”
However – Davis explained that the danger doesn’t exclusively lie in the amount of eelgrass consumed by green crabs, it also lies in how exactly they consume it.
“They don’t just dig up the eelgrass – they do eat the roots of the eelgrass while they’re digging it up – but they sort of clip off the fronts, and they’re basically digging up what’s hidden just beneath the surface of the seabed. They shred the blades and they rip up the seagrass. And as I said it’s such an important part of the nearshore ecosystem.”
Although early detection efforts have been in place in Alaska since 2006, the first discovery in 2022 marked a turning point in how local organizations approached the issue.
Thanks to additional funding from the Pacific States Marine Fisheries Commission, Fish & Game was able to supply trapping equipment to communities interested in monitoring invasive species.
“We’ve got new monitors in Ketchikan, Hoonah, Kasaan, Hydaburg, Klawock, Craig, Sitka, to name a few… We [also] planned and hosted a workshop in April 2023 in Ketchikan and in Metlakatla,” reported Davis.
“The idea was, let’s bring together entities in Southern Southeast [and] let’s provide information about green crab: what their life history is, their biology, why we’re so concerned about the fact that they’re now here in Alaska.”
Additionally – in late 2022 through early 2023 – a group of representatives from ADF&G, US Fish and Wildlife Service, NOAA Fisheries, Metlakatla, and Kachemak Bay National Estuarine Research Reserve collaborated with contractors funded by the Pacific States Marine Fisheries Commission to write a comprehensive early detection and rapid response plan, providing a framework for communities to respond to various green crab related threats under the standardized Incident Command System.
However – besides increasing monitoring and research efforts – there still is not a clear path for combating the threat green crab bring to coastal ecosystems.
According to ADF&G, the current best practice is to attempt to reduce green crab impacts through intensive trapping called “functional eradication.”
“Eradicating aquatic invasive species in a closed system like a lake that doesn’t have inflow or outflow can be done pretty straightforwardly. Fish and Game has done that for northern pike. We’ve also done eradication of northern pike in open systems where you can kind of close it off and treat an area and then restore the native fish populations and open the system back up,” explained Davis.
“In a marine environment that’s really impractical, right? You can’t really close off the area.”
Furthermore, effective functional eradication requires a complete understanding of the relationship between the density of the invasive population and the ecosystem’s responses to the changes it brings, as well as an understanding of the relationship between the invader’s density and their ability to recolonize.
“The goal of functional eradication is to reduce the population to the point that the negative impacts that that invasive population imposes on the ecosystem are either not disruptive or are minimally disruptive to the ecosystem,” said Davis.
“Right now for Alaska, we don’t have hard evidence on the direct impacts that green crab will have on coastal nearshore ecosystems, but we can expect them to be pretty similar to those that have occurred both on the Atlantic coast and in areas on the West Coast.”
But with so much still up in the air concerning European green crab, Davis again emphasized just how important public involvement and awareness can be in boosting monitoring efforts.
“My number one objective is to raise awareness about this invasive species,” she said.
Davis said if you find a crab that looks suspicious, call the invasive species hotline at 1-877-INVASIV (1-877-468-2748), or send her a picture of that crab at [email protected].