Invasive Species Make Us Crabby

A highly invasive kind of crab has been discovered in Metlakatla and it’s now all hands on-deck at HIA to respond to that threat. The Hoonah Stewardship Council met in September to discuss some of the dangers the crabs pose and how we’re planning on handling them, but given the magnitude of the danger, we wanted to clarify a bit more about what’s underway at the environmental office and how everyone in Hoonah can play a role in responding to invasive species.

An Enterprising Invader

European Green Crabs are ranked as one of the top one hundred worst invasive species because of their ability to seek out new habitats and obliterate them. While they’re known by many names, in French they’re called “le crabe enragé,” or enraged crabs, because of the unending havoc they wreak on nearshore ecosystems. One of the reasons green crab pose such a threat is because they’re an astoundingly hearty species and can adapt to live almost anywhere. While Dungees are able to live in waters with surface temperatures between 38 and 65 F, green crab can survive between 32 and 95 F, which gives them both a larger current potential range and a clear advantage when it comes to surviving climate change. They are also more tolerant of varying salinities and lower dissolved oxygen levels. While we talk a lot about human adaptation for the climate crisis and how we can adapt ourselves for a changing world, it’s a bit of a problem when our enemies can do the same

IGAP Manager Jeromy Grant deploys the first round of minnow traps in Hoonah Harbor. Since green crabs are much smaller than dungees, they require smaller traps but no less putrid of bait!

Green crabs are voracious predators, eating up to 40 juvenile clams and mussels per day. This unsurprisingly has disastrous effects on local populations, causing a clam calamity that can reduce bivalve biomass by up to 50 percent in just a few years. Perhaps the most important effect though is on eelgrass, which green crab shred while they’re burrowing for clams. Eelgrass is a vital part of the near-shore ecosystem, helping to stabilize sediment to prevent erosion and provides a nursery habitat for salmon fry that have departed from their natal streams. Eelgrass is also a key player in the fight against ocean acidification, as oceans absorb about a quarter of the carbon released by humans every year, and marine plants can fix it about three times more efficiently than trees. According to Fisheries and Oceans Canada (the federal branch equivalent to NOAA for our hockey-loving cousins), “because green crab has the ability to modify entire ecosystems, it is considered an ‘ecosystem engineer,’” which is a bit of a problem when the engineering works to directly dismantle the ecosystems we rely on.

Green crabs have spent the last 33 years slowly marching north; after being introduced in San Francisco in 1989, they’ve made it 1266 miles thus far. While they were only discovered in Metlakatla earlier this year, the shell sizes indicate they’ve most likely been there for three or four years already, and the numbers in recent days have been spiking. Earlier this summer they maxed out at 13 crabs a day, but just a few weeks ago had nearly five times that, which highlights the importance of hitting the problem hard as soon as it’s discovered. If they continue their northerly journey at the same rate they’ve had so far, we should expect to see them in Hoonah in a couple of years. By already having monitoring programs in place and knowing what to keep an eye out for, we’ll be ahead of the game when they do arrive.

Welcome to the Resistance

Members of NOAA and the Metlakatla Indian Community (MIC) Department of Fish and Wildlife conduct a shore survey in a traditional salmon trap in Tamgass Harbor in the Annette Island Reserve. The shallow water and marine plants make an almost ideal green crab habitat. Photo courtesy of NOAA/Barb Lake.

HIA has been developing a multi-pronged approaching to dealing with our potentially crabby neighbors. The first prong is by looking for larval green crabs using a light trap in Hoonah Harbor. Larval crabs spend several months floating through the water before settling down to the bottom, and during that time period they’re attracted to light like moths to a flame. We’re exploiting this behavior by using a light trap in Hoonah Harbor to collect larvae that should give us the earliest warning possible if green crabs decide Hoonah’s a desirable place to live (and really, who could blame them for that decision?). Second, we’re conducting shore surveys to look for crab shells washed up on the beach. Somewhat ironically, Metlakatla discovered their first green crab shell right behind one of their outreach posters, where it had been blown into tall grass by a recent storm. Green crab particularly like to live in intertidal marine vegetation, so next time you’re out harvesting cockles or beach asparagus, keep an eye out for unusual crabs and give us a shout if you find anything! Finally, we’ve recently started deploying traps around town. Since green crabs are much smaller than Dungees, we’re using modified minnow traps and the world’s smelliest herring to lure them in. In the next few months we’re hoping to find funding to expand our trapping program and cover sites throughout Port Fredrick, but as always the community remains our best tool for discovering environmental concerns, so please let us know if you find anything concerning! There are no native crabs in Alaska with five spines on either side of their shells, so count the spines!

Green crabs aren’t the only thing that ends up in the traps! Studying the bycatch can be another useful data set when understanding coastal waters and the impacts of the climate crisis. Photo courtesy of SHI Intern Natalie Bennett.

We’re also not going at it alone. Communities up and down the west coast are working together to understand and combat European Green Crabs and HIA is helping to get those collaborations off the ground in Alaska. The Southeast Alaska Tribal Ocean Research consortium (SEATOR), who is in charge of the shellfish and phytoplankton sampling programs in Hoonah, is now working with HIA to develop protocols and standards that we hope will spread to other communities throughout Southeast. Perhaps the most valuable partner we’ve worked with so far has been the Metlakatla Indian Community (MIC) who discovered the first green crabs in Alaska and has been another prime example of tribes leading the way on environmental work. While this is an alarming new threat to Hoonah, we are standing on the shoulders of giants, and are hoping to give a hand up to any other communities that may need it.

While green crab coming to Hoonah would lead to a whole host of repercussions, one small advantage is the opportunities it is providing for Hoonah youth to play an active role in environmental stewardship. A group of highschoolers from the Canoe Forest Science class, which seeks to combine traditional ecological knowledge and western science when studying environmental problems, joined our office to conduct shore surveys and learn more about the dangers the invasive crabs pose towards Hoonah. Students with the Rural Alaskan Students in One Health Research (RASOR) program are also looking to get involved with the protocol development part, testing out which types of traps and bait are most effective and comparing bycatch across various sections of the Hoonah beach front.

Map of green crab capture results in northern Washington near Seattle. While places like the Lummi Sea Pond (a tribally-owned artificial lagoon to support a salmon and shellfish production) have been heavily hit by green crab, they haven’t yet made it to some of the southern reaches of the bay area. Photo courtesy of the Washington Sea Grant.

Another ray of hope is that while the green crabs have a very wide range, it’s also with pretty patchy coverage. After the Lummi nation caught over 70,000 green crabs in a single year, Washington declared a state of emergency in the area to help combat the problem, but despite the scale of the issue in the lagoon, they haven’t yet detected green crabs in the southern-most reaches of Puget Sound. This provides hope that even if green crabs manage to travel as far north as Chichagof Island, they may not enter Port Fredrick.

In many ways though, green crabs arriving in Hoonah is almost inevitable. They have steadily marched up the west coast for the last 33 years nearly entirely unimpeded, but that doesn’t mean that we can’t react quickly and resolutely when they do arrive at our doors. While our current trapping work is just looking to detect the presence of green crabs, if we find any we’ll work with State and federal partners to set as many traps as physically possible and obliterate any invaders. With local monitoring, an aggressive trapping regime and many hands coming together to make light work, we will not go gentle into that good night.

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