The Silent Threat to Pacific Salmon: European Green Crabs

The Silent Threat to Pacific Salmon: European Green Crabs

Pacific salmon currently face an ever-growing list of existential threats, including a rapidly changing climate and widespread habitat destruction. To make matters worse, a silent new threat has been crawling its way up the Pacific coast, one with the potential to impact not only salmon but the ecosystem at large. This new threat: European green crabs.

What Are European Green Crabs?

As their name suggests, green crabs are native to the European coast, occupying shallow water with an abundance of plants and mollusks to eat. They’re small—with adults usually only around 4 inches wide, and weighing only 2.5 to 3 ounces—tiny by comparison to the blue, rock, or Dungeness crabs American fishermen are familiar with.

By hitching a ride in the ballast water of merchant ships traveling from Europe in the 1800s, they first established themselves on the U.S. Atlantic coast, only to again hitchhike their way to the west coast, where they have become firmly established. From there they dispersed northwards, through sea currents and fishing vessels starting in their original invasion point in San Francisco in 1989, into Oregon, Washington, and British Columbia, where they are now firmly established. They’ve even made it into Alaskan waters. Throughout this range, they concentrate their populations in eelgrass beds, consuming smaller crustaceans, mollusks, small fish, and aquatic plants at a voracious pace.

Green crabs breed incredibly fast, laying up to 185,000 eggs in a single clutch, and can reproduce after just a couple of years of growth. They even have a superpower that’s never been documented in other crabs; they can feed through their gills. Similar to how mussels or clams feed, green crabs can filter water through their gills to absorb essential nutrients, meaning that even when food is scarce, they can passively sustain themselves longer than any other crab.

In their native range, several fish, bird, and larger decapod species predate on green crabs, but in their new Pacific coast range, they have no major natural predators. Instead, they have been documented predating on all manner of fish or invertebrates throughout their life cycle.

What Threats Do They Pose?

Their threat to salmon is twofold. Firstly, they eat salmon smolt as they come down their birth river and enter the ocean. During this time, smolts are vulnerable to predation from larger salmonids, sea lions, and birds, and use coastal eelgrass beds as shelter—the same place green crabs tend to gather.

Secondly, green crabs have a habit of ripping up those same eelgrass beds, removing plants to reach the palatable below-ground rhizomes. This destroys the salmon smolts’ hiding place, making them open for predation by not just green crabs, but all of their natural predators.

Coast Restore, a non-profit partnering with the Ahousaht, Tla-o-qui-aht and T’Sou-ke First Nations to restore coastal ecosystems, has seen this first-hand.

“Green Crabs are ‘ecosystem engineers,’ altering environments to meet their needs,” the non-profit states. “Resulting in the destruction of native eelgrass beds, juvenile clam beds, and prime wild salmon rearing habitat. The damage to commercial fisheries, cultural keystone food sources, economic opportunities and healthy habitat is immense and long-lasting.”

The green crab’s impact on the Pacific Northwest extends well beyond salmon. Because of their impressive population growth rate, they have been documented eating copious amounts of native sea snails and mollusks, as well as out-competing native crabs like Dungeness or rock crabs. They’ve even been documented eating juvenile king crabs. They’re also seen as a threat to commercial shellfish farmers, eating the young oysters and mussels well before they can reach harvestable size.

How Do We Solve the Issue?

Despite being significantly smaller than other crab species, European green crabs still have a place on our menus. In Venice, they’re considered a delicacy, consumed in a variety of dishes, including fried crab, crab bisque, stock, and soft-shelled crab. Even the roe of pregnant females, referred to as Masinette, is a delicacy in Venice and is often eaten right out of the shell.

GreenCrab.Org, a non-profit dedicated to utilizing this introduced species, says, “Despite often being considered too small to pick for meat, green crabs have a rich flavor that’s gotten the attention of chefs for use in stocks, sauces, emulsions, and seasonings. Others look towards the crab’s bright orange roe, which tastes similar to blue crab roe, and is seasonally available in the fall and spring.” Distilleries on the east coast have also attempted to use the crabs to flavor small batches of whisky with reasonable success, but boutique whiskey alone isn’t enough to support an entire harvest.

Several states have attempted to transfer these eating habits over to the U.S. to encourage the harvest of as many crabs as possible. Excluding Washington state, which outlawed their recreational harvest and sale due to fears of accidental spread by fishermen, Pacific fisheries departments allow liberal or unlimited recreational green-crab harvest. In Oregon, the daily bag limit increased from 10 to 35 per person per day, and in California, British Columbia, and Alaska, unlimited numbers of crabs can be harvested by anyone with a fishing license. So far, commercial harvest is concentrated around the east coast, but it is an option that west coast agencies and fishermen agencies are considering.

Green crabs are harvested with the same gear used on Dungeness or rock crabs but can be harvested in much larger volumes. Just last year, the Washington Department of Fisheries and partners removed over a quarter million crabs from eelgrass beds. However, these high-volume harvests could have unintended consequences. In 2014, when Californian researchers removed almost all the breeding adult crabs from Seadrift Lagoon, there was a corresponding boom in crab numbers, from 90,000 to over 300,000 individuals. It turned out that adult green crabs cannibalize their young, as a form of self-inflicted population control. When breeding adults were removed, it left an unchecked population of juvenile crabs. Hence, any future efforts need to remove both adult and juvenile crabs to be effective.

One of those efforts could be through the food industry. Most soft-shelled crabs sold to restaurants or supermarkets in the U.S. are blue crabs, so the potential of switching invasive green crabs for native blue crabs could be a welcome break for the latter, especially in areas where their populations are suffering.

By all accounts, green crabs are here to stay. They reproduce too fast and are too adaptable to ever be completely eliminated. In the long-term, encouraging recreational and commercial harvests could help fund widespread population-reduction efforts, not only protecting salmon smolt and other wildlife but also injecting money into local economies. Plus, learning how to harvest and cook these invasive crustaceans presents a unique opportunity for anglers to utilize an endless bounty of wild food.

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