Claws for concern: The green crab epidemic threatening Washington clams

As Washington's coastal habitats face increasing pressure, scientists are intensifying their research into the European green crab, an invasive species wreaking havoc on local ecosystems.

Their focus is on understanding how these crabs contribute to the decline in Manila clam harvests, shedding light on the broader implications for the region's marine environment.

A newly published study has shed light on a crucial piece of the puzzle regarding Washington’s coastal ecosystem.

“The green crab represents a substantial and growing threat in our state figuring out what these impacts are, and getting ahead of them is really important,” said Emily Grason, a marine ecologist and the crab team program manager for Washington Sea Grant.

The study, by University of Washington and Washington Sea Grant, examined the diet of green crabs to assess their impact on declining Manila clam harvests.

Researchers have found that green crab populations in Washington’s coastal estuaries are about 50% higher in 2024 compared to 2023, even when accounting for increased monitoring efforts.

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This surge aligns with expectations given the recent El Niño (ENSO) event, which brought warmer winter conditions likely beneficial to green crabs. In contrast, green crab numbers along Salish Sea shorelines have remained relatively stable from 2023 to 2024.

Researchers said the data shows a significant drop in green crab numbers from 2022 to 2023 at only one of four collection sites in Willapa Bay, Washington. The decrease at the Stackpole site is likely due to intensive trapping efforts at that site starting in 2022.

While this reduction highlights the effectiveness of targeted trapping, it complicates using this site’s data to represent green crab populations across Willapa Bay, Grason added.

Prior to 2022, Stackpole's data aligned well with broader observations across the estuary, but the recent shift suggests that this site alone may not fully reflect overall trends in the bay.

Since 2020, shellfish growers in Willapa Bay have reported a troubling decline in Manila clam harvests. Coinciding with this decline has been a noticeable increase in European Green crab populations.

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These invasive crabs reproduce rapidly and pose a significant threat to local ecosystems. They damage eelgrass beds, which are crucial for protecting coastlines and reducing carbon dioxide levels in the water.

“[The study] tells us that larger male crabs do sometimes eat Manila clams, but probably not frequently enough for us to be able to point a finger yet at green crab as the only culprit in clam harvest declines,” explained Mary Fisher, the lead author of the diet DNA study now at the University of California Davis.

The findings indicate that while green crabs do consume Manila clams on occasion, more research is needed to fully understand their impact on clam populations.

Grason believes effective management of green crabs requires a combination of strong, strategic human efforts and favorable natural conditions.

Intensive local trapping can reduce green crab populations at specific sites, protecting valuable resources like shellfish and eelgrass beds. But long-term success depends on regionwide strategies that target key populations, which significantly influence overall spread.

While local predators such as other crabs, fish, otters, and gulls might help limit green crab habitat use, they are unlikely to solve the problem entirely.

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The Washington Department of Fish and Wildlife (WDFW) is not currently considering a recreational or commercial fishery for green crabs due to the invasion's early stage, but this could change if deemed beneficial in the future.

EDITOR'S NOTE: This story was changed on Tuesday, July 30, to correct Mary Fisher's affiliation from UC Berkeley to UC Davis.