Study Finds Crabs Destroying Coastal Saltmarshes In Rhode Island And Cape Cod

Apr 29, 2014

Hungry crabs eating grasses in coastal saltmarshes in Rhode Island and Cape Cod are behind the rapid die-offs of these salt marshes, according to a research team at Brown University.

Sesarma crabs have been decimating cordgrass in coastal salt marshes in Rhode Island and Cape Cod. Pictured here is a male crab.
Credit Photo Courtesy of Mark Bertness / Brown University

Crabs have been free to munch away at cordgrass, causing the death of coastal saltmarshes in southern New England, said Mark Bertness, a biology professor who leads that research team.

“We were able to show that it was intensive recreational fishing near every place you had the infrastructure for fishing that was depleting the populations of predacious fish, shellfish and fin fish," said Bertness. As a result, the disrupted food web in the saltmarshes left no checks and balances for the Sesarma crabs.

“This is not a crab just running around the surface," said Bertness. "This is a burrowing crab. It lives in communal burrows. And once these burrow complexes get going, they can just grow in expanse and cover the edge of entire marshes until the marshes actually erode away."

Salt marsh die-offs weren't reported in Cape Cod until 2004, said Bertness. But his team did historical reconstructions of saltmarshes from archived aerial photographs since the 1930s, and found that the die-offs actually began in the late 1970s, early 1980s. 

"Once we figured out that the die-offs were going on for a long time, we saw a really tight correlation between marshes where you had development going on, [such as] houses with boat docks, boat launches, or marinas, or anything where you had recreational fishing infrastructure, and protected areas," said Bertness. "In those [protected] areas, there was no history of die-offs. This was only happening in marshes where you had development."

Bertness' research team ruled out other hypothesis about what was causing the saltmarsh deaths.

In Rhode Island, those die-offs started to happen recently and quickly. Bertness said he and his team couldn't find any appropriate sites to study in Narragansett Bay as early as in 2010. "But over the last couple of years, marsh die-offs moved into Narragansett Bay with a vengeance," said Bertness. "It's gone, over the last six to seven years, from very little die-off, just little spots of die-offs about half the size of your desk, to entire marshes."

Professor Mark Bertness said with reduced numbers of predatory fish, Sesarma crabs destroy saltmarshes. "The lower part of the marsh is gone and it looks like Swiss cheese, and those are all burrows." The crabs live in communes in these burrows, according to Bertness
Credit Photo Courtesy of Mark Bertness / Brown University

Bertness said managing these die offs is critical, so that saltmarshes can continue to play important roles in our environment, including as storm buffers, carbon sequesters, nurseries for birds, and water quality filters.

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