Walk the beach long enough and you’ll see big balls of peat, festooned with a hair cut of cordgrass, washed up above the high water line, apparently uprooted to drift away from their starting point. This is a symptom of marsh die back, a phenomenon that scientists say is due to a break in the delicate equilibrium in marsh ecology.
The culprit is a small crab, the sesarma, or as we called them, Fiddler Crabs. These crabs live in burrows around the high water line and can be found in Cotuit in great numbers inside of Cupid’s Cove on Sampson’s Island and around the mouth of Little River. As kids we’d dig them up and set them on each other in gladiator battles in “coliseums” dug out of the sand.
Now scientists at Brown University are reporting that swathes of saltwater marsh are being killed off because of the negative effects of recreational fishing. They observed that marshes with dying cordgrass were not being affected by the foot traffic of anglers, but by the effects the fishing had on the sesarma’s natural predators, primarily striped bass. Saltwater flyfishermen have long known that flies patterned after small crabs can be used on stripers with deadly effect. Well, now it appears that if the striper population is reduced, the crab population thrives, and over time the crabs overfeed on the grass, and … well, the consequences are shown in the National Park Services photo below.
“With far fewer predators in areas where recreational fishing is prevalent, native Sesarma crabs have had relatively free rein to eat salt marsh grasses, causing the ecosystem to collapse, said Mark Bertness, chair of Brown’s Department of Ecology and Evolutionary Biology and the paper’s senior author. He led a series of experiments and measurements published online in the journal Ecology that he said unavoidably implicate recreational fishing in marsh die-off.
“We had to be so careful about dotting all the ‘i’s and crossing all the ‘t’s and making sure that we had ruled out all alternative hypotheses, because even within the scientific community, there are plenty of fishermen who don’t want this to be true,” Bertness said. “Certainly out in the general public there are plenty of people who are into recreational fishing who don’t want it to be the problem.”
I plead guilty to marsh fishing — I’ve done it on occasion in the past — the streams and muddy creeks up high in the marsh system can hold some great fish, and the surroundings can be beautiful and very peaceful. As well as buggy, muddy, and treacherous in the dark.
The condition of the marshes in and around Cotuit seems pretty healthy — there’s a large one at the head of the Mills River near Prince’s Cove, another around the Cow Yard in the Narrows, a third surrounding the “delta” of Little River, one by the town way to water at the confluence of Oceanview and Main. None see a lot of fishing traffic. Parking is tight and access limited.
Keeping the marshes and peat banks healthy is crucial to the health of an estuary. I’ve heard them described as the “lungs” of the system, filtering and absorbing a great deal of pollution and storm surge from the main shoreline.
Here are some links:
- National Park Service on New England die-back sites.
- New York Times blog post on the recent study.
- Phys.org story on the Brown study.