08 June, 2021

Restoring Salt Marsh through Human Crab Predation

 In many conservation and restoration projects, ecologists work hard to protect native things or bring native things back to a place. But this last year, Nantucket Conservation Foundation Staff acted as predators and aggressively removed a native crab from a few salt marshes in Polpis Harbor. Our prey? The purple marsh crab (Sesarma reticulatum).

The purple marsh crab (Sesarma reticulatum)

The purple marsh crab is a beautiful crab and native to Nantucket and other New England salt marshes. Unfortunately, purple crab population numbers have spiked in recent years and increased grazing on salt marshes grasses by these crabs has dramatically destroyed salt marshes.

Salt marsh dieback along a creek bank in Polpis Harbor, Nantucket

The term is salt marsh dieback and we’ve written about how devastating these purple marsh crabs can be for salt marsh health and even coastal resilience. With a reduction in natural predators, the purple crab population climbs and as the crabs eat the smooth cordgrass (Spartina alterniflora, the dominate low salt marsh grass), large areas of soil are laid bare with no plants or roots to hold that soil in place. This is risky for salt marsh resilience, causing increase erosion and loss of salt marsh to regular incoming tides and large storm events. And even riskier for coastal resilience as these valuable salt marshes are lost to erosion.

Salt marsh dieback as seen at Medouie Creek on Polpis Harbor, Nantucket (left) alongside a diagram showing the progression of salt marsh dieback and Sesarma impacts on marsh health (right).

While some areas in New England may be seeing a reduction in purple marsh crab populations, any potential recovery comes after over 10 years of extensive dieback and marsh erosion.

On Nantucket we want to jump ahead of this problem and reduce salt marsh dieback while protecting the marsh from erosion now. In 2019, NCF decided to become the predators in two salt marshes in Polpis Harbor, the area on island most dramatically impacted by salt marsh dieback.

Checking crab traps along the edge of salt marsh dieback in Polpis Harbor.

Tennis ball cans, sunk into the soil along the edge of smooth cordgrass dieback, act as crab traps and were placed throughout two marshes. These traps, checked 2-3 times a week, captured 439 purple marsh crabs (see the graph below). As an ecological bonus, we also trapped large numbers of the non-native, invasive European green crabs which were also removed from the marshes.

Total crabs trapped in 2019. Purple marsh crab (purple bars) and Asian green crabs (green bars) from May – October.

We trapped and removed a lot of crabs from the marshes, what does that mean for salt marsh health? Removing the crabs seemed to give the smooth cordgrass a chance to recover. By August, both salt marshes saw regrowth of the grass near the crab traps and even recolonizing in marsh areas bare for the last 5 years!

August 2019 – Smooth cordgrass regrowing in the middle of extensive salt marsh dieback at NCF’s Medouie Creek.

This isn’t a finished project though. Crab trapping will continue at both marshes in 2020 to determine if we really decreased crab populations and/or if they continue to migrate in from other salt marshes. Hopefully continuing to trap crabs will give the smooth cordgrass more of a chance to reestablish and stabilize the creek banks.

In 2020 we hope to plant out smooth cordgrass into the dieback areas to continue jump-starting the recovery process. Combining crab predation and vegetation restoration into the dieback areas, we hope to give the salt marsh a chance to recover and increase longer-term coastal resilience at these sites. We’ve just gotten back out in the marshes the last few weeks and so far – no purple crabs! It’s still early in the season but stay tuned!

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