Blue Crab Decline May Herald Salt Marsh Loss

PROVIDENCE, Rhode Island, August 7, 2002 (ENS) - Overharvesting of blue crabs may be triggering a massive die off of salt marshes now in progress across the southeastern United States, warns a new study by two Brown University biologists. The team says without blue crabs to control snail populations, the vegetation that anchors fragile salt marsh habitats is quickly eaten away.


Blue crabs may be crucial to the survival of coastal salt marshes. (Photo 2002 Virginia Institute of Marine Science (VIMS)/College of William and Mary)
In experiments along the Virginia and Georgia coasts, the researchers manipulated local populations of animals and found that when blue crabs disappeared from a salt marsh, one of their chief prey - periwinkle snails - flourished. Once free of predation from blue crabs, the periwinkles ate all of the cordgrass in a marsh.

Cordgrass dominates the southern marsh, anchoring it and providing its animals with habitat. Without the plants to bind sediment and protect wildlife, the salt marsh ecosystem collapses, the scientists found.

The study shows that overgrazing by periwinkle snails will convert a southern salt marsh into a barren mudflat within eight months, said lead scientist Brian Silliman.

"Cut back the blue crab harvest, because even if we're half right, the results of overharvesting could be disastrous," said Mark Bertness, the project's senior researcher. Their findings appear in the current issue of the journal "Proceedings of the National Academy of Sciences."


This healthy stand of cordgrass had a low density of snails. (Two photos by Silliman and Bertness, courtesy Brown University)
Southern salt marshes stretch from Chesapeake Bay to the central Florida coast. They are some of the most productive grasslands in the world, acting as nurseries for commercially important fish and other species.

The marshes also temper coastal flooding, filter polluted mainland runoff, and protect barrier islands from erosion. In turn, barrier islands buffer the shorelines from storm driven waves.

Hundreds of miles of southern salt marshes have died in recent years, particularly in Louisiana and Florida. Silliman and Bertness surveyed several of those dead and dying marshes and found very high densities of periwinkles.

"Blue crab populations have been in rapid decline due to overharvesting," Bertness said. The strong effects shown in these experiments suggest that southern marshes may already be suffering the consequences, said the authors.

For more than 50 years, ecologists assumed that the half to three-quarter inch long black or gray periwinkles ate only dead and dying plant materials in southern salt marshes. But Silliman and Bertness found that unchecked populations of the snail were happy to switch to eating living cordgrass.


This study plot has very little cordgrass - but lots of periwinkle snails.
The greater the nitrogen content of the grass, the more attractive the grass became to the periwinkles. Nitrogen is the prime nutrient in mainland runoff, carried in sewer overflows, agricultural wastes and runoff from fertilized lawns.

Marshes can choke to death through a process called eutrophication, which begins with the influx of too many nutrients in runoff water. The nutrients cause excessive aquatic plant growth. As these plants die, the bacteria feeding on the dead plants consume oxygen, eventually suffocating fish and other marine species.

"This research further suggests that human population growth and eutrophication may accelerate marsh dieoffs that are driven by snails consuming the cordgrass," Bertness said.

For decades, the prevailing model of marsh ecology was that bottom up forces, such as currents and nutrient flow, were the primary governors of plant productivity. The current study suggests, however, that a top down process - the control of grazers such as snails by consumers such as crabs - determines the growth of marsh grass.

Silliman and Bertness call this top down phenomenon a "trophic cascade."


Measuring a blue crab. Harvesters take a large portion of the adult crab population each year. (Photo courtesy Maryland Sea Grant)
"The discovery of this simple trophic cascade implies that overharvesting of snail predators, such as blue crabs, may be an important factor contributing to the massive die off of salt marshes across the southeastern United States," wrote the authors.

Their results contribute to a growing body of evidence that similar top down processes may regulate production in other saltwater ecosystems, including those dominated by kelps, other sea grasses or intertidal algae, they said.

In the Chesapeake Bay, North America's largest and most biologically diverse estuary, regional leaders have begun taking steps that could reduce the problems facing the Bay's salt marshes and blue crabs. More than 15.7 million people live in the Chesapeake Bay watershed, which also provides a home for more than 3,600 species of plants, fish and animals.

Cleanup of the Bay began in 1983 with the signing of the first Chesapeake Bay Agreement. Over the past 18 years nitrogen and phosphorus loadings have been reduced, bay grasses are recovering, and striped bass were saved from brink of extinction and restored to record levels.

Last fall, Maryland and Virginia leaders, and the Potomac River Fisheries Commission, agreed to a plan that would reduce the blue crab harvest over a three year period. But Maryland state officials, worried about the continuing decline in blue crab populations, have implemented regulations that aim to achieve a 15 percent reduction in the blue crab harvest in 2002 - one year before deadlines set by the agreement.


The Chesapeake Bay has lost much of its sea grass, but regional leaders are working to help salt marsh habitats make a comeback. (Photo courtesy U.S. Fish and Wildlife Service)
In December 2001, leaders signed three regional pacts designed to encourage regional planning, storm water management and low impact development in the Chesapeake Bay watershed. The agreements are designed to reduce the influx of polluted runoff into the Bay.

Washington DC Mayor Anthony Williams, who chairs the Chesapeake Executive Council, said the region is setting a national standard for ecosystem restoration.

"The challenges are significant, but the Chesapeake Bay Program is innovative and comprehensive," Williams said. "I am especially proud that this year the Bay Program established an Environmental Justice Task Force, integrating environmental revitalization with neighborhood revitalization and demonstrating real commitment to sustainable development - that which protects our environment, strengthens our economy and ensures social justice."