Blue crabs are running out of room.

The crabs overall are moving farther and farther upstream. Those are the early findings of an ongoing study of blue crabs in the ACE Basin.

The findings at least partially explain the recent drop in crab harvests and survey samples.

They're migrating partly because the brackish marsh estuaries they prefer have become saltier in the past decade or so. Their dilemma is one more sign that the coastal marsh -- whether from drought, sea rise or urban water withdrawals -- is vanishing. With it could go a lot of the local seafood.

The study by Michael Childress, a Clemson University assistant biological science professor, seeks to explain why the annual commercial blue crab catch since 1998 has dropped from more than 7 million pounds to 4 million pounds.

The blue crab is the delicacy of coastal living. The abrupt nosedive of numbers from 1998-2002 had commercial and recreational crabbers squabbling and led to state limits on licenses for commercial crabbers. It was blamed largely on too many people eating too many crabs.

But studies found viruses, bacteria and pollution exacerbated the decline; the five-year drought at that time depleted stream flow, aggravating an inward creep of saltwater that was already under way. The decline ended when rainfall recovered, but the crab numbers did not recover, remaining at about 4 million per year.

Salinity has risen in commercial crabbing waters from 24 parts per million to 30 parts per million, also since 1998, according to a state Department of Natural Resources sampling.

The salinity rise "is something we're seeing everywhere we look," said Larry DeLancey, Natural Resources crustacean monitoring program supervisor.

The saltwater intrusion at least partly explains the drop in harvest because the crabs' movement would put more of the population beyond the traditional saltwater demarcation line that is the commercial harvest boundary. It raises even more concern for maintaining the species because it has forced more crabbers to compete in narrower water bodies.

Eight of every 10 commercial pots, or traps, are now set within two miles of the salt line during the September prime harvest season, Childress said. The moving crabs must run "a real gantlet of traps."

"We have no choice but to crowd in," said James Island crabber Fred Dockery, who works the Stono and Kiawah rivers. "There's a dividing point and it does seem to be farther and farther upstream before you stumble into crabs."

Dockery also has noticed that the crabbing waters, which used to darken with silt after heavy rains, no longer do so.

"It's not flooding into the rivers like it used to, not where I am," he said.

Along with crabs, DNR biologists are starting to see shrimp and horseshoe crabs farther upstream. Childress' study is finding the crabs that remain in the saltier water appear to be more likely to get a lethal, saltwater-based virus, based on sampling.

Meanwhile, other testing has shown that stormwater pollution is worsening water quality in upstream feeder creeks, leaving them less able to support marine life.

There's an estimated 400,000 acres of coastal marshes in South Carolina. They are considered the nursery for much of the sea life and seafood of the region. A 2010 Boston University study of tidal creeks in Cape Romain National Wildlife Refuge was the latest in a series of alarming research that shows the coastal grasses literally are being eaten away by rising seas.

The crabs tend to move back and forth between salt and brackish water, depending on season and stage of life. But the study findings suggest an overall shift in habitat. Prime habitat for juvenile blue crabs is now as much as 11 miles farther upstream than it was a decade ago, Childress said.

"So far we're still holding our own; the crab catch has remained constant. That's reason to have hope," Childress said. But, "if it's salinity (that's causing the crab decline), the only solution is to let more fresh water downstream." Pointing to increased water demand and recent lawsuits over water usage rights among states in the Southeast, he said that wouldn't be easy to do.

Reach Bo Petersen at 937-5744.