By Ian Simpson
WASHINGTON (Reuters) - The ailing Chesapeake Bay is slowly recovering from pollution and overfishing but still has problems that include a drop by half in a key segment of the blue crab population, a report on the largest U.S. estuary said on Tuesday.
The health snapshot from the Chesapeake Bay Program said the 64,000-square-mile (166,000-square-km) watershed covering six states and Washington, D.C. was threatened by urban development, rising sea levels and warmer water.
"Yet in the face of these constantly changing challenges, we are witnessing signs of a system in recovery," Nick DiPasquale, the program's director, said in a statement.
Among signs of health, the program's Bay Barometer showed that between 2012 and 2013 the abundance of underwater grasses was up 24 percent, to 59,927 acres (23,371 hectares).
The abundance of American shad in the watershed rose in 2013 to 41 percent of the program's goal. The upturn was especially noted in the Potomac and York rivers, the report said.
Between 2013 and 2014, the abundance of juvenile striped bass went up significantly and is close to the historic levels for Virginia and Maryland.
On the down side, the abundance of spawning-age female blue crabs fell 53 percent between 2013 and 2014, to 68.5 million. The figure is well below the target of 215 million and means blue crabs, the official Maryland state crustacean and a local delicacy, are in a "depleted state," the report said.
Only 29 percent of the Chesapeake Bay's water quality standards were attained although river pollution fell below the long-term average in 2013.
The size and severity of the bay's annual "dead zone," or low-oxygen area caused by pollution, remained unchanged, although its duration has fallen over time, the report said.
For decades, overfishing, silting and pollution have taken a toll on the Chesapeake. Polluted runoff from urban areas and farms has been especially harmful to the bay, which produces only about 1 percent of the oysters it did in the late 19th century.
The Chesapeake Bay checkup comes less than two weeks after new Maryland Governor Larry Hogan, a Republican, pulled a pending rule that would have limited farmers' use of phosphorus-rich chicken manure on their fields.
The Chesapeake Bay Program is a regional partnership that includes state and federal agencies, governments, non-profits and schools.
(Reporting by Ian Simpson; Editing by Will Dunham)