State biologists have found "concentrated pockets" of dead oysters in the upper Chesapeake Bay, which they blame on a record-high influx of fresh water into the estuary this year. But the die-off appears so far to be limited to two areas north of the Bay Bridge, officials note, which together account for just 2 percent of the state's overall oyster harvest.
Reporting preliminary results from a continuing baywide survey, the Department of Natural Resources said biologists recently found three-fourths or more of the oysters dead on bars between the mouth of the Patapsco and Magothy rivers and across the bay in an area north of Rock Hall. The two northernmost bars had no live oysters on them at all, officials said, while those that survived elsewhere were in poor condition, apparently suffering from the lack of salt in the bay's water.
Michael D. Naylor, DNR shellfish program director, called it a setback for the upper bay, where oysters were spotty to begin with. But the die-off hit "a pretty small area," he added, while the rest of the bay, including other areas north of the Bay Bridge, appear to be relatively unharmed.
With the bay's oysters already just a fraction of their historic abundance because of overfishing, habitat loss and disease, scientists were worried about the shellfish population because of unprecedented weather conditions this year. There were record-high river flows in spring, followed by another surge of fresh water in late summer from Tropical Storm Lee. Oysters need salty water to grow and reproduce, and they swell up and die if exposed to fresh water for a couple weeks or more.
Watermen working around Annapolis and north of the Bay Bridge have reported finding relatively few live oysters, which they blamed on the flood of fresh water and mud produced by the early September tropical storm.
State biologists sampled 15 upper bay oyster bars, and found 79 percent dead in the four northernmost ones along the Eastern Shore, according to DNR. The mortality rate on six bars between the Patapsco and Magothy was 74 percent, a seven-fold increase in dead bivalves from what was seen last year.
Naylor said biologists believe most of the oysters were killed earlier this year, when fresh-water flows coming out of the Susquehanna River, the bay's largest tributary, hit record levels from March to May. Many of the dead bivalves found had barnacles and other fouling organisms growing inside their shells, indicating they've been gone for months.
The bay has seen large oyster kills before when heavy rains turn the water fresh, Naylor noted. Severe die-offs have occurred five times before since the beginning of the 20th century, most recently in 1993, according to DNR records.
"This is something that happens everywhere oysters grow," Naylor said, "wherever there's fresh water. It's a normal part of the bay."
Though a severe loss for watermen who harvest oysters in the upper bay, the die-off should not have a major impact on the state's overall catch, officials said. State records show 30 watermen reported any catch the past two years from the bars with the highest mortality, and together they harvested just a little more than 3,000 bushels annually – or about 2 percent of the baywide total.
However, Naylor said the die-off was a blow to hopes that oysters in the upper bay might be on the verge of a comeback because of a bumper crop last year of baby oysters. The "spat set" of juvenile bivalves settling on the bottom was the highest since 1997 and five times the 25-year average, Naylor said. The fresh water effectively wiped out that new generation, he said, and reproduction was poor almost everywhere this year.
State officials will be looking to restock an oyster sanctuary that was ravaged on Man O War shoal at the mouth of the Patapsco, Naylor said. But he declined to say if the state would make similar efforts to repopulate the public oyster bars that watermen are free to harvest.
Larry Simns, president of the Maryland Watermen's Association, said with upper bay watermen facing a meager harvest, he would ask the state to reopen the Chester River, which was converted to a sanctuary last year and closed to commercial harvest. But Naylor said that wasn't likely. "Allowing additional harvest of oysters in the face of widespread loss of oysters might not be the best strategy," the DNR official said.