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Oysters die off in Chesapeake Bay due to rainfall

ANNAPOLIS — As oyster growers prepare to harvest their maturing spat in the coming weeks, things are not looking good.

An unusually rainy fall lowered salinity levels in Pasadena creeks and rivers, apparently causing massive die-offs among young oysters planted as part of the state’s efforts to reinvigorate the oyster population of the Chesapeake Bay. In some areas all the spat planted were wiped out.

“It’s a very serious situation,” said Chris Judy, manager of the Marylanders Grow Oysters program. “We don’t want to see our oysters die. In in the four-year history of the program, this is the first time we’ve had such an event, so it was very unusual.”

Spat, larva that settles and begins to develop a shell, need water with a certain salinity — typically at least four parts-per-thousand. Heavy rains in late August and September made the water too fresh for many spat to survive.

Under the program, the Department of Natural Resources in September gave participants spat and cages to hang from their piers, in which larval oysters were supposed to grow. The plan was to recover the oysters next month and place them on reefs, where they would filter local waterways.

But things didn’t go as planned.

Growers in Rock Creek experienced what appears to be a nearly 100 percent mortality rate, coordinator Chris Wallis said. All of the oysters in Stoney Creek died, said Mario Ricci, the program coordinator for that area.

In the Magothy River, growers are seeing 90 percent mortality rates or worse, volunteer coordinator Carl Treff said. Exact numbers for Bodkin Creek aren’t available yet, but cages have produced very few spat.

Mortality rates for the program typically range between 30 percent and 40 percent.

For Judy, the die-offs were discouraging. But they couldn’t be helped, he said.

Hurricane Irene dropped heavy rain on Maryland in late August and Tropical Storm Lee added more rain shortly thereafter. Coupled with the opening of the Conowingo Dam, a mass of fresh water flowed into waterways north of the Bay Bridge.

Oysters typically need salinity rates above four parts-per-thousand to survive, though six parts-per-thousand or higher is optimal, Judy said. Readings in the Magothy River in late fall revealed salinity rates at or below four parts-per-thousand.

“Mother nature really put the nail in the coffin,” Treff said.

Cages are built using prison labor and the spat are taken from a University of Maryland hatchery on the Eastern Shore.

The initial cost to the state is about $30 per cage, including materials, labor and spat, Judy said. Cages last about 10 years.

The cages and spat are given to participants for free, on the condition the oysters will be taken to reefs after they have grown.

There were 600 cages placed in the Magothy River last fall, Treff said. Ninety cages were placed in Rock Creek, Wallis said. Stoney Creek had 20 cages and a total for Bodkin Creek wasn’t available, though eight are hanging from a marina at Bodkin Point.

For people who participate in the program, the duties are simple.

Cages must be kept in 4 feet of water or hung from docks or piers. Caretakers must provide occasional maintenance on the cages to shake sand and sediment off the oysters. Oysters also must deal with predators, such as worms and crabs.

Despite the high mortality of oyster spat, oysters on reefs in the Magothy survived “surprisingly well,” said Paul Spadaro, president of the Magothy River Association. An Earth Day dive revealed a relatively healthy population and several “very large” oysters on existing reefs, he said.

“We were thinking of the worst, but I was pleasantly surprised that the oysters we recovered in April were doing surprisingly well and surviving,” Spadaro said. “I thought 90 percent would have died, but they hadn’t. The oysters were continuing to grow and survive in the river, which is what we need.”

A fully grown oyster can filter up to 50 gallons of water a day.

Many participating in the spat program are looking forward to giving it another shot this fall. The state has no plans to cancel the program in any of the four Pasadena waterways that participated this year, Judy said.

“There’s still a lot of interest within Stoney Creek,” Ricci said. “Everybody I talked to said they would like to do it again. We had back luck this year, but it has been a great learning experience. I think we’ll have better luck this year.”