Seven bottlenose dolphins have washed up dead on Maryland shores in recent weeks, part of a larger mystery along the Mid-Atlantic coast, where alarmed scientists are working to find the cause of more than 120 dolphin deaths since June.
The seven dolphin carcasses were found in the Chesapeake Bay and on beaches on Maryland's Atlantic coast during July.
In a typical July, marine biologists might respond to a single report of a dead dolphin, said Dr. Cindy Driscoll, veterinarian and fish and wildlife health program director for the state Department of Natural Resources.
Several of the carcasses were "very decomposed" by the time they were found, Driscoll said, making it difficult to determine their cause of death. But state officials have sent several tissue samples to labs in the hope of gaining more information.
Scientists believe that toxic algae blooms or a virus could be to blame.
Dolphin deaths have been reported this summer from New Jersey to Virginia, with the greatest numbers in those two states. The total count troubles scientists, who are waiting on lab results to determine whether it is, indeed, a crisis.
"It is alarming since it's much higher than normal and in such a short amount of time," said Jennifer Dittmar, the stranding coordinator for the National Aquarium in Baltimore. "As far as an overall effect it's having on the population, it's hard to tell right now."
In Maryland, Driscoll said, 15 dead dolphins have washed ashore since the beginning of the year. In a normal year, the tally is more like eight to 10.
In Virginia, 87 dead dolphins have been found since June, most of them clustered along that state's portion of the Chesapeake. In a normal year, about 50 deaths would be reported there by this time of the year.
In New Jersey, 21 dolphin carcasses have washed up this summer, compared with about a dozen in a normal year.
Bottlenose dolphins live in pods and can be found along the East Coast from New Jersey to Florida. The mammals spend the winter in the temperate waters off the Southern states, then come north to the bays, sounds and open waters off the Mid-Atlantic coast from May to October.
In Maryland, Dittmar said, pods of as many as 20 to 30 dolphins follow schools of fish as far up the Chesapeake Bay as the Choptank River. Her job is to rescue stranded dolphins, but she said none have been found alive this year.
Necropsies — the veterinary equivalent of autopsies — are being performed on the animals that have been found. Officials say it could take several months to determine causes of death — or whether and how they might be linked.
Early findings indicate a sickness, said Susan Barco, research coordinator at the Virginia Aquarium and Marine Science Center Foundation.
"This is really frightening because these animals are sentinels of ocean health," she said. "Strandings have been much more common in the past few decades, and we think it's an indication of the health of our ecosystem."
Suspected causes include pneumonia, E. coli infection and morbillivirus, which was responsible for mass dolphin deaths in the 1980s, said Matthew Huelsenbeck, a marine scientist with the conservation group Oceana.
Environmental factors such as the accumulation of heavy metals or exposure to biotoxins through food make dolphins "some of the most toxic animals on the planet" — and more vulnerable to diseases, Huelsenbeck said.
In other cases involving mass deaths of large marine mammals, causes have included biotoxins from algae, other suspected environmental factors, or changes in the food chain.
Manatees are dying in record numbers in Florida this year — nearly 700 through late July, according to a state tally — in a case linked to a red tide of toxic algae.
Red tides have also killed sea lions in large numbers on the U.S. Pacific Coast in recent years. In some cases, Huelsenbeck said, young sea lions have died of starvation because they weren't able to adapt quickly enough when their food supply moved elsewhere.
About 900 dolphins and other large mammals, including porpoises and whales, have died in the northern Gulf of Mexico since December 2010. Some have linked the deaths to the Deepwater Horizon oil spill that began in April 2010, but litigation over the spill has limited the release of information about the causes of the deaths, Huelsenbeck said.
There have been no indications of harmful algae blooms in the Chesapeake or in the coastal bays west of Ocean City and Assateague Island, according to the Maryland Department of the Environment. Brevetoxins, neurotoxins emitted by algae blooms that can be deadly to mammals, have not been detected in Maryland this year, department spokeswoman Samantha Kappalman said.
Scientists are working to gather evidence before placing blame in the case of the dolphin deaths.
"They're collecting as much information as possible from all the stranding networks," Driscoll said. "Hopefully there is some more information on numbers from other states and some of causes."
It could be months or even years before an answer is found. Or a cause might never be determined: About half of the cases of mass marine mammal deaths that the National Oceanic and Atmospheric Administration deems "Unusual Mortality Events" remain unsolved.
It's also likely the number of deaths will grow, Huelsenbeck said.
He called the 120 deaths reported so far "the lowest-end estimate." He said the actual tally could number 1,000.
"This could have a more population-level impact that we don't know about yet," he said.
The state Department of Natural Resources asks anyone who finds a dead or stranded dolphin in Maryland to call (800) 628-9944.
Reuters contributed to this article.