With dozens of dead and dying dolphins washing up on northeastern beaches in recent weeks, federal officials are warning local stranding responders to be on the alert for even more cases.

The National Oceanic and Atmospheric Administration has sent alerts to marine stranding response centers in the Northeast as part of a growing effort to figure out what is causing the deaths of unusually high numbers of Atlantic bottlenose dolphins.

Since July 9, at least 21 dolphins have washed up either dead or dying in New Jersey. In Delaware, 10 baby dolphins have been found dead since June. The NOAA said last week that it is investigating an increase in bottlenose dolphin deaths between New Jersey and Virginia.

Exactly how many dolphins have died along the East Coast is difficult to determine. Many have not been counted in New Jersey, for instance, because their bodies floated away after being reported to the Marine Mammal Stranding Center in Brigantine.

"We're trying to collect any info we can to see if there are any trends," said Maggie Mooney-Seus, spokeswoman for the NOAA's National Marine Fisheries Service. "Right now we don't have enough info to say we're seeing anything out of the ordinary. We're still collecting the data."

Necropsy results for at least four of the dolphins found in New Jersey determined the animals died of viral pneumonia, Marine Mammal Stranding Center co-director Bob Schoelkopf said. However, full results still will take more time as animal pathologists continue to finalize tests, he said.

About twice as many baby dolphins washed up on Delaware beaches than in a typical year, said Suzanne Thurston, executive director of the Delaware-based Marine Education, Research and Rehabilitation Institute. In a typical year, five or six baby dolphins wash up in the First State, but this year 10 were picked up between June and early July.

Most of those dolphins were badly decomposed when discovered, but one was still barely alive when it was found, Thurston said. That animal died from severe pneumonia linked to an E. coli infection, Thurston said.

Since then, Thurston said, the center has not had any dolphin strandings.

"We're on alert. I'm notifying our stranding team to try to prepare the best we can for an increase," she said.

Maryland also has seen an increase in bottlenose dolphin strandings, said Jennifer Dittmar, stranding coordinator with the National Aquarium in Baltimore, which handles live strandings. So far, Dittmar said, only dead dolphins have washed up on Maryland beaches, but she did not know how many there were.

The 20th dolphin to wash ashore in New Jersey was found Wednesday morning in Atlantic City. An additional dolphin was reported Wednesday off Cape May, but the stranding center cannot retrieve the animal because of a lack of resources.

"It's hard for us to go out 10 to 12 miles and tow an animal back. The boat can't handle it, and our insurance won't allow it," Schoelkopf said.

The stranding center also has received reports from fishermen of dead dolphins floating on the surface, sometimes caught in fishing line. However those animals have not been found because currents can carry away the carcass quickly or other wildlife will eat the remains.

A fisherman reported watching a shark eat a dead dolphin about 7 miles off Atlantic City.

It is illegal for anyone without a permit to pull a dead dolphin from the water to carry it to shore. The federal Marine Mammal Protection Act also forbids touching or moving a marine mammal - dead or alive - on land. Schoelkopf said anyone who sees a dead dolphin on land or in the water should call the stranding center's 24-hour response line at 609-266-0538.

The last mass die-off of bottlenose dolphins on the East Coast was in 1987 and 1988, when the morbillivirus, a virus similar to measles, killed nearly 750 dolphins between New Jersey and Florida. Reports following the die-off found that some of those dolphins had lesions, shrunken lymph nodes and pneumonia.

"Pneumonia is a common cause of death year-round in several species of marine mammals in both temperate and tropical waters," Mooney-Seus said. "Viral, fungal and bacterial forms of pneumonia are commonly found in bottlenose dolphins."

Dolphins are social animals and can catch penumonia from each other the way people can catch the flu from being around each other. "As they come up for air, one exhausts and passes it on to the one behind as they come up for air," Schoelkopf said.

Schoelkopf could not say what a normal number of dead dolphins found in a month would be, but 21 is a high number. The number, he said, is a cause for concern.

Necropsies for six of the washed-up dolphins are pending, in addition to the four completed. Six other dolphins already sent to the University of Pennsylvania, where the necropsies are performed, were too decomposed to determine a cause of death, Schoelkopf said.

Each necropsy costs the stranding center $100, plus the cost of fuel to transport the animal, Schoelkopf said.

Two distinct populations of bottlenose dolphins live in Mid-Atlantic waters, Thurston said. Smaller dolphins that weigh about 300 pounds are found close to shore, typically no more than a mile at sea. Larger dolphins, generally weighing about 500 pounds and having a thicker bone structure, are found farther at sea, she said.

Genetics testing will be done to identify what types of dolphins have washed ashore. So far, the ones tested are a mix of deep sea and inshore dolphins, Schoelkopf said. The type can also be identified by pictures, which Schoelkopf suggested fishermen and others take if they find one.

Schoelkopf said those who see a dead dolphin should report it immediately and keep pets and children away from any that wash up along the shore.

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