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Sixty-seven bottlenose dolphins have washed up on Gulf Coast beaches over the past few weeks. And more than half were babies. Scientists are trying to determine if there's a link between a spike in dolphin deaths and last year's Gulf of Mexico oil spill.

NPR's Elizabeth Shogren reports that the large number of dead calves is particularly unusual and alarming.

ELIZABETH SHOGREN: Dolphins have an 11 or 12-month gestation period. These dead baby dolphins were conceived just before the Deepwater Horizon blew up.

Dr. TERI ROWLES (Marine Mammal Scientist, National Oceanic and Atmospheric Administration): So, these animals were undergoing development during the height of the oil spill.

SHOGREN: Teri Rowles is the top marine mammal scientist at the National Oceanic and Atmospheric Administration. She says it's very worrisome to see such a large number of marine mammals die.

Dr. ROWLES: The oil spill is definitely on our list of potential causes, but we're certainly not ruling in or out any causes at this point.

SHOGREN: At least two of the dead calves found over the last few days had what looked like oil on their faces. Mandy Tumlin is the marine mammal strandings coordinator for the Louisiana Department of Wildlife and Fisheries. She says those calves were discovered by cleanup crews surveying the beach of Grand Terre Island in southeast Louisiana.

Ms. MANDY TUMLIN (Louisiana Department of Wildlife and Fisheries): We're concerned but we really can't speculate. There's a lot of factors that could play a role in an animal's death.

SHOGREN: Tumlin doesn't expect to be able to confirm any cause of death until all the samples from the animals come back from labs. Most of the dead dolphin calves were found on the beaches of Alabama and Mississippi. Staff from the Institute for Marine Mammal Studies collected them.

Moby Solangi directs that institute. He says some of the calves were stillborn, some were premature and some died shortly after birth.

Mr. MOBY SOLANGI (Executive Director, Institute for Marine Mammal Studies): It is very, very strange. Usually we see one or two calves, but this year it's just a very, very large number.

SHOGREN: His staff took samples from the decomposed carcasses and is doing autopsies on the dead dolphins that were still intact.

Mr. SOLANGI: We are doing a forensic study and we're trying to put all the pieces of the puzzle together.

SHOGREN: Imagine "CSI" or "Bones," but the victims are dolphins. One of Solangi's working theories is these dolphins' mothers ate fish contaminated with BP oil. Those contaminants passed through the mothers' bloodstreams to the fetuses.

But there are other possibilities. Researchers are looking for signs of an infectious disease. They'll also investigate whether the unusually cold winter played a role. A toxic algal bloom is another suspect. Even if the oil spill did not directly cause these deaths, it still could be a factor.

Veterinary pathologist Greg Bossart is a dolphin expert at the Georgia Aquarium. He says researchers are still trying to tease out all the ways BP oil affected the Gulf's ecosystem.

Dr. GREG BOSSART (Dolphin Expert, Georgia Aquarium): When those interactions become unbalanced from the oil, then you're prone to seeing, you know, new diseases emerge, predator-prey relationships change, temperatures change, and chemistry of the ocean change. And all those indirectly affect the health of organisms.

SHOGREN: Experts say since dolphins are at the top of the food chain, they reflect what has happened to their environment.

Dr. BROSSART: What we do know is dolphins can be very good sentinels for what's happening in our oceans and even what's happening in our own bodies.

SHOGREN: Dead dolphins keep washing up day after day. Scientists say they'll investigate every animal they find.

Elizabeth Shogren, NPR News.

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