CLEARWATER (CBSMiami) – Scientists are puzzled by a disturbing trend in the waters along four Gulf Coast states.

Almost 300 dead and dying dolphins have washed ashore since February.

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Off the coast of St. Petersburg, Erin Fougeres with NOAA Fisheries took CBS News out on their dolphin research vessel.

We found a pod of eight or nine dolphins, all healthy, swimming a half-mile from shore.

But from Louisiana to the Florida Panhandle, coastline residents keep seeing a dolphin die-off.

They’re all bottlenose dolphins, which live closest to shore. Many were adults. The single-day record is nine.

Last week, NOAA’s Marine Mammal Stranding Program – which Fougeres oversees — designated this die-off an Unusual Mortality Event, or UME.

“I would say it’s concerning and bordering on alarming primarily because it’s a group of dolphins that have been impacted because of other unusual mortality events,” said Fougeres. “So this is the same area that was impacted by the Deep Water Horizon spill.”

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Scientists are exploring several possible causes, from the lingering effects of the BP oil spill in 2010 to skin lesions on many recovered dolphins that indicate freshwater exposure.

Heavy rainfall has flowed from the Mississippi into the Gulf.

Or it could be chemicals, pollutants, or a combination.

“It’s an area where dolphins have been previously exposed to oil, they have compromised health,” said Fougeres. “They have lingering health issues and um, so they are more susceptible to any additional stressor; anything could tip them potentially over the edge.”

What researchers want to see throughout the Gulf is healthy dolphins unthreatened by a mysterious danger lurking hundreds of miles away.

“Even if the numbers stopped right now, we still have a long way to go, so the work isn’t gonna end,” said Fougeres. “We’ll be figuring out which tests and which analyses we need to conduct and we’re going to move forward with that.”

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Whatever is happening, there is no known threat to people swimming.