(CNN) A total of 261 bottlenose dolphins were found stranded between Louisiana and the Florida Panhandle, officials said.

Authorities discovered the dolphins between February 1 and May 31. A majority of them -- 98% -- were dead, according to the National Oceanic and Atmospheric Administration.

That number is "three times the historical average in the northern gulf," said Erin Fougères, a marine mammal stranding program administrator for NOAA.

Because of the dolphin deaths and strandings, the agency declared an unusual mortality event.

"We are seeing higher numbers in Mississippi and Louisiana and we are concerned about fresh water," Fougères said. "It's an exceptionally wet winter for the entire United States and it's the wettest winter in the Mississippi Valley in the past 124 years."

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