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."
Fougères told reporters Friday it was too early to say what was causing the deaths, but investigators are looking at the salinity -- or salt -- levels in the water. Bottlenose dolphins are usually found in waters with high saline levels, according to the NOAA .
"Some of the dolphins have had freshwater skin lesions that are common with freshwater exposure," she said.
Fougères said dolphins have small home range and don't leave despite environmental changes. "They won't typically leave their resident area," she said.
An investigative team has been created to look into the unusual number of dolphin deaths, said Teri Rowles, national coordinator at the marine mammal training response program for NOAA.
Investigators are exploring everything, including freshwater spilling into the Gulf from the wet winter, food supply and possible lingering impacts from the 2010 Gulf oil spill.
"We urge the public to report any sick, stranded or dead dolphins to the local stranding network," Rowles said.