U.S. scientists say they still don't know why dead baby bottlenose dolphins are washing up on the shores of the Gulf of Mexico in record numbers. Since February 2010, 406 dolphins were either found stranded or sighted dead offshore, CNN reported Friday. The National Oceanic and Atmospheric Administration has declared the deaths an "unusual mortality event," defined as an unexplained stranding incident or one involving a significant loss of marine mammal numbers. NOAA is trying to determine why so many of the dolphins are so young, officials said. "These were mostly very young dolphins, either pre-term, neonatal or very young and less than 115 centimeters (45 inches)," said Blair Mase, the agency's marine mammal investigations coordinator. Marine life in the area has been monitored closely since the BP oil disaster sent millions of barrels of crude into the Gulf of Mexico nearly a year ago. However, the dolphin deaths may be completely independent from the oil spill, Mase said, noting bottlenose dolphins are the marine mammal most-frequently found stranded.