( CNN ) -- Dead baby bottlenose dolphins are continuing to wash up in record numbers on the shores of the Gulf of Mexico , and scientists do not know why . Since February 2010 to April 2011 , 406 dolphins were found either stranded or reported dead offshore . The occurrence has prompted the National Oceanic and Atmospheric Administration to designate these deaths as an `` unusual mortality event '' or UME . The agency defines a UME as a stranding incident that is unexpected or involves a significant loss of any marine mammal population . `` This is quite a complex event and requires a lot of analysis , '' said Blair Mase , the agency 's marine mammal investigations coordinator . Mase said NOAA is working closely with a variety of agencies to try to figure out not only why the bottlenose dolphins are turning up in such large quantities but also why the mammals are so young . `` These were mostly very young dolphins , either pre-term , neonatal or very young and less than 115 centimeters , '' she said . Marine mammals are particularly susceptible to harmful algal blooms , infectious diseases , temperature and environmental changes , and human impact . `` The Gulf of Mexico is no stranger to unusual mortality events , '' Mase said . Sensitivity surrounding marine life in the area is particularly high after the BP oil disaster that sent millions of barrels of crude into the Gulf of Mexico nearly a year ago . The incident occurred on April 20 , 2010 , when a Deepwater Horizon rig leased to BP exploded , killing 11 workers and leading to the worst oil spill in U.S. history . As recently as two weeks ago , scientists documented a dead dolphin with oil on its remains , Mase said . Since the start of the oil spill , a total of 15 bottlenose dolphins have been found with either confirmed or suspected oil on their carcasses . Even after the gushing well was capped , the agency said nine oiled dolphins have been found since November 2 , 2010 . Of those nine , six were confirmed to contain oil from the incident ; one was found with oil that did not match the Deepwater Horizon samples , and two have not yet been tested . The dolphin deaths may be completely independent from the oil spill , Mase said . `` Even though they have oil on them , it may not be the cause of death , '' she said . `` We want to look at the gamut of all the possibilities . '' The agency said bottlenose dolphins are actually the most-frequently found stranding marine mammal . Scientists say they are equally concerned about the number of sea turtle strandings . Similar to the dolphin deaths , an abnormally high number of turtles have been found either floating close to shore or washed up on shores in Louisiana , Mississippi and Alabama . `` The vast majority of these are dead , with states in moderate to severe decomposition , '' said Barbara Schroeder , NOAA Fisheries national sea turtle coordinator . The majority of them are Kemp 's ridley sea turtles , an endangered species since 1970 . But some strandings included loggerheads , which are also endangered . `` Since January 1st , we 've had just under 100 strandings , '' Schroeder said . `` About 87 of those have been documented since the middle of March . '' Only about a third of those found were in good enough shape to perform necropsies , she said . Seven turtles showed indications that they had been in accidents involving watercrafts , while another displayed injuries consistent with being caught on a hook . Results from the rest appeared to indicate they had drowned near the bottom of the Gulf -- possibly either from forced submergence or an acute toxic event . NOAA Fisheries Stranding Program Coordinator Dr. Teri Rowles said tissue samples from both turtles and dolphins are being carefully documented due to the civil and criminal litigation ongoing with BP . `` We are looking at what is the impact of the oil spill and the response activities to the oil spill event , and what impact they had on the Gulf of Mexico ecosystem , '' she said . `` We did not say that the dolphins have died because of the oil , just that they have come back with oil on them . ''
Since February 2010 , no dolphins have been found dead or stranded . Sensitivity about marine life in the area is high after the BP oil spill . People are also concerned about sea turtle strandings .