2019 Bottlenose Dolphin Unusual Mortality Event Along the Northern Gulf of Mexico
2019 Bottlenose Dolphin Unusual Mortality Event Along the Northern Gulf of Mexico (CLOSED)
The Unusual Mortality Event involving bottlenose dolphins in the northern Gulf of Mexico (NGOM) that began in February 2019 is over. The event was investigated by NOAA Fisheries and partners and was defined as occurring from February 1 through November 30, 2019 along the NGOM coastline from the eastern border of Taylor County, Florida through Alabama, Mississippi, and Louisiana. The largest number of mortalities occurred in Mississippi and eastern Louisiana.
337 bottlenose dolphins stranded during the event with only nine dolphins documented stranding alive. The majority of carcasses were in moderate to advanced states of decomposition, limiting the analyses that could be conducted. A high prevalence of dolphins exhibited skin lesions consistent with exposure to low salinity waters (Mississippi = 47%, Louisiana = 30%, Alabama= 10%, Florida= 7%, of fresh to moderately decomposed cases). Based on necropsy, histopathology, and diagnostic findings and the extreme environmental conditions documented in the NGOM during this time period, the cause of the mortality event was determined to be environmentally driven by exposure to low salinity waters resulting from extreme freshwater discharge from watersheds that drain into the NGOM, including rivers in Florida, Alabama, Mississippi and Louisiana. This unprecedented amount of freshwater discharge during the winter, spring and summer months of 2019 resulted in a drop in salinity levels across the coastally associated waters in the region, which was most pronounced and prolonged in the western Mississippi Sound. Prolonged exposure to low salinity water (e.g., <10ppt) has been documented to have harmful health impacts on bottlenose dolphins, ranging from skin lesions and serum electrolyte abnormalities to acute mortality.