Dolphin with suspected freshwater lesions. Photo credit: Institute for Marine Mammal Studies.

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.

Bottlenose Dolphin Data

Figure 1: Graph shows total number of reported bottlenose dolphins (Tursiops truncatus) strandings from Florida Panhandle (Taylor County) through Louisiana by month. The final dates of the 2019 Northern Gulf of Mexico Bottlenose Dolphin Unusual Mortality Event are shown within the red bracket area (01 FEB 2019 to 30 NOV 2019). Historic average of stranding numbers from 2002-2018 with previous UMEs excluded (blue bars) with +2 times the standard deviation (2SD) from historical average (black error bars) and 2019 UME stranding numbers (orange bars). “*” indicates total number in 2019 month is at or above historical average + 2SD. Map