Analysis of data from the Northern Gulf of Mexico Unusual Mortality Event (UME) found that this longest running UME is actually a cluster of events involving four distinct groups of dolphins. One of those clusters is strongly linked to the Deepwater Horizon (DWH) blowout.
Initial investigations suggested that exposure to oil played a part in the large dolphin mortality event in the Gulf of Mexico but the overall event, affecting dolphins in Florida, Alabama, Mississippi, Louisiana and Texas, started before the DWH oil spill occurred.
The initial cluster centered in northern Louisiana and western Mississippi, largely around Lake Pontchartrain, beginning in March 2010. Unusual cold and low salinity from flooding in the lake, which is actually a bay off the Gulf appear to be a major factor for this group. This cluster from the UME may have little or no connection with the oil spill.
The dolphins in Barataria Bay were not included in this UME until August 2010 but they have the dubious distinction of the longest sustained losses, which are ongoing, as is the UME investigation. Barataria Bay was heavily oiled during the DWH event and the dolphin population is resident, not transient or migratory, thus they had the highest exposure to oil of all the dolphin groups being monitored. In addition, an assessment of live dolphins in this area found health conditions consistent with oil exposure.
Previous UMEs have been linked to severe weather (cold water temperatures or low salinity in enclosed waters where flooding added significant fresh water), viral and bacterial infections and harmful algal blooms (HABs) such as red tides.
As scientists separate out the data from the UME based on regional variation in the start of the event, it may be easier to tease out what other factors beyond the oil spill were at play. A combination of spatial, temporal and demographic data will need to be taken into account to get the most accurate picture possible for this UME or group of UMEs.
Resources:
Venn-Watson, S. et al. 2015. Demographic Clusters Identified within the Northern Bottlenose Dolphin (Tursiops truncatus) Unusual Mortality Event: January 2010-June 2013. PlosONE 10(2):e0117248. doi:10.1371/journalpone.011728
Litz, J.A. et al.2014.Review of historical unusual mortality events (UMEs) in the Gulf of Mexico (1990-2009): providing context for the multi-year northern Gulf of Mexico cetacean UME declared in 2010. Diseases of Aquatic Organisms. 112:161-175. Doi:3354/dao02807