Large numbers of dolphins have died as a direct result of the Deepwater Horizon oil spill, new evidence suggests.
Dead bottlenose dolphins stranded in the northern Gulf of Mexico since the spill have lung and adrenal gland injuries consistent with petroleum exposure, scientists have discovered.
High numbers of dolphin deaths had already been reported within the oil spill’s footprint, but the new findings are the strongest so far linking them to the environmental disaster.
Deepwater Horizon was a semi-submersible off-shore drilling rig which exploded and sank off the coast of Louisiana in April 2010. It left oil gushing from the sea floor for 87 days, causing the world’s worst accidental oil spill at sea.
In 2011, a health assessment of living dolphins in Barataria Bay, Louisiana, which was severely polluted after the spill, showed they were in poor health and afflicted with adrenal and lung disease.
This coincided with unusually high numbers of marine mammal deaths in the Gulf of Mexico and record numbers of dead bottlenose dolphins being found stranded on beaches.
Half the dead dolphins from Barataria Bay stranded between June 2010 and November 2012 had thinly-walled adrenal glands, indicative of poor adrenal function, the new research shows.
The same effect was seen in one in every three dolphins examined across Louisiana, Mississippi and Alabama.
In comparison, only 7% of dead dolphins collected from coastal regions away from the oil spill had similarly damaged adrenal glands.
One in five dead dolphins from within the oil spill zone, but only 2% of those from other areas, also had signs of severe lung damage caused by pneumonia.
US lead scientist Dr Stephanie Venn-Watson, from the National Marine Mammal Foundation in San Diego, California, said: “Animals with adrenal insufficiency are less able to cope with additional stressors in their everyday lives, and when those stressors occur, they are more likely to die.”
In other mammals, exposure to petroleum-based polycyclic aromatic hydrocarbons, or PAHs, can lead to injured lungs and reduced immune function, increasing susceptibility to bacterial pneumonia.
Co-author Dr Kathleen Colegrove, from the University of Illinois, said: “These dolphins had some of the most severe lung lesions I have seen in the over 13 years that I have been examining dead dolphin tissues from throughout the United States.”
The research is reported in the online journal Public Library of Science ONE.
Dr Teri Rowles, head of the Marine Mammal Health and Stranding Response Program at the US National Oceanic and Atmospheric Administration (Noaa), said: “This is the latest in a series of peer-reviewed scientific studies, conducted over the five years since the spill, looking at possible reasons for the historically high number of dolphin deaths that have occurred within the footprint of the Deepwater Horizon spill.
“These studies have increasingly pointed to the presence of petroleum hydrocarbons as being the most significant cause of the illnesses and deaths plaguing the Gulf’s dolphin population. This study carries those findings significantly forward.”