By Letitia Stein
TAMPA, Fla., May 20 (Reuters) - A record dolphin die-off in the northern Gulf of Mexico was caused by the largest oil spill in U.S. history, researchers said on Wednesday, citing a new study that found many of the dolphins died with rare lesions linked to petroleum exposure.
Scientists said the study of dead dolphins tissue rounded out the research into a spike of dolphin deaths in the region affected by BP Plc's oil spill that was caused by the 2010 Deepwater Horizon oil rig explosion.
Millions of barrels of crude oil spewed into Gulf waters, and a dolphin die-off was subsequently seen around coastal Louisiana, Alabama and Mississippi, according to the National Oceanic and Atmospheric Administration (NOAA).
"Dolphins were negatively impacted by exposure to petroleum compounds," from the spill, said Stephanie Venn-Watson, a veterinary epidemiologist at the National Marine Mammal Foundation and lead author of the study published in the scientific journal PLOS ONE.
"Exposure to these compounds caused life-threatening adrenal and lung disease that has contributed to the increase of dolphin deaths in the northern Gulf of Mexico," she added.
More than 1,200 cetacean marine mammals, mostly bottlenose dolphins, have been found beached or stranded since the spill, according to NOAA, which has declared an ongoing "unusual mortality event" under 1972 Marine Mammal Protection Act.
To understand the cause of death, researchers compared tissue samples from 46 dolphins in areas affected by the spill with 106 dolphins that were not exposed.
One-third of the oil-exposed dolphins suffered rare adrenal damage, consistent with petroleum exposure, the study found.
Yet only seven percent of the dolphins away from the spill had similar adrenal disease, which would hurt their ability to produce critical hormones and make them vulnerable to stressors, researchers said.
Dead dolphins near the spill were more likely to have lung damage and pneumonia, researchers found, explaining the mammals would have inhaled contaminants coming up for air near the water's surface.
"These dolphins had some of the most severe lung lesions I have ever seen in wild dolphins throughout the United States," said Kathleen Colegrove, the University of Illinois-based lead veterinary pathologist for the study.
Yet BP questioned whether other factors were to blame.
"The data we have seen thus far, including the new study from NOAA, do not show that oil from the Deepwater Horizon accident caused an increase in dolphin mortality," said Geoff Morrell, BP's senior vice president for U.S. communications, in a statement. (Reporting by Letitia Stein; Editing by Marguerita Choy)