Surprise: BP's Record-Breaking Oil Spill Decimated Dolphins

Dolphins have been dying off at unprecedented rates in the Gulf of Mexico over the last five years, and a new study helps confirm what researchers suspected: the 2010 Deepwater Horizon oil spill is to blame.

After the 2010 disaster, which was the largest marine-based oil spill in US history, scientists documented the highest number of dead bottlenose dolphin strandings on record in the northern Gulf of Mexico, with more than 1,300 washed ashore as of May 2015.

Scientists have been investigating the unusual mortality event over the last five years to see if it can be tied to the spill. Previous research showed dolphins in heavily affected areas showed signs of adrenal and lung disease, and the most recent study, published today in PLoS One, found exposure to oil was the most significant cause for these diseases and death.

"There is no feasible alternative that could reasonably explain the timing, location, and nature of the increase in deaths," Dr. Stephanie Venn-Watson, veterinary epidemiologist at the National Marine Mammal Foundation and the study's lead author, said of the oil spill.

Researchers in the study assessed tissue samples from 46 dolphin carcasses washed ashore in Louisiana, Mississippi, and Alabama after the spill and compared them to samples from 106 dolphins who washed to shore in the same area years prior. They found dolphins who died after the spill had rare lung and adrenal lesions consistent with petroleum product exposure.

In addition to the lung and adrenal lesions, Venn-Watson said researchers found a variety of other health issues related to oil exposure in dolphins who died after the spill.

"One of the major abnormalities we found is a thin adrenal gland cortex, which is indicative of deficiency," she said. "That evidence is very striking and indicative that the adrenal gland lesions we are seeing are consistent with oil exposure."

The adrenal gland cortex, a thin layer around the hormone-producing adrenal gland, was found to be unusually thin in one in three of the dolphins who died after the oil spill compared to only 7 percent in species not affected by the spill. The condition makes the dolphins more susceptible to disease and death.

"Animals with adrenal insufficiency are less able to cope with additional stressors in their everyday lives," Venn-Watson said in a statement. "and when those stressors occur, they are more likely to die."

The affected dolphins also had significant increase lung disease. More than one in five dolphins surveyed had bacterial pneumonia, which contributed to their deaths.

"These dolphins had the most severe lung lesions I have ever seen in dolphins in the US," Kathleen Colegrove-Calvey, a veterinary pathologist at the University of Illinois at Urbana-Champaign and author on the study, said.

Venn-Watson said the dolphin mortality rate saw an unprecedented increase in years after the oil spill, in some periods deaths were four to five times greater than baseline. rates. At one point, there were 18 consecutive months of elevated dolphin deaths.

She said although many species were harmed by the oil spill, dolphins are particularly susceptible to the effects.

"Dolphins, which breathe at the water's surface, are particularly high risk of chemical aspiration and injury," she said. "They take big breaths at water surface and hold their breaths for a long time."

Although this is one in a series of several studies devoted to researching the aftermath of the oil spill, Venn-Watson said it could be years before we know the full effect.

"More assessment of these trends and research over a period of years will be needed to better understand the long-term effects on dolphins and the full impact of the Deepwater Horizon oil spill," she said.

Update 5/21: In a response to the study from BP, a representative said, "According to NOAA, the Gulf 'unusual mortality event' (UME) began in February 2010, months before the spill. Even though the UME may have overlapped in some areas with the oil spill, correlation is not evidence of causation."