Study Links Record Dolphin Die-Off In The Gulf Of Mexico To Deepwater Horizon Spill


Exposure to oil and dispersants from the 2010 Deepwater Horizon oil spill caused bottlenose dolphins in the Gulf of Mexico to develop lesions and die, according to a new study.

The study, published Wednesday in PLOS One, looked at that the unusually high number of dolphins that died off the coast of Louisiana, Alabama, and Mississippi between June 2010 and December 2012. The researchers compared the 46 dead dolphins they looked at to 106 dolphin carcasses they found outside either outside of the “unusual mortality event” region — dolphins found in places such as South Carolina, Texas, and North Carolina — or found before the Deepwater Horizon spill. The disaster, which killed 11 men and sent millions of gallons of oil into the Gulf of Mexico, occurred in April 2010.

The study found that the dolphins associated with the unusual mortality event — which, according to the National Oceanic and Atmospheric Administration, is the highest bottlenose dolphin die-off in the Gulf of Mexico, and is ongoing — were more likely to have certain forms of pneumonia and adrenal problems than other dolphins, and that these problems were consistent with exposure to oil and dispersants. The dead dolphins that had been affected by the spill were found with lung, liver, and adrenal lesions.

“These dolphins had some of the most severe lung lesions I’ve ever seen,” Kathleen Colegrove, one of the study’s authors and associate professor at the University of Illinois, said during a press call Wednesday.

These dolphins had some of the most severe lung lesions I’ve ever seen

According to the study, the dolphins found off the coast of Alabama, Louisiana and Mississippi had “rare, life-threatening and chronic adrenal gland and lung diseases,” and these diseases are “are consistent with exposure to petroleum compounds as seen in other mammals.” Because of this consistency, the study concluded that the Deepwater Horizon spill helped cause the dolphins’ deaths, and that the lung and adrenal problems made the dolphins more susceptible to other stressors such as cold water and infections. The scientists looked at other possible causes of the dolphin die-off, including diseases that have resulted in dolphin deaths in the past, but ruled them all out.


“No feasible alternative causes remain that can reasonably explain the timing, location and nature of these distinct lesions,” Stephanie Venn-Watson, lead author of the study and veterinary epidemiologist said on the press call Wednesday.

The researchers said in the call Wednesday that bottlenose dolphins are particularly susceptible to oil and dispersant exposure because they take “big breaths” at the surface of the water, meaning that they’re likely to breathe in any contaminants that rest there.

The study is the latest to tie a drop in dolphin health to the Deepwater Horizon disaster, and is part of the Natural Resource Damage Assessment on the spill, which is being conducted by NOAA, BP, and federal and state agencies. In 2013, a study found that dolphins living in Barataria Bay, Louisiana were suffering from significant lung damage and hormonal problems — health issues that the study noted were “consistent with petroleum hydrocarbon exposure and toxicity.” Earlier this year, researchers also linked the unusual number of dolphin deaths and strandings to the oil spill, but the link wasn’t as definitive as in the most recent study.

“What this study really does is create a really strong link in the chain that shows that not only did you have an oil spill, but the habitat, particularly in Barataria Bay was impacted,” Alisha Renfro, staff scientist at the National Wildlife Federation’s Mississippi River Delta Restoration Campaign, told ThinkProgress of Wednesday’s study.

The fact that the researchers were able to eliminate all other possible causes of death of the Gulf dolphins was important, Renfro said, because it strengthens the conclusion that the oil spill contributed to the dolphin’s deaths.


BP disputes the study’s findings — as it has in past cases of research linking the drop in dolphin health to the Deepwater Horizon spill.

In this photo taken May 10, 2015, a dead dolphin washes ashore in the Gulf of Mexico on Grand Isle, La. CREDIT: AP Photo/Cain Burdeau
In this photo taken May 10, 2015, a dead dolphin washes ashore in the Gulf of Mexico on Grand Isle, La. CREDIT: AP Photo/Cain Burdeau

“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,” Geoff Morrell, BP’s Senior Vice President for U.S. Communications and External Affairs, said in a statement. “This new paper fails to show that the illnesses observed in some dolphins were caused by exposure to Macondo oil.”

Morrell also noted in the statement that the dolphin strandings in the Gulf began in February 2010 — a few months before the Deepwater Horizon spill. The researchers acknowledged this fact on the press call, but said that those strandings were more localized than the die-off seen after the spill.

As BP is still wrapped up in court proceedings over the spill, the studies from the Natural Resource Damage Assessment have been slow to come out. But Renfro said that, as more and more of these studies come out, the country will be able to better understand how much the spill affected the Gulf ecosystem. There are still many creatures that may have been affected by the spill that scientists don’t have as clear of data on as bottlenose dolphins, including sperm whales and sea turtles.

“We’re still trying to understand what happened and what the status of the Gulf is and what it’ll be in the future,” Renfro said. “I don’t think we have all the information yet, and I think that BP is a little premature in trying to brush it off and say the Gulf is fine…I think we have to wait and see, and I think the science isn’t quite there yet.”

The true impact on the ecosystem isn’t the only question surrounding the Deepwater Horizon disaster, five years after the spill occurred. The total oil spilled, the impacts of human health, the total amount BP will have to pay and the future of the deep sea environment are all still unclear.