Julie Dermansky/Corbis

More than 1,300 bottlenose dolphins have stranded themselves in the northern Gulf of Mexico since early 2010. Research now links this unusual mortality event to the massive Deepwater Horizon oil spill.

The spike in dolphin deaths began shortly before the spill in April 2010, and scientists have struggled to understand whether the two events are related. A study published on 20 May in PLoS ONE finds that many of the dead animals had lung and adrenal-gland lesions that are consistent with exposure to petroleum compounds1.

That led the study's authors to conclude that the Deepwater Horizon spill probably drove the mass deaths. The study builds on a 2011 assessment of live dolphins in Barataria Bay, Louisiana, that revealed widespread adrenal and lung disease and general poor health2. The spill, which began with the explosion of a BP drilling rig, caused heavy and prolonged oiling in the bay.

In the latest study, researchers analysed lung and adrenal-gland tissue samples from 46 dolphins that were found dead in Louisiana, Mississippi and Alabama — areas that experienced significantly elevated levels of petroleum compounds. The team compared these animals with a reference group of 106 dolphins that stranded before the mass deaths began, or outside of the area where these strandings took place.

The dolphins that died in the footprint of the Deepwater Horizon oil spill were more likely to have lesions in their lungs and adrenal glands than animals from outside that area were, says Kathleen Colegrove, one of the study’s authors and a veterinary pathologist at the University of Illinois at Urbana–Champaign.

Catastrophic colds

Other differences were also apparent. More than one in five dolphins from the mass-death group had bacterial pneumonia, and the disease caused or contributed significantly to the deaths of 70% of these animals. In contrast, just 2% of the animals in the reference group had pneumonia.

“What's been really striking to me as a pathologist has been the severity of some of these pneumonias,” Colegrove says. “They've been some of the most severe pneumonias that myself or some of the other pathologists involved in this investigation have ever seen.”

Dolphins in the Deepwater Horizon footprint were also more likely to have a thin adrenal cortex, which can increase the risk of death and disease — especially in animals that are already fighting infection, exposed to cold temperatures or are pregnant. The condition was observed in a third of the animals from the mass-death group, including half of the dolphins from Barataria Bay.

“Animals with untreated adrenal dysfunction can essentially be balancing precariously on a ledge, waiting for the right stressor to push them into an adrenal crisis, including rapid death from shock,” says Stephanie Venn-Watson, a veterinary epidemiologist at the National Marine Mammal Foundation in San Diego, California, and the study's lead author.

Oil trigger

These results, taken with the findings of the 2011 assessment, show that dolphins were significantly affected by exposure to petroleum compounds after the Deepwater Horizon spill, Colegrove says. That exposure caused the animals' adrenal and lung disease, and those conditions were major factors in increasing dolphin deaths, she adds.

Frances Gulland, a veterinarian at the Marine Mammal Center in Sausalito, California, is pleased to see concrete data on the potential impacts of the spill after years of debate. “It’s really nice to see real confirmation from histology,” she says.

But the book is not closed on the dolphin deaths. Because the animals are long-lived, and slow to mature and reproduce, Colegrove says that it will be many years before scientists understand the full effects of the Deepwater Horizon spill on the Gulf of Mexico population.

In a prepared statement, BP spokesman Geoff Morrell disputed any connection between the oil spill and the dolphin deaths. “The data we have seen thus far, including the new study from [the US National Oceanic and Atmospheric Administration], do not show that oil from the Deepwater Horizon accident caused an increase in dolphin mortality,” he said.