Here's Why Dolphins Continue to Die in the Gulf of Mexico

Researchers say they've now firmly connected the high rates of dolphin deaths seen in the northern Gulf of Mexico to the 5-year-old Deepwater Horizon oil spill, which damaged the mammals' lungs and adrenal glands.

"No feasible alternative causes remain that can reasonably explain the timing, location, and nature of these distinct lesions and increase in deaths," veterinary epidemiologist Stephanie Venn-Watson said. The dead dolphins "show unusual and life-threatening adrenal and lung diseases" that weakened them and contributed to their deaths, she said.

Dolphins in the northern Gulf have been dying at rates four to five times higher than before the spill. Finding out why is one of the goals of the National Resource Damage Assessment that federal agencies have been conducting since the April 2010 blowout, which killed 11 men aboard the drill rig Deepwater Horizon and uncorked a subsea gusher that spewed for three months.

Related: Scientists say an area the size of Rhode Island is coated with oil from the BP well explosion

Scientists compared the remains of 46 dolphins found along the shores of Louisiana, Mississippi, and Alabama to those of more than 100 dolphins found dead in other states at various times.Veterinary pathologist Kathleen Colegrove said a third of the northern Gulf dolphins — including half of those from Louisiana's Barataria Bay, one of the most heavily oiled sites — had thinner adrenal cortices, what she called "a very unusual abnormality."

"This was significantly higher than in the reference population, in which less than one in 10 had this lesion," she said. While that condition can also be caused by cancer, autoimmune diseases or fungal infections, "We did not find any evidence of these alternative causes in the dolphins," she said.

More than a fifth of the animals suffered from bacterial pneumonia, another condition linked to oil exposure.

The findings were published Wednesday in the peer-reviewed scientific journal PLOS One. The National Wildlife Foundation (NWF) called the study a "smoking gun" that undercuts well owner BP's efforts to avoid blame for the ongoing deaths.

"The facts are clear: the company's 'grossly negligent' actions killed dolphins in the northern Gulf," NWF President Collin O'Mara said in a statement. "And while we may never know the full extent of the damage, because only a small fraction of dolphin carcasses are ever found, [the National Oceanic and Atmospheric Administration's] scientific research indicates a direct link between BP's oil spill and the horrible deaths experienced by dolphins. No amount of public relations spin can change these scientific facts."

Vice on HBO: The lasting effects of the BP oil spill

The disaster has already cost BP well over $30 billion, and it faces billions more in penalties in an ongoing court case in New Orleans. The company vigorously disputed NOAA's findings, saying the study failed to trace oil to the ruptured well and that the "unusual mortality event" (UME) began two months before the blowout.

"Even though the UME may have overlapped in some areas with the oil spill, correlation is not evidence of causation," chief spokesman for BP Geoff Morrell said in a written statement. A BP expert testified in the court case that of the 130 dolphin post-mortems he reviewed, none listed oil exposure as the cause of death. And Morrell said the National Oceanic and Atmospheric Administration has hundreds more of those reports that it hasn't released.

NOAA has said the pre-spill deaths included in the UME involved a small number of animals in Lake Pontchartrain and nearby Mississippi Sound, most likely due to cold temperatures and lower concentrations of saltwater runoff in those waterways. After the spill, the deaths spread around the north-central Gulf.

Follow Matt Smith on Twitter: @mattsmithatl