Large numbers of bottlenose dolphins have been washing up on the shores of Louisiana, Mississippi, and Alabama since the 2010 Deepwater Horizon oil spill, the largest spill to take place in US waters. While it seems like it should be a safe assumption to link the spill to the dolphins' deaths, there's been a holdup: the dolphins actually started washing up in elevated levels two months before the spill, potentially because of a cold winter.

"Some of the most severe lung lesions I have seen."

But the deaths have continued into 2015, and researchers have been trying all the while to determine what the actual cause is. In the latest study, researchers from the National Oceanic and Atmospheric Administration describe serious health issues in washed-up dolphins that are linked to petroleum product exposure, strengthening the link between dolphins' deaths and the BP-owned well's spill.

"These studies have increasingly pointed to the presence of petroleum hydrocarbons as being the most significant cause of the illnesses and deaths plaguing the Gulf’s dolphin population," Teri Rowles, head of NOAA’s health and stranding program for marine mammals, says in a statement. "This study carries those findings significantly forward." The findings were published this week in PLOS One.

The researchers looked at 46 dolphins that washed up between 2010 and 2012, finding lung and adrenal lesions as well as an increased likelihood that they had of bacterial pneumonia. In 70 percent of the dolphins with pneumonia, "the condition either caused or contributed significantly to death," the researchers write. Close to 1,400 dolphins or whales have been stranded in total, the vasty majority of which — 94 percent — were found dead.

About one-fifth of all examined dolphins had pneumonia or adrenal issues, more than 10 times as many as a normal population is expected to see. The effects are all said to be consistent with what other mammals experience when exposed to oil.

Dolphins swimming through the oil spill in 2010.

During a conference call discussing the findings, lead author Stephanie Venn-Waston said that "no feasible alternatives remain" for explaining the cause of these lesions, according to Smithsonian. BP disagrees, saying, "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."

But the findings back up earlier studies that say dolphins in the Gulf of Mexico are experiencing lung diseases and seeing increased deaths because of the spill. The researchers describe these effects as particularly harmful. Kathleen Colegrove, a veterinary pathologist who worked on the study, says in a statement, "These dolphins had some of the most severe lung lesions I have seen in the over 13 years that I have been examining dead dolphin tissues from throughout the United States."