Dolphin deaths in the Gulf of Mexico were caused by a perfect storm of cold water temperatures coupled with the 2010 BP oil spill, according to a new study, published in the journal PLoS ONE. (Photo: Creative Commons: Mrs. Gemston)

Dolphin deaths in the Gulf of Mexico were caused by a perfect storm of cold water temperatures coupled with the 2010 BP oil spill, according to a new study, published in the journal PLoS ONE.

More than 185 bottlenose dolphins died between January and April 2011, most washing ashore between Louisiana and Florida.   Nearly half were calves, which is more than double the normal proportion of calves to older dolphins, researchers said.

"Unfortunately, it was a 'perfect storm' that led to the dolphin deaths," Graham Worthy, study author and biologist at the University of Central Florida, said, according to NBC News. "The oil spill and cold water of 2010 had already put significant stress on their food resources. It appears the high volumes of cold freshwater coming from snowmelt water that pushed through Mobile Bay and Mississippi Sound in 2011 was the final blow."

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The Gulf of Mexico is the site of the Deepwater Horizon oil spill, which killed 11 people and caused the largest offshore oil spill in American history in April 2010.

Just before calves began washing ashore in January 2011, the Gulf was hit by an usually heavy snowfall. Researchers found that dolphins washed ashore two to three weeks after, indicating that the temperature drop caused the dolphins to become stressed, die and wash ashore.

The findings don't prove that the BP oil spill contributed to the dolphin die off, but the evidence strongly suggests that the oil spill helped weaken them, researchers said. In addition to stressing them out, the oil also affected the dolphin's prey, making food       scarcer.

"When we put the pieces together, it appears that the dolphins were likely weakened by depleted food resources, bacteria or other factors as a result of the 2010 cold winter or oil spill, which made them susceptible to assault by the high volumes of cold freshwater coming from land in 2011," Ruth Carmichael, study coauthor and marine scientist at the Dauphin Island Sea Lab, told NBC News.