The largest oil spill ever caused the highest number of dolphin deaths in the Gulf of Mexico, concludes a two-year scientific study.

Most troubling to scientists was the exceptionally high number of young dolphins that made up nearly half of the 186 dolphins that washed ashore from Louisiana to western Florida from January to April 2011.

The number of “perinatal” (near birth) dolphins stranded during this four-month period was six times higher than the average number of perinatal strandings in the region since 2003 and nearly double the historical percentage of all strandings.

“Unfortunately, it was a ‘perfect storm’ that led to the dolphin deaths,” said Graham Worthy, University of Central Florida, Orlando and professor of biology and study co-author, the journal Public Library of Science ONE reports.

“The oil spill and cold winter of 2010 had already put significant stress on their food resources, resulting in poor body condition and depressed immune response. 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,” said Worthy, according to a Central Florida statement.

The cold winter of 2010 was followed by the historic BP Deepwater Horizon disaster in April 2010, which dumped millions of gallons of oil into the Gulf of Mexico, likely disrupting the food chain.

This was in the middle of the dolphins’ breeding  season. A sudden entry of high volumes of cold freshwater from Mobile Bay in 2011 imposed additional stress on the ecosystem and specifically on dolphins that were already in poor body condition. “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,” said Ruth Carmichael.