A higher than normal mortality rate for bottlenose dolphins (Tursiops truncatus) in the Gulf of Mexico during 2011 has been found to correspond with the Deepwater Horizon oil spill as well as colder weather conditions during 2010 leading to an influx of colder water into the Gulf from snowmelt. From January through April 2011, 186 bottlenose dolphins were found ashore between Louisiana and Florida, 46% of which were calves. This number is nearly twice that of preceding years (2003-2010). Using tissues collected from the animals, the actual cause of death is still under investigation.

Figure 1 from article published in PLoS ONE showing the number of stranded dolphins. http://www.plosone.org/article/info:doi/10.1371/journal.pone.0041155

New report ties cold Gulf water, dolphin deaths: fox10tv.com

According to physical examinations of 32 dolphins in the Gulf by scientists with the National Oceanographic and Atmospheric Administration, the animals have been showing signs of illness including weight loss, anemia, low blood sugar, as well as symptoms of liver and lung diseases and adrenal insufficiency (indicated by low levels of hormones related to stress responses, immune function and metabolism). Although these tests could not confirm a direct link between the oil spill and the declining health of the dolphins, a control group of dolphins from an area of the Atlantic coast not affected by the spill did not exhibit similar symptoms. They did not mention whether there were potential differences in the water temperature of the two areas preceding the increased mortality.

 
For information on how an oil spill can affect the physiology of marine life, the EPA has a nice paper you can access here.
 
Sources:
Carmichael RH, Graham WM, Aven A, Worthy G, et al. (2012) Were Multiple Stressors a ‘Perfect Storm’ for Northern Gulf of Mexico Bottlenose Dolphins (Tursiops truncatus) in 2011?. PLoS ONE 7(7): e41155. doi:10.1371/journal.pone.0041155
 
 
Fox10 News, Florida