Marine scientists are debating whether 80-plus bottlenose dolphins found dead along the U.S. Gulf Coast since January were more likely to have perished from last year’s massive oil spill or a winter cold snap.

The National Oceanic and Atmospheric Administration declared “an unusual mortality event” last week when the number of dead dolphins washing up in Louisiana, Mississippi, Alabama and Florida had reached nearly 60, about half of them newly born or stillborn calves.

The death toll along 200 miles of shoreline has climbed to at least 82 since then, many times the normal mortality rate for dolphins along the Gulf Coast this time of year.

Although none so far showed outward signs of oil contamination, suspicions immediately turned to petrochemicals that fouled Gulf waters after a BP drilling platform exploded in April 2010, rupturing a wellhead on the sea floor.