(Reuters) - Marine scientists are debating whether 80-plus bottlenose dolphins found dead along the U.S. Gulf Coast since January were more likely to have died from last year's 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 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.
Eleven workers were killed in the blast, and an estimated 5 million barrels (206 million gallons) of crude oil spewed into the Gulf over more than three months.
Scientists in the Gulf already were in the midst of investigating last year's discovery of nearly 90 dead dolphins, most of them adults, when officials became alarmed at a surge in dead baby dolphins turning up on beaches in January.
The latest spike in deaths, and a high concentration of premature infants among them, has led some experts to speculate that oil ingested or inhaled by dolphins at the time of the spill has taken a belated toll on the marine mammals, possibly leading to dolphin miscarriages.
The die-off has come at the start of the first dolphin calving season in the northern Gulf since the BP blowout.
But scientists at the independent Dauphin Island Sea Lab in Alabama suggested Thursday that unusually chilly water temperatures in the Gulf may be a key factor.
"Everyone wants to blame toxicity due to the oil spill, said Monty Graham, a senior scientist at the Dauphin Island lab. "The oil spill ... very well could have been the cause of the dolphin deaths. But the cold weather could have been the last straw for these animals."
He noted that water temperatures abruptly plunged from the upper 50s into the 40s off Dauphin Island in January, just before the first two stillborn calves found there were recovered. He said a second wave of dolphin carcasses washed ashore after temperatures dipped again.
Fellow Dauphin Island scientist Ruth Carmichael called the arrival of the cold snap "incredibly compelling."
"The timing of the cold water may have been important because the dolphins were late in their pregnancies, about one to two months from giving birth. That might render them more vulnerable to temperature shocks," she said.
But NOAA officials discounted the significance of chilly weather, saying a similar cold snap in February 2010, months before the oil spill, was accompanied by higher-than-normal mortality among a range of wildlife, including fish and sea turtles. They also cited research showing bottlenose dolphins tend to swim away from extremely cool waters.
"These animals have the ability to move away from cold. They don't stay around in cold water," said Moby Solangi, director of the Institute for Marine Mammal Studies in Gulfport, Mississippi.
Scientists on both sides of the argument agreed that if frigid weather were to blame, the end of the die-off is likely at hand as warmer temperatures return.
But NOAA experts are bracing for the number of deaths to jump further as the bottlenose calving season reaches full swing in the coming weeks, said Blair Mase, a marine mammal scientist for the agency. Some 2,000 to 5,000 dolphins in the region typically bear their young this time of year.