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Bottlenose Dolphin Deaths Debated

March 4, 2011

Marine scientists are debating whether the 80-plus dead bottlenose dolphins that have washed up along the U.S. Gulf Coast since January died from last year’s oil spill or of a winter cold snap.

The National Oceanic and Atmospheric Administration (NOAA) declared “an unusual mortality event” last week when the death toll of the dolphins that washed up in Louisiana, Mississippi, Alabama and Florida coasts reached nearly 60.

Since then, the death toll has climbed to at least 82.

The dolphins have no outward signs of oil contamination, but suspicions immediately turned to petrochemicals that were placed into the Gulf after a BP drilling platform exploded in April 2010.

Eleven workers were killed in the explosion, and about 206 million gallons of crude oil spewed into the Gulf last year.

Scientists in the Gulf were in the midst of investigating last year’s discovery of about 90 dead dolphins when officials became alarmed at a surge in dead baby dolphins turning up on beaches in January.

The latest spike in deaths 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.

The die-off has come at the start of the first dolphin calving season in the northern Gulf since the BP spill.

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 said that water temperatures abruptly plunged from the upper 50s into the 40s off Dauphin Island in January, just before the first two stillborn mammals were discovered.  He said a second wave of dolphin carcasses washed ashore after temperatures dipped again.

Fellow Dauphin Island scientists 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 told Reuters.

However, NOAA officials cited research showing that 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,” Moby Solangi, director of the Institute for Marine Mammal Studies in Gulfport, Mississippi, told Reuters.

Blair Mase, a marine mammal scientists for NOAA, said that the organizations is bracing for the number of deaths to jump further as the bottlenose calving season reaches full swing in the coming weeks.

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