Rising dolphin deaths linked to Deepwater Horizon spill

The April 2010 Deepwater Horizon oil spill helped spark a massive, ongoing die-off of dolphins in the Gulf of Mexico, a new study suggests.

Dead common bottlenose dolphins (Tursiops truncatus) examined in the region had lung lesions and adrenal gland damage, injuries previously linked to oil exposure, researchers report May 20 in PLOS ONE. Following the blow out at BP’s Macondo well five years ago, researchers have tracked a rising number of unusual deaths and health issues in Gulf dolphins, but have struggled to definitively pin the problems on the spill (SN: 4/18/15, p. 22).

“No feasible alternative causes remain that can reasonably explain the timing, location and nature of these distinct lesions and increase in deaths,” said Stephanie Venn-Watson, lead author of the report and a veterinary epidemiologist with the National Marine Mammal Foundation in San Diego, at a press conference May 20.

Since early 2010, the number of stranded cetaceans (dolphins, whales and porpoises that are dead or in need of assistance) in the Gulf has nearly quadrupled. From 2002 to 2009, an average of about 74 cetaceans became stranded each year. As of May 17, 2015, the five-year toll had reached 1,395.

Venn-Watson and colleagues collected 46 dead dolphins from the Louisiana, Mississippi and Alabama coasts from June 2010 to December 2012. The researchers also examined records of 106 other dolphin deaths clearly not related to the spill, including some from the Gulf coasts of Florida and Texas that died before the spill, plus others from North Carolina and South Carolina.

Of the spill-impacted dolphins, 33 percent had thinning of their adrenal gland cortex, which produces hormones essential for regulating metabolism and other functions. Only 7 percent of dolphins in the other group had such thinning. Adrenal gland abnormalities have been linked to oil exposure in mink and some kinds of birds.

Such damage could leave dolphins more vulnerable to infection and diseases, the researchers note. In fact, 22 percent of dolphins from the spill zone had bacterial pneumonia, compared with only 2 percent of dolphins not in the oil’s path. “These dolphins had some of the most severe lung lesions I have ever seen in wild dolphins,” said veterinary pathologist Kathleen Colegrove of the University of Illinois at Urbana–Champaign, a study coauthor.

Dolphins may be particularly vulnerable to lung damage from oil slicks because the animals take deep breaths at the surface and often hold their breath for long periods, which could extend exposure to toxic compounds, the authors suggest.

The lung and adrenal data for these dolphins are distinct from data for other dolphin die-offs, says molecular toxicologist Michael Twiner of the University of Michigan–Dearborn. Overall, the study makes a convincing link between the Deepwater Horizon oil spill and the health problems seen in the dolphins, he says.

Geoff Morrell, BP’s senior vice president for U.S. communications and external affairs, doesn’t agree. “This new paper fails to show that the illnesses observed in some dolphins were caused by exposure to Macondo oil,” Morrell said in a statement. 

Editor’s Note: This story was updated June 8, 2015, to correct a quote by Stephanie Venn-Watson. The phrase “increase in deaths” was misquoted as “injuries in death.”