A Dolphin Baby Bust in the Gulf of Mexico Worries Scientists

Researchers find that only 20 percent of bottlenose dolphins captured in an area contaminated by the Deepwater Horizon oil spill were able to reproduce.
(Photo: Wolfgang Kaehler/LightRocket via Getty Images)
Nov 9, 2015· 2 MIN READ
David Kirby has been a professional journalist for 25 years. His third book, Death at Seaworld, was published in 2012.

The devastating impact of the 2010 Deepwater Horizon oil spill on dolphins in the Gulf of Mexico may last for generations, a new study has found.

The research, published in the journal Proceedings of the Royal Society, reported that only 20 percent of pregnant dolphins captured in oil-contaminated Louisiana’s Barataria Bay in 2011 had produced viable calves. A study of similar dolphins in Florida’s Sarasota Bay reported a pregnancy success rate of 83 percent.

Longevity among the Barataria Bay dolphins was also lower than other populations. The annual survival rate was just 87 percent, compared to a 96 percent survival rate found in a similar bottlenose population, according to the study.

We are very concerned about the high rate of reproductive failures among Barataria Bay dolphins, as recovery of the population depends on successful reproduction,” the study’s co-author Cynthia Smith, executive director of the National Marine Mammal Foundation, said in an email.

“Barataria Bay dolphins were more likely to be underweight, have moderate-severe lung disease, and have an impaired stress response. Any one of these conditions could put a pregnancy at risk, as well as make it difficult to care for a newborn,” she added.

The damage could last for decades.

“We are currently estimating that recovery of the Barataria Bay dolphin population would take nearly 40 years,” Lori Schwacke, a scientist at the National Oceanic and Atmospheric Administration (NOAA), said in an email. “Monitoring the health of the Barataria Bay population over time and throughout the restoration process will be especially critical to ensure the effectiveness of the restoration efforts.

Researchers from NOAA the marine mammal foundation looked at data from 32 Barataria Bay dolphins that were temporarily captured and examined for ill-health effects in August 2011.

Among the dolphins examined, 10 females were determined to be pregnant through ultrasound testing. Of those 10, only two were observed swimming with their calves after 47 months of follow up monitoring.

“Experienced marine mammal veterinarians gave approximately half of the sampled dolphins a guarded or worse prognosis for survival, while 17 percent received a poor or grave prognosis,” the scientists wrote.

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Other dolphin populations in the Gulf suffered similar losses, Smith said.

“We also studied the reproductive rate of dolphins living in Mississippi Sound, which was also impacted,” Smith said. “We determined that the reproductive success rate in Mississippi Sound was equally as low during 2013-2014.”

“Whether the observed reproductive failures are directly related to oil exposure or indirectly related to the oil through a cascade of other health impacts to the adult females, cannot currently be determined,” the authors wrote. “However, given the documented poor health of Barataria Bay dolphins…it is unsurprising to find impacts on reproduction as well.”

The BP Deepwater disaster released some 4.9 million barrels of crude oil into the Gulf, making it the worst marine oil spill in U.S. history. Potential routes of oil exposure in the dolphins include ingestion, inhalation and absorption through the skin.

In February, a government-funded study reported that, since early 2010, 1,305 dolphins were discovered stranded on Gulf shores, about 94 percent of which were found dead, making it the longest marine mammal die-off in the Gulf in recorded history.

In addition to damage from the oil itself, dolphins and other marine mammals in the Gulf were also likely harmed through exposure petroleum dispersants, particularly a product known as Corexit.

If history is any guide, the long-term impact of the Deepwater Horizon spill could be devastating. In the year following the 1989 Exxon Valdez oil spill in Alaska, 33 percent of one resident killer whale pod and 41 percent of a transient group vanished, “with neither group recovering to pre-spill numbers even two decades later,” the new study said.