Oil impaired gulf dolphins’ ability to deliver healthy calves

Y35 wasn’t supposed to die.

Like other dolphins exposed to oil from beneath the Deepwater Horizon, she was ill, but she was supposed to be one of the lucky ones. Wildlife veterinarians gave her a “good” prognosis. Even better, along with nine others assessed that year, she was pregnant. Knowing that dolphins have a gestation period of 12.5 months, the biologists assessing her gave her a due date of approximately February 29, 2012.

Y35 was last spotted on February 13, just a couple weeks before her due date and six months after her veterinary exam. Dolphins rarely leave Louisiana’s Barataria Bay, so her disappearance could only mean one thing. Her pregnancy was classified as a loss: either she died shortly before giving birth, or she delivered and then promptly died thereafter. Without its mother, the newborn dolphin would have quickly died as well.

For 87 days in 2010, some 210 million gallons of oil from wells below the Deepwater Horizon poured into the Gulf of Mexico. Last year, a group of governmental, academic, and non-governmental scientists reported as part of the post-spill Natural Resource Damage Assessment that as a result, bottlenose dolphins in Louisiana’s Barataria Bay were suffering from a host of maladies, including lung disease and adrenal problems. Many were underweight, some had low red blood cell counts, and quite a few were losing their teeth. It’s been five years since the blowout, and the bay’s dolphins continue to suffer.

In 2011, months after the leaky well had been capped, Lori H. Schwacke, chief of the Ocean and Human Health Branch within NOAA’s National Centers for Coastal Ocean Science, and her team conducted health assessments on 32 Barataria Bay dolphins. The site was chosen because it was heavily oiled both during and after the spill. The researchers compared those dolphins to 27 from Sarasota Bay, Florida, a site that remained oil-free following the spill, and one in which there already was a long-term ongoing study of marine mammal health.

Upon concluding those health assessments, the researchers tagged the dolphins and outfitted them with satellite and radio transmitters. Ten females at the time were confirmed pregnant through ultrasound, including Y35. As part of their ongoing monitoring through July 2015 in Barataria Bay, the researchers worked to identify whether or not those pregnancies resulted in the delivery of viable offspring. The approximate age of dolphin fetuses can be estimated from the size of their developing skulls. Therefore, the researchers were able to infer the approximate due dates for each of the pregnant females.

If a calf was spotted swimming along its presumed mother, then the pregnancy was considered successful. If, however, a calf was not spotted between two weeks and one year after its estimated due date, the pregnancy was classified as unsuccessful – either due to neonatal death or to spontaneous abortion.

Only 20% of pregnant dolphins produced viable offspring.

One dolphin, called Y01, was spotted pushing the carcass of a neonatal calf some ten months after her due date. That means that she aborted her 2011 fetus before the end of her third trimester, allowing her to become pregnant again by the spring of 2012. It also means that she had two reproductive failures in a row.

By contrast, dolphin pregnancies in Sarasota Bay, Florida have an estimated success rate of 83%.

And it isn’t just newborn calves that continue to suffer. As a part of this study, Schwacke and her colleagues discovered an annual survival rate of 86.8%. Of those dolphins assessed in 2011, 86.8% were still alive one year later. But in Sarasota Bay, a similar study showed an annual survival rate of 96.2%, and in Charleston, South Carolina, the annual survival rate was 95.1%. That’s an additional mortality rate of 8-9% beyond what is expected for coastal dolphin populations in the southeastern United States.

While the oil seems clearly to blame, it’s not the only threat that marine life on the gulf coast has to contend with. As marine mammal researcher Chris Parsons, professor of environmental science and policy at George Mason University, told me last year, “the [unusual mortality event] seems to have started before the Deepwater Horizon spill, and there may be other issues going on, too.” Those other issues include shipping noise, toxic algal blooms, nutrition problems, and more. The oil may be exacerbating a set of problems already present in and around Barataria Bay.

Still, the impacts on bottlenose dolphins – not to mention on fish, birds, and even corals – from the Deepwater Horizon spill will continue to be felt for years or even decades. Indeed, say the authors, “dolphin reproduction and survival is being impacted by chronic disease, indicating that the effects of the [Deepwater Horizon] oil spill have been long-lasting.” – Jason G. Goldman | 04 November 2015

Source: Lane, S.M. et al. (2015). Reproductive outcome and survival of common bottlenose dolphin sampled in Barataria Bay, Louisiana, USA, following the Deepwater Horizon oil spill. Proc. R. Soc. B. DOI: 10.1098/rspb.2015.1944.

Header image: Dolphin Y01 pushes her deceased offspring, via Louisiana Department of Wildlife and Fisheries.