Study documents high rate of reproductive failure in dolphins hit by Deepwater Horizon oil spill

Impacts of oil pollution expected to affect Barataria Bay populations for a long time

Dolphin Y01 pushes a dead calf in March, 2013. This behavior is sometimes observed in female dolphins when their newborn calf does not survive. Credit Louisiana Department of Wildlife and Fisheries
A dolphin pushes a dead calf in March, 2013. This behavior is sometimes observed in female dolphins when their newborn calf does not survive. Photo courtesy Louisiana Department of Wildlife and Fisheries.

Staff Report

There’s already a wealth of research showing that the Deepwater Horizon oil disaster in the Gulf of Mexico was very bad for coastal dolphins. One study, for example, showed dolphins in Barataria Bay exposed to BP’s oil suffered lung disease and hormone deficiencies.

In a report released this week, a team of researchers led by National Ocean and Atmospheric Administration scientists is reporting a high rate of reproductive failure in dolphins exposed to the 2010 spill. The biologists monitored bottlenose dolphins in heavily-oiled Barataria Bay for five years following the spill.

The findings, published in Proceedings of the Royal Society today, suggest that the effects of the Deepwater Horizon oil spill will be long-lasting. Only 20 percent of the sampled dolphins that were pregnant produced viable calves. By contrast, the pregnancy success rate in Sarasota Bay — not affected by the spill — was about 83 percent during the study period.

The overall survival rate of dolphins in Barataria Bay is about 86.8 percent, compared to other populations with a 95 percent survival rate. The reduced reproductive potential, along with decreased survival, will have long-term consequences for the Barataria Bay dolphin population, the study found.

The scientists looked closely at individual dolphins by giving them regular checkups. In August 2011, a team of independent and NOAA scientists gave dolphin Y35 a good health outlook. Based on the ultrasound, she was in the early stages of pregnancy, but unlike many of the other dolphins examined that summer day, Y35 was in pretty good shape. She wasn’t extremely underweight or suffering from moderate-to-severe lung disease, conditions connected to exposure to Deepwater Horizon oil in the heavily impacted Barataria Bay.

Veterinarians did note, however, that she had alarmingly low levels of important stress hormones responsible for behaviors such as the fight-or-flight response. Normal levels of these hormones help animals cope with stressful situations. This rare condition–known as hypoadrenocorticism–had never been reported before in dolphins, which is why it was not used for Y35 and the other dolphins’ health prognoses.

Less than six months later, researchers spotted Y35 for the last time. It was only 16 days before her expected due date. She and her calf are now both presumed dead, a disturbingly common trend among the bottlenose dolphins that call Barataria Bay their year-round home.

Of 10 Barataria Bay dolphins confirmed to be pregnant during the 2011 health assessment, only two successfully gave birth to calves that have survived. This unusually low rate of reproductive success— only 20 percent — stands in contrast to the 83 percent success rate in the generally healthier dolphins being studied in Florida’s Sarasota Bay, an area not affected by Deepwater Horizon oil.

At least half of the dolphins with failed pregnancies also suffered from moderate-to-severe lung disease, a symptom associated with exposure to petroleum products. The only two dolphins to give birth to healthy calves had relatively minor lung conditions.

The study was conducted as part of the Natural Resource Damage Assessment (NRDA) for the Deepwater Horizon oil spill. The complete results from the NRDA are documented in the Draft Programmatic Assessment and Restoration Plan, which is currently out for public comment. We will accept comments on the plan through December 4, 2015.

This research was conducted under the authority of Scientific Research Permit nos. 779-1633 and 932-1905/MA-009526 issued by NOAA Fisheries pursuant to the U.S. Marine Mammal Protection Act. Protocols for the dolphin health assessments were reviewed and approved by the NOAA Institutional Animal Care and Use Committee.


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