Sea Turtles, Dolphins, and Whales - 10 years after the Deepwater Horizon Oil Spill

Deepwater Horizon Oil Spill

The 2010 Deepwater Horizon oil spill adversely affected wildlife throughout the Gulf of Mexico, including marine mammals, sea turtles, birds, fish, and invertebrates. 

NOAA, other federal and state natural resource agencies, and non-governmental organizations responded to the Deepwater Horizon oil spill to assist affected wildlife. Following the oil spill, many of the same groups contributed to the Natural Resource Damage Assessment. This process estimated the extent of natural resource losses, including wildlife and habitat, caused by the oil spill and related activities. This assessment ultimately informed plans for efforts to restore Gulf resources to the condition that would have existed if the incident had not occurred.

How did the Deepwater Horizon oil spill affect sea turtles and marine mammals?

Oil from Deepwater Horizon contaminated sea turtle habitat from open sea to shore.Kemp’s ridley, green, loggerhead, hawksbill, and leatherback sea turtles were affected. Oil on turtles restricts their movements, causing them to become exhausted, exposing them to harsh temperatures, and making them vulnerable to predators. Also, many sea turtles ingested oil or oil-contaminated prey and water, leading to potential toxic effects. During the spill, more than 300 oiled sea turtles were rescued from offshore areas. They were brought into rehabilitation, cleaned, and released back into the wild. In addition, eggs from 275 sea turtle nests were collected from beaches under threat from oil. They were relocated to the Atlantic coast where 14,000 hatchlings, mostly loggerheads, were later released.

Researchers used information collected during these rescue operations and subsequent studies to determine the number of sea turtles that died as a result of the spill. An estimated 4,900–7,600 large juvenile and adult sea turtles and between 56,000–166,000 small juvenile sea turtles were killed by the spill. Furthermore, an estimated 35,000 hatchlings were lost due to the effects of the spill and associated clean-up activities on sea turtle nesting beaches.

During response activities and surveys, workers observed more than 1,400 marine mammals in the surface slick. All 21 species of cetaceans found in the Gulf were exposed to oil through subsurface, surface, and air contamination. These cetaceans were exposed to oil when they inhaled contaminated air; aspirated liquid oil; ingested contaminated sediment, water or prey; or swam in contaminated waters. Lung impacts were the most obvious adverse health effect but impaired stress response and compromised immune function were also noted. 

The Natural Resource Damage Assessment found that at least 22 stocks (representing at least 15 species) of dolphins and whales overlap with the oil spill footprint. They had demonstrable, quantifiable injuries. Bay, sound, and estuarine bottlenose dolphin stocks, coastal and shelf dolphin stocks, and oceanic whale and dolphin stocks were all injured. Injuries included increased mortality, increased reproductive failure, and adverse health effects. The Barataria Bay and Mississippi Sound common bottlenose dolphins were two of the most severely injured stocks.

How have sea turtle populations been affected since the Deepwater Horizon oil spill?

We estimate a substantial number of sea turtles were killed as a result of the Deepwater Horizon oil spill. All impacted sea turtle species are threatened or endangered. Because turtles mature slowly, the populations will require decades of sustained restoration efforts to recover the numbers that were lost.

It is much more difficult to state specific effects from the oil spill on sea turtle populations. They are challenging to reliably count in the water, especially over large areas like the Gulf of Mexico. Also, sea turtles have complex life cycles ranging over thousands of miles. Places where they grow up, feed, and later reproduce can be very far apart.  

The number of nesting females is often used to study population trends. However, multiple factors influence the numbers of female turtles that nest from year to year. It can be difficult to use nesting trends to monitor sea turtle populations in the short-term and to evaluate the effects of a single event like the oil spill.  

Since 2010, the number of Kemp’s ridley nests has fluctuated annually, with a record high in 2017 and a 10-year low in 2019. It is very difficult to tease apart all of the factors that affect annual nest counts. The combined effects of the deaths of juvenile and adult Kemp’s ridleys caused by Deepwater Horizon and other causes of sea turtle mortality are both contributing factors. Environmental and biological factors also influence reproduction. It will take a longer period of monitoring and additional studies to better understand the causes of the current Kemp’s ridley nesting trend.

How have marine mammal populations been affected since the Deepwater Horizon oil spill?

The initial Natural Resource Damage Assessment studies focused on live and dead common bottlenose dolphins in Mississippi and Louisiana. They provided more detailed information on the exposure, injuries, and outcomes from the oil spill. These studies examined Barataria Bay (heavily oiled), Mississippi Sound (moderately oiled), and Sarasota Bay (un-oiled) dolphins. We learned that dolphins in Barataria Bay and Mississippi Sound, which were exposed to oil, suffered significant health effects. These effects included lung disease from inhalation and  aspiration of oil, impaired stress response, and compromised immune function. They may have led to increased infection and disease. 

Many dolphins did not survive and became part of the largest and longest lasting cetacean mortality event ever recorded for the northern Gulf of Mexico. A total of 1,141 dolphins died from March 2010 through July 2014. Photographic surveys of live dolphins from 2010 through 2014 showed low survival and poor reproductive success for dolphins from Barataria Bay and Mississippi Sound. Additionally, the stranding network documented high numbers of perinatal strandings since the spill, especially in 2011.

Based on studies conducted from 2010 through 2015, bottlenose dolphins in Barataria Bay suffered:

• Increased mortality (35 percent greater than expected based on studies of other bottlenose dolphin populations)
• Increased failed pregnancies (46 percent greater than expected)
• Increased likelihood of having adverse health effects (37 percent greater than expected). 

Scientists estimated that the Barataria Bay stock of dolphins would take 39 years to recover in the absence of active restoration. They also estimated a 51 percent population loss in the population size approximately 10 years after the spill. The results from the coastal common bottlenose dolphin studies and information on exposure of offshore cetaceans were used to estimate the impacts to continental shelf and oceanic cetaceans. For these species, the number of years to recovery without active restoration ranged from 10 to 105 years.

Are there any ongoing problems for sea turtles and marine mammals caused by the Deepwater Horizon oil spill?

Long-term and potentially life long effects of oil exposure experienced by sea turtles as a result of the Deepwater Horizon oil spill are unknown. Sea turtles live in the ocean and spend most of their time underwater, so it is extremely difficult to monitor them over the course of their lives. We are largely constrained to studying turtles that are stranded, that come ashore to nest, and that are captured during field studies or counted by aerial surveys. We have not seen greater numbers of diseased turtles or those with other health problems after 2010 compared to years before the spill. Nonetheless, many of the sea turtles exposed to the Deepwater Horizon oil spill were small turtles; some would just now be approaching maturity. Others will still take many years to mature. It is important to continue monitoring sea turtle populations in the Gulf as part of an ongoing, long-term effort.  
Studies into longer term impacts on dolphin health post-Deepwater Horizon have been funded by the Gulf of Mexico Research Initiative. These studies specifically focus on Barataria Bay common bottlenose dolphins, examining causes of reproductive failure and long-term health impacts from the spill. These studies developed new tools, techniques, and analyses that will be useful for future assessments. These tools were developed and refined in managed care facilities, then adapted for use in wild bottlenose dolphins. These new methods will be used in emergency response, conservation medicine efforts, future NRDA assessments and in health assessments.

What is being done to restore sea turtles and marine mammals lost due to the Deepwater Horizon oil spill?

The Deepwater Horizon oil spill Trustees released the Strategic Framework for Sea Turtle Restoration Activities and the Strategic Framework for Marine Mammal Restoration Activities in 2017. These frameworks provide summaries of the overarching restoration goals, approaches to restoration, and considerations for the selection of restoration projects to benefit each resource.  

Based on the framework, the Trustees are prioritizing sea turtle restoration projects that reduce primary threats on land and in the marine environment. The Trustees established several approaches to restoration, including:

• Reducing bycatch in commercial and recreational fisheries
• Enhancing and protecting nesting beach habitat
• Increasing survival through improved stranding network capacity and enhanced mortality investigation
• Reducing injury and mortality from vessel strikes

Similarly, for marine mammals, the Trustees are prioritizing marine mammal restoration projects that reduce stressors and increase marine mammal survival. The restoration approaches include:

• Reducing bycatch in commercial fisheries
• Reducing bycatch in hook and line fisheries
• Increasing survival through better understanding causes of illness and early detection of human-caused and natural threats
• Reducing impacts from noise
• Reducing impacts from vessel collisions
• Reducing impacts from illegal feeding and harassment activities
• Increasing Marine Mammal Protection Act state enforcement
• Protecting key marine mammal habitats

More Information

• Deepwater Horizon restoration efforts
• NOAA’s Sea Turtle Early Restoration Project
• Open Ocean TIG Restoration Planning Efforts - OO TIG’s Restoration Plan #2 (marine mammal and sea turtle projects)
• Alabama TIG Restoration Planning Efforts - AL TIG’s Restoration Plan #2 (marine mammal and sea turtle projects)
• Individual restoration projects