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An inside look at the science of cleaning up and fixing the mess of marine pollution

Latest NOAA Study Ties Deepwater Horizon Oil Spill to Spike in Gulf Dolphin Deaths


Group of dolphin fins at ocean surface.

A study published in the journal PLOS ONE found that an unusually high number of dead Gulf dolphins had what are normally rare lesions on their lungs and hormone-producing adrenal glands, which are associated with exposure to oil compounds. (NOAA)

What has been causing the alarming increase in dead bottlenose dolphins along the northern Gulf of Mexico since the Deepwater Horizon oil spill in the summer of 2010? Independent and government scientists have found even more evidence connecting these deaths to the same signs of illness found in animals exposed to petroleum products, as reported in the peer-reviewed online journal PLOS ONE.

This latest study uncovered that an unusually high number of dead Gulf dolphins had what are normally rare lesions on their lungs and hormone-producing adrenal glands.

The timing, location, and nature of the lesions support that oil compounds from the Deepwater Horizon oil spill caused these lesions and contributed to the high numbers of dolphin deaths within this oil spill’s footprint.

“This is the latest in a series of peer-reviewed scientific studies, conducted over the five years since the spill, looking at possible reasons for the historically high number of dolphin deaths that have occurred within the footprint of the Deepwater Horizon spill,” said Dr. Teri Rowles, one of 22 contributing authors on the paper, and head of NOAA’s Marine Mammal Health and Stranding Response Program, which is charged with determining the causes of unusual mortality events.

“These studies have increasingly pointed to the presence of petroleum hydrocarbons as being the most significant cause of the illnesses and deaths plaguing the Gulf’s dolphin population,” said Dr. Rowles.

A System out of Balance

In this study, one in every three dead dolphins examined across Louisiana, Mississippi and Alabama had lesions affecting their adrenal glands, resulting in a serious condition known as “adrenal insufficiency.” The adrenal gland produces hormones—such as cortisol and aldosterone—that regulate metabolism, blood pressure and other bodily functions.

“Animals with adrenal insufficiency are less able to cope with additional stressors in their everyday lives,” said Dr. Stephanie Venn-Watson, the study’s lead author and veterinary epidemiologist at the National Marine Mammal Foundation, “and when those stressors occur, they are more likely to die.”

Earlier studies of Gulf dolphins in areas heavily affected by the Deepwater Horizon oil spill found initial signs of this illness in a 2011 health assessment of dolphins living in Barataria Bay, Louisiana. NOAA scientists Dr. Rowles and Dr. Lori Schwacke spoke about the results of this health assessment in a 2013 interview:

“One rather unusual condition that we noted in many of the Barataria Bay dolphins was that they had very low levels of some hormones (specifically, cortisol) that are produced by the adrenal gland and are important for a normal stress response.

Under a stressful condition, such as being chased by a predator, the adrenal gland produces cortisol, which then triggers a number of physiological responses including an increased heart rate and increased blood sugar. This gives an animal the energy burst that it needs to respond appropriately.

In the Barataria Bay dolphins, cortisol levels were unusually low. The concern is that their adrenal glands were incapable of producing appropriate levels of cortisol, and this could ultimately lead to a number of complications and in some situations even death.”

Swimming with Pneumonia

Ultrasounds showing a normal dolphin lung, compared to lungs with mild, moderate, and severe lung disease.

Ultrasounds showing a normal dolphin lung, compared to lungs with mild, moderate, and severe lung disease. These conditions are consistent with exposure to oil compounds and were found in bottlenose dolphins living in Barataria Bay, Louisiana, one of the most heavily oiled areas during the Deepwater Horizon oil spill. (NOAA)

In addition to the lesions on adrenal glands, the scientific team discovered that more than one in five dolphins that died within the Deepwater Horizon oil spill footprint had a primary bacterial pneumonia. Many of these cases were unusual in severity, and caused or contributed to death.

Drs. Rowles and Schwacke previously had observed significant problems in the lungs of dolphins living in Barataria Bay. Again, in 2013, they had noted, “In some of the animals, the lung disease was so severe that we considered it life-threatening for that individual.”

In other mammals, exposure to petroleum-based polycyclic aromatic hydrocarbons, known as PAHs, through inhalation or aspiration of oil products can lead to injured lungs and altered immune function, both of which can increase an animal’s susceptibility to primary bacterial pneumonia. Dolphins are particularly susceptible to inhalation effects due to their large lungs, deep breaths, and extended breath hold times.

Learn more about NOAA research documenting the impacts from the Deepwater Horizon oil spill and find more stories reflecting on the five years since this oil spill.

Author: Office of Response and Restoration

The National Ocean Service's Office of Response and Restoration (OR&R) provides scientific solutions for marine pollution. A part of the National Oceanic and Atmospheric Administration (NOAA), OR&R is a center of expertise in preparing for, evaluating, and responding to threats to coastal environments. These threats could be oil and chemical spills, releases from hazardous waste sites, or marine debris.

5 thoughts on “Latest NOAA Study Ties Deepwater Horizon Oil Spill to Spike in Gulf Dolphin Deaths

  1. Thanks for this report. So what is being done about this? And how do we stop more disasters from existing rigs, and stop future rigs from even starting up?

    • From NOAA’s end of things, quite a few things are being done. The state and federal trustees, including NOAA and our scientific colleagues at universities and institutions around the Gulf, are engaged in a rigorous, scientific process of injury assessment (known as Natural Resource Damage Assessment) and are still analyzing the data, conducting studies, and evaluating what happened. The first round of health assessments began the summer after the well was capped. These studies, which have taken place at different times over the past five years, aim to understand and verify the potential impacts to dolphins and other marine life affected by the oil spill, which will inform the restoration required to help these creatures.

      Our obligation under the Oil Pollution Act is to restore the public’s natural resources injured by the Deepwater Horizon spill to the condition they would have been in but for the spill and to compensate the public for the services of those natural resources that were injured or lost. In addition to assessing the damage, we are undertaking early restoration and developing a long-term restoration plan with public involvement to meet that responsibility.

      The assessment is a thorough and time-consuming process by which we evaluate the best scientific evidence available to ensure we understand the injuries caused by the spill, as well as the most appropriate means to restore for those injuries and to compensate for the lost use of the Gulf’s resources while they are injured. The restoration planning effort involves a great deal of public outreach to ensure we consider the public’s perspective when making restoration decisions. You can learn more and stay tuned to the restoration efforts here:

      In fact, today the trustees just released 10 proposed early restorations projects that would benefit sea turtles, birds, and fish; increase recreational opportunities; and improve nearshore and reef habitats. You can review and give feedback on these projects at

  2. Out of all the money that the tax payers pay to the government, there surely is enough, for a team to determine how to fix this problem, and if it’s found to be the cause of the previous oil spill from BP…surely they will want to be a part of the process that gives vaccines to dolphins, or other ways, to keep these glorious creatures from baring an extremely violent death. God gave breath to these wonderful creatures, leave it up to mankind to take it away

  3. What we have to remember is that in the bible it states that we will have dominion over all creatures….. and that also means that these creatures are our wards it is up to us to take care of them…. shame on us for allowing this to happen and holding no one responsible. Monetary levee’s are a joke to these company’s . We have to realize that when these catastrophe’s happen we are just destroying ourselves.

  4. I want to Thank Everybody that works at NOAA for choosing to make a Difference in Our World. No one joins an Organization like NOAA to find fame and fortune, You joined because You Care about Our Environment and all the Lifeforms within it and that makes Each and Everyone of You very Special and irreplaceable. GOD Bless You All.

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