The Die-Off of Bottlenose Dolphins: Unusual Mortality Events

By Roberta Attanasio, IEAM Blog Editor

During the past year, bottlenose dolphins – known sentinels of ocean health – have been dying at an alarming rate along the US East and Gulf Coasts, albeit for apparently different causes. Along the East Coast, a major cause of death is dolphin morbillivirus, a pathogen related to human measles and canine distemper, which is responsible for an epidemic probably fueled by current lack of immunity and perhaps aided by global warming and pollution – factors that may increase the dolphin susceptibility to infection.

Bottlenose dolphins

Bottlenose dolphins (Tursiops truncatus), by Allison Henry, Northeast Fisheries Science Center, NOAA

The measles-like virus also caused an outbreak in 1987-1988, wiping out 50 percent of the coastal migratory dolphins. During the ongoing epidemic, more than 1,000 dolphins have already succumbed to the infection, although the toll may be higher because of uncounted dolphins that die offshore. The epidemic started in June 2013 and prompted the declaration of an Unusual Mortality Event – an unexpected event that involves a significant die-off and demands immediate response under the Marine Mammal Protection Act of 1972.

Additional causes of concern are the possible spread of dolphin morbillivirus from East Coast to Gulf Coast dolphin populations as well as its spread from dolphins to other marine mammals, such as humpback and pygmy sperm whales.

Meanwhile, on the Gulf Coast, from the Texas-Louisiana border through the Florida Panhandle, more than 1,000 dolphins have succumbed since the first deaths were reported four years ago, before the British Petroleum (BP) Deepwater Horizon oil spill occurred. However, the causes of these deaths, which have also been labeled as Unusual Mortality Events, are not clear.

The Deepwater Horizon spill does contribute to the current dolphin die-off, though, as shown by a study recently published (December 18, 2013) in the journal Environmental Science and Technology and entitled “Health of Common Bottlenose Dolphins (Tursiops truncatus) in Barataria Bay, Louisiana, Following the Deepwater Horizon Oil Spill.

The research was carried out by a team of government, academic, and non-governmental scientists as part of the Natural Resource Damage Assessment (NRDA), in which response agencies work in cooperation with BP to identify potential injuries to natural resources as well as loss of services resulting from the spill, to develop a restoration plan, and to implement and monitor its effectiveness.

For the study, scientists captured and examined bottlenose dolphins from Barataria Bay, Louisiana (an area that received heavy and prolonged oiling), as well as dolphins from Sarasota Bay, Florida, which represented the reference site (no oil was observed in this area.) All dolphins were released after the examination.

Results show that dolphins from Barataria Bay present evidence of hypoadrenocorticism, which indicates adrenal toxicity – a finding previously reported for laboratory mammals exposed to oil. In addition, Barataria Bay dolphins were 5 times more likely to develop moderate-to-severe lung disease when compared to Sarasota Bay dolphins. In total, 29 dolphins were examined in Barataria Bay – of these, 48% were given a guarded or worse prognosis, and 17% were not expected to survive.

Many disease conditions observed in Barataria Bay dolphins are considered uncommon but are consistent with exposure to petroleum hydrocarbon.

Lori Schwacke, the study’s lead author and an experienced investigator of dolphin health, said in a public release, “I’ve never seen such a high prevalence of very sick animals — and with unusual conditions such as the adrenal hormone abnormalities.”  In addition, she told NPR “Many of the Barataria Bay dolphins were underweight. Their blood tests showed abnormalities such as elevated liver enzymes or markers of inflammation. Many were hypoglycemic, which is low glucose or low blood sugar, and several were anemic.”

The scientists examined alternative hypotheses to explain the conditions observed in Barataria Bay dolphins, as for example exposure to other toxic chemicals known to induce adverse health effects in marine mammals. However, blubber samples from the Barataria Bay dolphins contained relatively low concentrations of these toxic chemicals – including polychlorinated biphenyls (PCBs) and pesticides – as compared to other coastal bottlenose dolphin populations.

Erin Fougeres, a marine mammal biologist with the National Oceanic and Atmospheric Administration’s fisheries service, told the New York Times “We can’t conclusively say what role oil played at this time, but the event is pretty unprecedented in terms of how long the die-off has been occurring and how many have died as part of that event.”

Scientists are currently carrying out studies based on continued photographic monitoring – as part of the Deepwater Horizon NRDA – to understand the impact on dolphin reproduction and long-term survival.

14 thoughts on “The Die-Off of Bottlenose Dolphins: Unusual Mortality Events

  1. Wayne Landis

    First, it is great to see a blog for Integrated Environmental Assessment and Management. My environmental toxicology students will find it very interesting. I hope the blog provides a forum for a lot of interesting and productive discussion.
    Second, the introduction to the this particular entry starts with “known sentinels of ocean health “. I certainly understand that the idea of “ocean” or “ecosystem health” is well ingrained in the public mind and that it is a powerful metaphor. But I would maintain that it is not applicable to a factual or scientific framework for thinking about the relationships between chemicals and ecological effects. There are a number of investigators that have convincingly written that the construct of “ecosystem health” is an inappropriate term. Suter (1993) led with a detailed evaluation of the idea of ecosystem health and found it wanting. He particularly tied the idea of health to the critique of the various indexes that were and are still being used to measure “stream health or integrity”.
    Larry Kapustka and I weighed in (Kapustka and Landis 1998) and asked how can an construct that does not have the properties of an organism have a property such as health? It is the same as asking if your car is healthy. A car is healthy because of the value we place on it taking us to work or the grocery store so it is a valuation statement, not an intrinsic property of the car. Similarly “ecosystem health” is a reflection of how human cultural systems value the services of that particular ecological structure and is dependent on that particular set of cultural values.
    Robert Lackey has been eloquent in his critique of terms such as “ecosystem health” and identifies them as part of normative science (Lackey 2001, 2004). Normative science is defined by Lackey (2004) as “information that is developed, presented, or interpreted based on an assumed, usually unstated, preference for a particular policy or class of policy choices.” In other words the use of such terms is advocacy disguised as science. He argues for the use of policy neutral terms such as alteration or change as opposed to “good, bad, degradation, improvement or integrity”.

    It is not desirable to use terms such as health, degradation or other terminology in describing why a particular organism should be studied or that the toxicological effects reported. In this instance dolphins and the other toothed whales are iconic species deeply valued by our society. The ecological resources and environmental conditions necessary to maintain the dolphins are important.

    Three, the article discusses what is essentially ecoepidemiology. This is an important part of the science and essential in connecting laboratory science to being able to describe cause-effect in open ecological systems. Epidemiology has proven essential to describing the environmental factors that are linked to human well-being. I hope that the blog and the journal can publish similar papers.

    Kapustka LA and W. G. Landis WG. 1998. Ecology: the science versus the myth. Hum. Ecol. Risk Assess.4:829-838.

    Lackey, RT. 2001. Values, policy, and ecosystem health. BioScience. 51: 437-443.

    Lackey, RT. 2004. Normative science. Fisheries. 29: 38-39.

    Suter, GW 1993. A critique of ecosystem health concepts and indexes. Environ. Toxicol. Chem. 12: 1533-1539.

    1. Ray Kinney

      Wayne, when thinking about the statement: “He particularly tied the idea of health to the critique of the various indexes that were and are still being used to measure “stream health or integrity”, very few environmental assessments really put the emphasis on trying to understand the toxicologic influences that have changed the stream conditions from what they seemed to be pre-human contact. If ‘stream health’ were an attempt to understand actual stream conditions before our societies came along and introduced 85k new chemicals to release into the air and waters, I don’t have a quarrel with the terminology. But, almost all assessments of stream ‘health’ are done with much more limited scope and are more arbitrary in setting health parameter indicators that are only pertaining to more recent ‘more altered’ conditions to define as ‘healthy’. Regulatory politics and short term thinking dominate such assessment scope parameters. It seems valid to think about prehistoric site specific stream characteristics that span thousands of years, with indications of relative stability of assemblages, wide biodiversity, and vibrant appearance, as generally being something approaching “health”. Once the politics and short term thinking dominate discussion of ‘stream health’ I become much more disenchanted with the terminology used and consider that politics was once again muddling ‘science’. Richard Feynman was quoted to having said, “The first principle is that you must not fool yourself- and you are the easiest person to fool” so, it may be that I’m not adequately comprehending your “a factual or scientific framework for thinking about the relationships between chemicals and ecological effects’ statement. I agree that it is very tricky to communicate well about the term ‘environmental health’ or ‘ocean health’ if we don’t have very similar constructs about what ‘health’ is, but I guess I’ll have to try to read Suter ’93 and your paper to clarify more particulars of your point.

  2. Roberta Attanasio

    Thank you, Wayne, for your thoughtful and interesting comment. You make a very important point, and the car analogy explains very well the concept at the basis of your concern for the use (or misuse) of “ocean health”. The term is indeed ingrained all around and my take is that, right or wrong, it’s going to stay. For example, we have an Ocean Health Index, a Global Ocean Health Program, the Wendy Schmidt Ocean Health XPRIZE, a New York Times article (December 22, 2013) with the revealing title of “Focus on Ocean’s Health as Dolphin Deaths Soar”. It looks like “ocean health” is the way to go – we can’t swim against the current.

  3. Tylah Hankerson

    If humans want to preserve the ocean and its inhabitants, then we must watch how we use and abuse the water. The disappearance of these dolphins is sad and alarming, especially since we play a huge part in why they are disappearing. Infectious disease in marine life is no different than infectious disease in humans. If a disease strikes a marine ecosystem and the animals have no immunity to it then they will die at a rate that catches the attention of the public. The same happens when a disease strikes humans. However, once a disease affects humans, doctors and scientist do everything to try to slow the spread of the disease and find cures. The same should go for marine animals. If global warming, pollution, and oil spills are acting as a catalyst in the spread of an infectious disease, in this case dolphin morbillivirus, then we should go straight to the source and fix how we abuse the ocean. More laws and stricter fines should be implemented for those who pollute the ocean. More consequences should be placed on oil companies who are careless. Laws, fines, and consequences won’t completely eliminate infectious disease from affecting marine life but it could slow the spread significantly.

    1. Rebecca Park

      You made a good point, I didn’t even think about how humans could played a huge part in the decline of the dolphins. People need to realize that dolphins are bio-indicators that let scientists know the status of the dolphin’s environment, With a clean marine environment, the dolphin’s immune system will surely boost as well. As well as more laws and stricter fines (to help create a clean marine environment), there should be a stronger law enforcement. When you go to the beach there are laws that doesn’t allow people to litter, but you see litter everywhere because those laws are not really enforced.

    2. Maria Mbugua

      You made a very good point on how human beings are using and abusing water. The dolphins may be dying because of all the waste industries lob in the waters nearby. This is seen particularly in the decline of the dolphins immune system. Researchers need to look closely at dolphin morbillivirus before it wipes out the entire population of dolphins. There should have strict guidelines that enforce clean water for the marine so that the marine life longer lives . People should be troubled by diseases like dolphin morbillivirus where no one understands them because they can cross barriers and infect and kill many humans.

    3. dpitts5

      You made some excellent points in this post. I must agree that humans do play a very significant part of role in the disappearance of dolphins with their careless littering and pollution. There should definitely be stricter fines and consequences placed on big oil companies and anyone who cause some of these detrimental affects to dolphins . In particular I think the money they should have to pay should go directly to funding research for dolphins and other marine animals which their careless behavior affects. Some of the money should also go to creating artificial habitats for dolphins who are suffering from pollution more than others until their permanent home is clean enough for them to return.

      1. Tylah Hankerson

        That’s really good idea. Good is actually an understatement. That’s a brilliant idea. I think that is definitely something that should be looked into.

  4. Samantha Deochand

    The post begins by introducing dolphin morbillivirus as the pathogen causing an alarming amount of death among bottlenose dolphins before the BP oil spill. The article continues to explain the effects of the oil spill on dolphins, including adrenal toxicity and lung disease, and highlights the fact that these diseases are “considered uncommon.” While reading this article, I began to wonder about the big picture. Before the oil spill, there was a pathogen wreaking havoc and causing death in the bottlenose dolphin population that was so alerting. The oil spill occurred and this had a devastating effect on marine life in general. Bottlenose dolphins specifically suffered from diseases that were previously uncommon to them. Because they are uncommon, the body may not have mechanisms to compensate and cope with the onset of these disease. Therefore, these disease could have compromised the immune system’s ability to protect the dolphins. The dolphins could have died from the disease itself or pathogens like dolphin morbillivirus. Looking at this situation from an evolutionary standpoint, dolphin produced during this time and successfully mature may have slightly evolved to adapt to their new environment. Subsequently, offspring produced here on out may have evolutionary adaptations to help combat strains from their environment. These adaptations could very well be in the immune system to enhance the ability to combat multiple stresses.

    1. sarah

      I agree on the idea of evolutionary adaptions, or some may call it, natural selection. It is a probable event that can happen and will happen eventually. However, base on the article, out of the 29 dolphins that were studied at Barataria Bay, nearly half of the dolphins were given a worse prognosis and on top of that 17% of the dolphins are not expected to survive. The dolphin morbillivirus is an uncommon disease that affected dolphins and in addition to the disease, there was an oil spill that further contribute to the high death rate of the dolphins. We need to find a solution that will quickly resolve the situation. I don’t think evolutionary adaptation will happen fast enough to overcome the rate of dolphins dying every year.

  5. Rebecca Park

    The decline of the dolphins is pretty serious so maybe drastic measure need to be taken. Since dolphin morbillivirus is closely related to the human measles maybe there is a possibility dolphin morbillivirus can be eradicated through a vaccine. Since scientist fear that the dolphin morbillivirus will spread to whales, scientists should find a way to naturally isolate the dolphins and give them the vaccine to boost the dolphin’s immune system. As Tylah stated cleaning up the marine environment would be the best and natural way for the dolphins to have a healthy life.

  6. Aaron Alcala

    While using the word “health” may be controversial when describing the ocean or ecosystem, it may actually refer to the observation of the health of the actual organisms living in the ocean/ ecosystem. Studying the health of these organisms and how they are affected from ecological changes is important. Studying various ecosystems, such as oceans, may be useful to humans for various direct and indirect reasons. The obvious direct reason is that we use some marine animals as food. Dolphins may be affected by changes in their diets due to ecological changes, such as global warming or pollution. They are predators, so they may eat some of the same type of animals humans fish for. Studies stated above did show a correlation between pollution and decreased dolphin health, so we should continue to monitor the health of organisms in the ocean affected by pollution.

    Dolphins are also mammals, so studying the rise of morbillivirus, the measles-like virus, could definitely be useful for humans. Studying how changes in the ecology might have affected dolphin immunity could help humans. We can find out how to prepare ourselves for any ecological changes.

  7. cns2392

    It can be expected that changes in human activities will cause changes in entire ecosystems as a result. While we may think that minor oil spill or contamination leak only bothers the algae and organismal level biota, it is often forgotten that smaller species need those small organisms for energy, and larger species need the smaller, and so on. With that being said, all species have the potential to build up pollutants in their tissues. Pollutants have been shown to cause suppressed immune systems and reproductive failures, according to a study of bottlenose dolphins near Galveston Bay, TX. ( Since dolphins have the ability to acquire relatively high levels of toxins in their organs, muscles, and blubber, as a higher order species like humans, the dolphins are bioindicators of high pollutant levels in coastal areas. Coastal ecosystems are already quite fragile due to high volume of tourism, water sport activities, and overfishing. Recognizing pollutants as a preventable threat and introducing risk management for contaminated sites would be the first step in reducing threats to bottlenose dolphins and ultimately improving their entire ecosystems.

  8. Nhan Ta

    The dolphin morbillivirus outbreak, like many other deadly outbreaks, will probably last until animals are no longer susceptible. In an article that I’ve read, it was stated that the dolphins that survived the morbillivirus outbreak of 1987-1988 had antibodies against the virus, enabling them to become resistant ( These dolphins then prevented other young dolphins without immunity from being infected through what is known as “herd immunity”. After the massive die-off of infected dolphins, the number of dolphins with the resistant antibodies increased, so the chances of a susceptible dolphin coming into contact with the infection decreased as a result. The article also mentioned that besides pollutants, one of the reasons why bottlenose dolphins are now experiencing a similar outbreak is that the population of dolphins with immunity have left or died, leaving the new population susceptible. I thought this gave a more interesting insight as to why dolphins are currently dying from a viral infection that appeared decades ago. I agree with Rebecca, who suggested that scientists should develop a vaccine for the virus. Hopefully, once a successful vaccine has been created and given to the dolphin population, another instance of herd immunity can occur.


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