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Cetacean Unusual Mortality Event in Northern Gulf of Mexico Investigation Results

April 12, 2016: Gulf of Mexico perinatal dolphin deaths likely result of oil exposure

Study finds higher rate of illness in dead fetuses and newborns after Deepwater Horizon oil spill

A dead prenate dolphin stranded on Grande Terre Beach, Louisiana, in June 2013. Photo credit: Louisiana Department of wildlife and Fisheries

The increased number of stranded stillborn and juvenile dolphins found in the Gulf of Mexico from 2010 to 2013 was likely caused by chronic illnesses in mothers who were exposed to oil from the Deepwater Horizon spill, scientists said today.

The paper, published in the journal Diseases of Aquatic Organisms, is part of an effort to explain the unusual mortality event in the Gulf involving bottlenose dolphins, between early 2010 and continuing into 2014. The investigations into both the fetal dolphin deaths, and the overall the effects of the oil spill, are continuing. The long-term effects of the spill on dolphin reproduction are still unknown.

“Our new findings add to the mounting evidence from peer-reviewed studies that exposure to petroleum compounds following the Deepwater Horizon oil spill severely harmed the reproductive health of dolphins living in the oil spill footprint in the northern Gulf of Mexico,” said Dr.Teri Rowles, veterinarian, co-author on the study, and head of NOAA’s Marine Mammal Health and Stranding Response Program , which is charged with determining the causes of these events.

“In contrast to control populations, we found that Gulf of Mexico bottlenose dolphins were particularly susceptible to late term pregnancy failures, signs of fetal distress and development of in utero infections including brucellosis ,” said Dr. Kathleen Colegrove, Ph.D., the study’s lead author and veterinary pathology professor at the University of Illinois Chicago-based Zoological Pathology Program.

Scientists saw higher numbers of stranded stillborn and juvenile dolphins in the spill zone in 2011 than in other years, particularly in Mississippi and Alabama. “The young dolphins, which died in the womb or shortly after birth, were significantly smaller than those that stranded during previous years and in other geographic locations,” said Dr. Stephanie Venn-Watson, study co-author and veterinary epidemiologist from the National Marine Mammal Foundation.

Bottlenose dolphins are pregnant for about 380 days, so stillborn and juvenile dolphins found in the early months of 2011 could have been exposed in the womb to petroleum products released the previous year. “Pregnant dolphins losing fetuses in 2011 would have been in the earlier stages of pregnancy in 2010 during the oil spill,” said Colegrove.

The researchers report that 88 percent of the stillborn and juvenile dolphins found in the spill zone had abnormal lungs, including partially or completely collapsed lungs. That and their small size suggest that they died in the womb or very soon after birth – before their lungs had a chance to fully inflate. Only 15 percent of stillborn and juvenile dolphins  found in areas unaffected by the spill had this lung abnormality, the researchers said.

A previous study from lead authors Venn-Watson and Colegrove revealed that non-perinatal bottlenose dolphins that stranded in the spill zone after the spill were much more likely than other stranded dolphins to have severe lung and adrenal gland damage “consistent with petroleum product exposure.”

The study team included researchers from the University of Illinois; National Marine Mammal Foundation; NOAA; the Dauphin Island Sea Lab and University of South Alabama; the Institute for Marine Mammal Studies in Mississippi; the Louisiana Department of Wildlife and Fisheries; Audubon Aquarium of the Americas; Animal Health Center in British Columbia; the Mote Marine Laboratory in Florida; the University of Georgia; and the University of North Carolina.

This study was conducted in conjunction with the Natural Resource Damage Assessment for the Deepwater Horizon oil spill, as well as the investigation into the northern Gulf of Mexico unusual mortality event. These results are included in the injury assessment documented in the Programmatic Damage Assessment and Restoration Plan. The marine mammal findings are found between pages 4-584 and 4-647. The restoration types laid out in the plan will address injuries to dolphins due to the oil spill.

November 4, 2015: Deepwater Horizon oil spill contributed to a decrease in reproductive success and survival in Barataria Bay, LA.    

Dolphin Y01 pushes dead calf through waters of Barataria Bay, Louisiana, in March 2013.
Photo credit: Louisiana Department of wildlife and Fisheries

Reproductive study finds lung disease and adrenal dysfunction decreasing reproductive success and survival 

For the five years following the DWH oil spill, a team has been monitoring bottlenose dolphins in Barataria Bay, LA to assess reproductive success and survival rate in the population. In 2011 during the health assessments of the free-ranging dolphin population, the animals had a high prevalence of lung disease and adrenal dysfunction. Today  this team reported more results from this ongoing study, published in the peer-reviewed journal Proceedings of the Royal Society, that the yearly mortality rate for the Barataria Bay dolphins was roughly 13% as compared to annual mortality rates of 5% or less that have been previously reported for other dolphin populations. Additionally the reproductive success of Barataria Bay dolphins was only 20% compared to 83% for other dolphin populations. Find out more information in this NOAA Fisheries web story and the NOAA Office of Response and Restoration Blog.


May 20, 2015: Deepwater Horizon oil spill contributed to high number of Gulf dolphin deaths

Louisiana Department of Wildlife and Fisheries collects data from one of the more than 1000 dolphins that have stranded in the Northern Gulf of Mexico since spring 2010. Photo Credit: Louisiana Department of Fisheries and Wildlife

Tissue study finds petroleum contaminants likely source of lung, adrenal lesions causing deaths

As part of an unusual mortality event investigation, a team of scientists has discovered that dead bottlenose dolphins stranded in the northern Gulf of Mexico since the start of the Deepwater Horizon oil spill have lung and adrenal lesions consistent with petroleum product exposure according to a paper published today in the peer-reviewed online journal PLOS ONE.

These findings support those of a 2011 health assessment of live dolphins in Barataria Bay, Louisiana,  a heavily oiled area during the spill which showed those resident dolphins had poor health, adrenal disease, and lung disease.

The timing, location, and nature of the detected lesions support that contaminants from the Deepwater Horizon oil spill caused these lesions and contributed to the high numbers of dolphin deaths within this oil spill’s footprint. Increased dolphin deaths following the oil spill are part of the northern Gulf of Mexico unusual mortality event investigation.

“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 Teri Rowles, Ph.D., 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, also known as UMEs. “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. This study carries those findings significantly forward.”

Direct causes of death, during this period, likely included:

● Chronic adrenal insufficiency resulting from adrenal gland effects;

● Increased susceptibility to life-threatening outcomes due to adrenal insufficiency, especially when challenged with pregnancy, cold temperatures, and infections; and

● Increased susceptibility to primary bacterial pneumonia, possibly due to lung injury, or alterations in immune function.

Animals with untreated adrenal insufficiency are at risk of life-threatening adrenal crises. 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.”

Since early 2010, there has been an ongoing cetacean unusual mortality event involving primarily bottlenose dolphins in the northern Gulf of Mexico. Three out of four groupings of elevated dolphin strandings identified within this event followed the Deepwater Horizon oil spill.

This ongoing die-off, with the highest number of dead bottlenose dolphin strandings on record in the northern Gulf of Mexico, coincided with the largest marine-based oil spill in the United States.

Barataria Bay, Louisiana, was one of the most heavily oiled coastal areas from the Deepwater Horizon oil spill, and the new study shows that half of the dead dolphins examined from Barataria Bay that stranded between June 2010 and November 2012 had a thin adrenal gland cortex, indicative of adrenal insufficiency. One in every three dolphins examined across Louisiana, Mississippi and Alabama had this lesion. In comparison, only 7 percent of the dead stranded reference dolphins, collected from other coastal regions outside the Deepwater Horizon oil spill area and time frame, had a thin adrenal cortex. In fact, almost half of the dolphins with this otherwise rare adrenal lesion appeared to have died without another clear explanation for their death.

In addition to the adrenal lesions, 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.

“These dolphins had some of the most severe lung lesions I have seen in the over 13 years that I have been examining dead dolphin tissues from throughout the United States,” said Dr. Kathleen Colegrove, the study’s lead veterinary pathologist based at the University of Illinois. In comparison, only 2 percent of reference dolphins had these lesions.

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.

The prevalence of Brucella and morbillivirus infections, which were investigated as potential alternative causes for increased dolphin deaths, was low in UME dolphins after the oil spill and was no different compared to the reference dolphins. Additionally, biotoxin levels were either low or below the detection limit in the UME dolphins.

Ongoing studies assessing changes in these lung and adrenal gland lesions over time will help to address questions regarding how long these chronic conditions may last.

This work was completed as a part of the Northern Gulf of Mexico unusual mortality event investigation and a part of the Deepwater Horizon Natural Resource Damage Assessment being conducted cooperatively among NOAA, other federal and state trustees, and BP.

Feb. 11, 2015: Researchers identify distinct groupings in the location and timing of Gulf common bottlenose dolphin deaths

New study provides possible clues in search for causes of on-going unusual mortality event

A new study, published on February 11, 2015 in the journal PLOS ONE, indicates that the current multi-year marine mammal unusual mortality event (UME) in the Northern Gulf of Mexico has multiple groupings of high bottlenose dolphin mortalities and may be due to different contributing factors, including the Deepwater Horizon oil spill.

The researchers found that the largest most prolonged cluster was in Barataria Bay followed by Mississippi and Alabama in 2011, consistent with timing and spatial distribution of oil. The number of annual dolphin deaths was not elevated for the Gulf coasts of Florida or Texas, which were not as heavily oiled.

The peer-reviewed study was conducted in collaboration with marine mammal stranding network organizations across the Gulf region, as well as nationally-recognized marine mammal research institutions and the Working Group for Marine Mammal Unusual Mortality Events. The scientists compared the number and demographics of dolphin deaths across the Gulf from January 2010–June 2013 to patterns from baseline data (1990–2009) to examine changing characteristics of the UME over time.  They looked for distinct groupings of high numbers of dolphin deaths, both before and after the spill.

"In the current study we found that dolphin deaths in Louisiana for 2010 and 2011 were the highest ever recorded for that state," said Teri Rowles, Ph.D., one of 16 authors on the paper, and head of NOAA’s Marine Mammal Health and Stranding Response Program, which is charged with determining the causes of UMEs.

The new findings add to several earlier science papers looking at the potential impacts of the Deepwater Horizon oil spill on dolphins, and it is anticipated that additional results of the on-going research into the causes of the UME deaths will be presented in future papers.

The researchers identified four distinct groupings of dolphin deaths, three of which occurred after the April to July 2010 Deepwater Horizon oil spill and in areas exposed to the oil.  

"Interestingly, several clusters of dead dolphins identified during this mortality event were consistent with the timing and spatial distribution with the DWH oil spill," said Dr. Stephanie Venn-Watson, the paper’s lead author, veterinarian, and director of translational research at the National Marine Mammal Foundation. Among those, the grouping in heavily oiled Barataria Bay (August 2010 thru December 2011) had the longest duration of sustained, high numbers of dead or stranded dolphins. A previously published study showed that Barataria Bay dolphins had adverse health impacts consistent with expected effects of exposure to petroleum products.

The study builds on a recent paper released in December 2014  that compares the current UME to 11 historical mortality events. The December study provides “evidence that the most common causes of previous UMEs, dolphin morbillivirus or brevetoxicosis, are unlikely to be associated with the current UME.” It also found that the elevated dolphin mortalities are continuing, lasting longer than any other UME in the Gulf since NOAA has been recording strandings in the region.

Causes of a distinct grouping of dolphin deaths that occurred before the spill in Lake Pontchartrain and western Mississippi are also being investigated. Combined observations of tell-tale skin lesions and known low salinity uniquely associated with the Lake Pontchartrain group support that short term, fresh water exposure in addition to cold weather may have been key contributors to those pre-spill deaths.

Identification of spatial, temporal, and demographic groupings within the UME suggest that this mortality event may involve different contributing factors varying by location and time including a potential contributing role of the DWH oil spill that will be better discerned by incorporating diagnostic information, including histopathology and other tissue analysis.

This PLOS ONE study is part of the Deepwater Horizon NRDA being conducted cooperatively among NOAA, other Federal and State Trustees, and BP.


Updated: April 12, 2016