Bottlenose dolphins living near the Deepwater Horizon oil spill in the Gulf of Mexico are showing striking signs of genetic changes associated with a wide range of bodily functions, according to a study published Wednesday.
The discovery highlights how scientists are discovering the lasting consequences of the unprecedented disaster in April 2010, which released an estimated 210 million gallons of crude oil off the Louisiana coast and killed 11 people. It is also estimated to have killed more than 80,000 birds and nearly 26,000 marine mammals.
The study focused on dolphins in the highly polluted Barataria Bay near New Orleans, and used blood samples to compare these dolphins to those living in the less polluted waters of Sarasota Bay, Florida.
The researchers later discovered gene expression changes in the Barataria Bay dolphin population, including genes involved in immunity, inflammation, reproductive failure, lung problems and heart dysfunction. The results were published in the journal PLoS ONE.
These changes are consistent with previously documented health effects, said co-author Sylvain De Guise, a professor in the Department of Pathobiology at the University of Connecticut. He also co-authored another study that found the dolphin population in Barataria Bay has declined by 45 percent since the disaster. In an assessment of dolphins that lived through the oil spill, De Guise and his colleagues found that nearly 80 percent still experience some form of ill health, with lung disease being the most common problem.
This new study utilized data collected from dolphin health evaluations conducted between 2013 and 2018. The team analyzed blood from 60 dolphins from Barataria Bay and 16 from Sarasota Bay and looked for molecular differences through a process called gene expression profiling. This method is a new way of understanding an organism’s health because it has the potential to enable early detection of disease and is easier to perform than traditional veterinary assessments.
The dual purpose of this study was to test and refine this method while attempting to understand the underlying causes of the health consequences of Barataria Bay dolphins. In the future, the study team hopes that this method can help to find out which marine mammals are susceptible to disease.
“We can say that dolphin populations are experiencing effects, but we don’t really know what’s underlying the disease and dysfunction that we’re seeing,” said first author Jeanine Morey, a research biologist working with the National Marine Mammal Foundation at the time of the study. “Through this molecular work, we are beginning to understand the root of the problem.”
Different triggers can cause changes in gene expression, she said. Changes in gene expression in turn lead to the body’s response. Because factors such as water temperature can also cause a change in gene expression, comparing the Barataria Bay dolphins with the Sarasota Bay dolphins helped the team identify exposure to oil pollution as a crucial difference between the two groups.
Furthermore, due to the previous studies evaluating the health of Barataria Bay dolphins since the spill, the team was able to identify which dolphins in their cohort were exposed to oil and which were born after the incident. They found that the significant differences they observed stemmed primarily from the dolphins that lived through the disaster.
But younger dolphins are not necessarily in the open. Some of the most pronounced differences observed in the Barataria Bay group relate to genes related to the immune system. A previous study found that these dolphins experienced problems with their immune systems as recently as 2018, and subsequent laboratory tests on dolphin cells and mice suggested that these immune differences could be passed on to future generations. Changes in the immune system increase susceptibility to infectious diseases, which can also affect the dolphin’s reproductive success.
“The bottlenose dolphin population in Barataria Bay is not doing very well,” De Guise said. “If recovery is underway, it will be at the very beginning and will be conditioned by no additional stressors.”
A cessation of stressors is unlikely. A study published in August found that traces of the Deepwater Horizon spill are still detectable, and new drilling and flood protection plans are expected to result in the death of Barataria Bay dolphins if they go ahead as planned. Even minor man-made problems continue to interrupt dolphin life.
“When we went out on these health assessments, we would find dolphins entangled in fishing lines and nets,” Morey said. “It’s very tough to see these animals suffer.”