Bottlenose dolphin strandings in the Gulf of Mexico are at a troubling high. An analysis on the cause of their deaths since 2010 has revealed that many had injuries and illnesses consistent with petroleum exposure. The timing, location, and cause of their deaths are congruous with the BP Deepwater Horizon oil spill, suggesting that the event contributed to their deaths
“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 the contributing authors on the paper. The study was conducted by the National Oceanic and Atmospheric Administration.
The study, published in PLOS ONE, discovered that the stranded dolphins had much higher rates of lung and adrenal lesions after the spill than before it. This is in line with an earlier study conducted in 2011 that found dolphins living in the region heavily affected by the spill showed signs of lung and adrenal disease.
The adrenal gland is found just above the kidneys, and produces a number of different hormones, such as cortisol and aldosterone. These regulate the body's metabolism and blood pressure, amongst other things. Previous studies have shown that petroleum from anthropogenic sources can lethally damage the adrenal glands in wild birds and mammals.
One of the dead dolphins found stranded along the Louisiana coast in 2012. Credit: Louisiana Department of Wildlife and Fisheries
The researchers found that half of the dolphins stranded in Barataria Bay, Louisiana—one of the worst affected stretches of the coast—had a thin adrenal gland cortex, which is indicative of the gland not being able to function correctly. In other regions along the coast, they found that around a third of the dolphins had adrenal lesions. Before the spill, only around 7% of stranded dolphins showed this.
“Animals with adrenal insufficiency are less able to cope with additional stressors in their everyday lives,” explained 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.”
Whilst mass stranding events have occurred in the Gulf of Mexico before, most notably one in 1991 that lasted for over a year and a half, the stranding event thought to be triggered by the most recent spill is still ongoing to date, lasting for over 48 months so far. The previous major mortality events can be explained by infections of either morbillivirus or brevetoxicosis, but this new paper rules out these as major contributors to the ongoing strandings.
This report is just the latest in a long line of studies looking into the ecological impact of the Deepwater Horizon oil spill that killed 11 people and spewed 5 million barrels of oil into the Gulf of Mexico. The researchers plan to continue monitoring the lesions over time to assess how long these health problems last.