A dolphin swims in Louisiana’s Barataria Bay near oil from the2010 Deepwater Horizon spill. (CHARLIE NEIBERGALL/ASSOCIATED PRESS)

Researchers studying sea life in the aftermath of the 2010 oil spill in the Gulf of Mexico say that bottlenose dolphins in Louisiana’s Barataria Bay were suffering from a host of maladies, including lung disease and adrenal problems.

In particular, they say that the Barataria Bay dolphins were underweight and had low red blood cell counts, lung disease and pneumonia.

“There’s disease in any wild population,” said lead researcher Lori H. Schwacke, branch chief of the National Oceanic and Atmospheric Administration’s Centers for Coastal Ocean Science. “But we just haven’t seen animals that were in such bad shape as what we saw in Barataria Bay.”

To understand what the effects of oil contamination were on marine mammals such as dolphins and whales, researchers conducted health assessments on 32 dolphins in Barataria Bay, a site chosen because that area was heavily oiled during and after the spill from the Deepwater Horizon well, which released millions of gallons of oil into the gulf over the course of 87 days. The team compared those dolphins to 27 from Florida’s Sarasota Bay, where researchers have been conducting a decades-long study of dolphin health and which remained oil-free following the spill. Bottlenose dolphins, known for their playfulness and intelligence, are one of 29 species of whales and dolphins in the gulf.

The research was conducted in August 2011 by a team of governmental, academic and nongovernmental scientists as part of a project called the Natural Resource Damage Assessment.

The dolphins were exposed to the oil in three ways, the researchers said. The first was through direct contact; in the months following the spill, dolphins were spotted swimming through surface oil and with oil stuck to their skin. The second was through ingestion, either directly from the water or from eating fish that had been contaminated. The third was through inhaling aerosolized compounds derived from oil, the result of breathing air that came from directly above the water’s surface. (Dolphins can hold their breath during extended dives, but they breathe by inhaling air, as all mammals do.)

While previous research has shown that dolphins can detect the presence of oil in the water, they can’t avoid it entirely. Until now, little was known about what oil exposure might mean for the health of marine mammals.

The researchers were particularly surprised to find that the Barataria Bay dolphins had unusually low levels of the adrenal hormones cortisol and aldosterone, compared with the Sarasota dolphins. When faced with a stressful situation, the adrenal glands typically pump out hormones, which in turn increase heart rate and blood sugar, preparing the animal for a fight-or-flight response. That response allows the dolphins to respond appropriately to stressful or threatening situations, such as being attacked by a predator.

Being captured for a health assessment, as was done for this study, ought to activate that fight-or-flight system. Even a humane capture would be fairly stressful for a wild dolphin. “To conduct these assessments, we had to encircle the dolphins with a net; then the dolphins would usually hit into the net trying to escape, and at that point we’d have a team of handlers jump into the water and restrain the animals,” Schwacke explained. “In a wild animal, this would prompt an acute stress response.”

But the Barataria Bay dolphins did not show the expected cortisol or aldosterone increases, which the researchers measured via blood test taken aboard a research vessel. Similar muted responses are seen in people with Addison’s disease, and they occur in domestic dogs, but this marks the first time they have been documented in dolphins.

While the researchers can’t rule out the possibility that these effects were the result of something other than oil from the spill, they found, through blubber biopsies, that the Barataria Bay dolphins showed lower levels of pollutants such as the banned pesticide DDT than the Sarasota Bay dolphins did. (In other experiments, minks exposed to oil showed a similar set of adrenal abnormalities.)

About 25 percent of the Barataria Bay dolphins were also found to be significantly underweight in comparison with just one of the Sarasota Bay dolphins. Many had low blood sugar, and several had low red blood cell counts. For six of the Barataria Bay dolphins — but none of those from Sarasota Bay — blood tests revealed liver abnormalities.

Ultrasound exams also allowed the team to diagnose lung disease and pneumonia in many of the Barataria Bay dolphins. In fact, the Barataria Bay dolphins were five times as likely as those from Sarasota Bay to suffer from moderate to severe lung disease.

“Some of the animals’ lung disease [was] so severe that we did consider those conditions life-threatening,” said researcher Cynthia R. Smith, executive director and director of medicine for the National Marine Mammal Foundation. At least one of the dolphins that the team examined died about five months later; a necropsy resulted in the relatively rare diagnosis of bacterial pneumonia.

Another surprising result from the checkups given to the Barataria Bay dolphins: They were losing their teeth. “The extensive loss of teeth in their entirety and often without evidence of wear in remaining teeth in both young and old dolphins was unexpected,” the researchers wrote. It is perhaps telling that beluga whales in Quebec that were exposed to some of the same toxic compounds found in crude oil also suffered from tooth loss and periodontitis. Dolphins rely on their teeth for capturing prey, which they then swallow whole. Without their teeth, they have a much harder time feeding.

The illnesses found in the Barataria Bay dolphins come amid a larger pattern in the northern Gulf of Mexico described by the National Oceanic and Atmospheric Administration as an “unusual mortality event,” or UME, which began prior to the oil spill. In the past four years, more than 1,080 whales and dolphins have become stranded; the majority of them were dead when they were found.

“This paper is very much a smoking gun as far as the Deepwater Horizon spill is concerned,” said Chris Parsons, a professor of environmental science and policy at George Mason University. “But the UME seems to have started before the Deepwater Horizon spill, and there may be other issues going on, too.”

He suggested that the animals may not be dying because of the spill alone but that the oil serves as an additional stressor that, when pooled with shipping noises, toxic algal blooms, nutrition problems and so on, could lead to death.

Goldman is a freelance science writer for Scientific American and other publications and has a PhD in psychology.