Determining an oil spill’s toll on wildlife is never an easy feat—and the challenging conditions of the current gulf spill make it all the more complicated. While most of the animals collected alive have been visibly covered in oil, the majority of those that have been found dead have had no oil visible on their bodies, making the cause of death difficult to ascertain. According to an official report, of the 1,571 birds, 449 sea turtles, and 54 marine mammals found dead so far, 35 percent, 3 percent, and 6 percent, respectively, clearly had oil on them. “I’ve actually been surprised at the relatively low number of oiled animals being seen so far,” says Michael Ziccardi, director of the Oiled Wildlife Care Network at the University of California, Davis.
That’s where veterinary pathologists come in. To tease out causes of death for the animals collected since the spill began, necropsies—animal autopsies—are being done on as many of the carcasses as possible, says Erin Fougeres, a marine- mammal biologist at the National Oceanic and Atmospheric Administration (NOAA).
There’s always the possibility that higher-than-normal reported animal casualties are due to factors other than oil exposure. For instance, NOAA says that more dolphins than usual were being stranded prior to the spill—62 of them were found stranded in March, compared with the usual 18. Additionally, people are looking a lot harder for injured or dead animals than they ordinarily would, says Robert MacLean, a veterinarian at the Audubon Nature Institute. With so many cleanup workers, government officials, and environmental advocates swarming across the outlying beaches of the Gulf Coast since the spill, animal carcasses that would otherwise have gone unnoticed are more likely to be discovered.
But then again, response workers are probably collecting only a small portion of the affected animals out there. “I think we’re only seeing the tip of the iceberg. The question is how much of the iceberg is actually submerged,” says Ziccardi, a veterinarian who has been working with injured animals in the gulf since late April. “We don’t know whether we’re finding one in five animals that are affected or one in 50.”
A full necropsy involves opening the animal up and examining all of its internal organs. Tissue samples of each organ are taken, along with samples of stomach content, urine, bile from the liver, and blood. For a large mammal like a dolphin, the necropsy can take up to several hours. Later, the samples are analyzed under a microscope and subjected to chemical analyses. Microscopic examination allows pathologists to see, among other things, changes in tissue architecture that might be associated with oil exposure, and chemical testing can check for the presence of hydrocarbons, even at low levels, or point to the presence of other exposures that could have caused death, such as biotoxins from algal blooms like the ones that cause red tides.
Although Fougeres says the goal is to conduct a necropsy on every animal, that’s not always possible because so many of the carcasses are badly decomposed by the time they’re recovered. In the heat of the gulf region, they can degrade within a day or two, and most animals have already degraded to the point where they no longer have distinct organs by the time they are discovered, making internal examinations difficult, if not impossible. Even in these cases, though, at least a limited assessment is done. Carcasses are checked for visible oiling, and external samples are collected to check for hydrocarbons that aren’t immediately visible (the bodies are usually swabbed, much like a throat culture). A skin sample and a tooth are taken from marine mammals, as these allow for later testing to determine where the animal came from and its age. Depending on how degraded the animal is, internal samples might be taken as well.
Response teams make every effort to get animal carcasses to a lab as quickly as possible after they’re discovered to minimize further decomposition. After the examination or necropsy, carcasses are frozen as evidence, in keeping with the legal requirements that all samples be collected, transported, and stored under very specific protocols as part of the Natural Resource Damage Assessment (NRDA) process. When animals that are too decomposed to be moved are found, the carcasses are examined in the field and then either buried or marked to ensure that they don’t get counted twice.
To date, about half the turtles found have been somewhat degraded, with a subset so decomposed that nothing could be used to determine cause of death beyond external observation, says Ziccardi. Even fewer dolphins have been able to get full necropsies, since they tend to decompose more quickly than turtles.
Although many necropsies have been conducted, complete results are not yet in. Analysis of the tissue samples is key to determining a cause of death with as much certainty as possible, but these samples, at least for marine mammals, haven’t been sent out yet. Fougeres says there’s been a delay in choosing labs to analyze these samples. “People really want to know right away what killed these animals, but from the building-the-evidence and building-the-case side of things, we want to make sure that the samples are analyzed at the best labs so that the evidence stands up in court,” she says. The results will ultimately be passed to the people working on the NRDA, who, with the help of mathematical models, will generate estimates of the wildlife impact of the spill.
Meanwhile, responders are, with good reason, focusing on helping the animals that are found alive. “This spill’s been very different from any other I’ve been involved in,” Ziccardi says. “The sheer magnitude—the volume of oil that is coming out, the overall risk to threatened and endangered species—the potential impact is enormous.”
Even as the body count continues to rise, we may not have a solid estimate of the spill’s toll on wildlife for a while, probably not until long after the oil flow is stopped.