Oil Spill Wildlife Spotlight: Sperm Whales

There are approximately 1,665 sperm whales in the northern Gulf of Mexico. A preferred feeding ground is along the continental slope, right in the area where oil is leaking.

Deepwater Horizon oil has killed birds along a 100-mile stretch of Louisiana coast and led to fisheries closures near Florida but there is an endangered species known to feed in the murky depths right where oil is leaking: sperm whales. A 2004 count estimated there were 1,665 sperm whales in the northern Gulf of Mexico. They are slightly smaller than typical sperm whales and have a distinct dialect. Overall, very little is known about the population. With oil still spewing, they may be at risk.

“There is potential for direct impact,” says Keith Mullin, a marine biologist with NOAA’s Southeast Fisheries Science Center. But though sperm whales in the northern Gulf may be feeding in areas affected by oil, little research has been done on just how oil affects whales. “Not to make a bad pun,” says Mullin, “but a lot of this is untested waters.”

Sperm whales are the largest toothed whales on earth. They are found at the edge of icepacks and along the equator and can weigh 125,000 pounds and measure more than 60 feet in length—females are pregnant for 14-16 months and give birth to 13-foot long calves. The whales eat mainly squid but will also hunt fish, skates and deep water sharks, feeding in depths from 1,000 feet to nearly 10,000 feet. In the Gulf of Mexico, the preferred feeding ground is along the continental slope, the margin several dozen miles offshore where the seafloor descends from shallow to very deep. This is exactly where the oil is leaking.

Both female and immature whales remain in the Gulf year-round but males migrate to higher latitudes for food. Exactly when they come back and where they go when they’re gone is unknown, says Mullin, although he assumes they travel to the north Atlantic to feed along the edge of the icepack. Because sperm whales don’t frequent the coast, monitoring them from land, as is done with grey whales in California or humpbacks off Massachusetts, is impossible and day trips out in boats are expensive. “There is quite a bit about them that we don’t know,” says Mullin.

And that is in the best of times. Very little at all is known about how oil affects whales. For one, ethical issues prevent studies from being done on whales in captivity, and a study of whales in the wild is all but impossible since they are so hard to locate. Even post-mortem studies are difficult, carcasses sink and beachings are rare and difficult to access.

Studies performed in the 1980s on captive dolphins found that dolphins could sometimes detect thick oil, but could not detect sheen or light slicks. Gray whales off the coast of California have been known to swim through oil seeps. The lack of an olfactory system likely contributes to the difficulty cetaceans have in detecting oil, says a 2008 paper in the Marine Ecology Progress Series.  

The study examined two pods of killer whales in Alaska, both before and after the 1989 Exxon Valdez spill. It found that numbers declined by about one-third in both groups. In one stunning photo, four members of one of the pods examined were photographed directly astern of the grounded tanker, less than 24 hours after the accident. The paper concludes that it is “highly unlikely that this mortality was simply coincidental.”

The paper suggests several possible explanations for how oil may have led to the whales' deaths. One is that poisonous hydrocarbon vapors evaporating from the slick were inhaled by whales surfacing to breathe. This would have acted as a narcotic, an effect well-documented in humans (why some kids sniff gas to get high). The whales would not necessarily have died from the vapors themselves, but could have lost consciousness and drowned. The authors also suggest that the killer whales may have died as a result of eating oiled harbor seals. At some haul out locations in western Prince William Sound, from 80 to 100 percent of seals were oiled. The seals developed brain lesions, which caused them to become lethargic, making them an easy yet toxic meal for killer whales.

For now, there is little way of knowing exactly how the more than 1,600 sperm whales in the northern Gulf will be affected by the spill. “I think there is reason for concern,” says Mullin.