Counting corpses underestimates Deepwater Horizon whale toll

Nature.com: The Great Beyond blog: Counting corpses underestimates Deepwater Horizon whale toll
paper published today in Conservation Letters suggests that the number of whales and dolphins killed during the Deepwater Horizon oil spill could be as much as 50 times that originally reported.

During the disaster, the US government compiled figures of injured and dead wildlife based on reports from US Fish and Wildlife Service and other authorized sources. Those numbers include approximately 115 whale and dolphin carcasses.

But after analysing data on abundance, mortality rates and strandings for whale and dolphin species in the Gulf, Rob WIlliams and his colleagues have concluded that that only two percent of the whales and dolphins that die in these waters are ever recovered.


“We used the default values for survivorship and natural mortality that are used in standard US stock assessment reports for marine mammals,” says Williams, a marine biologist at the University of British Columbia in Vancouver, Canada. “Our calculations are rough, but they are a good starting point, and far better than assuming, implicitly or explicitly, that the bodies on the beach represent the sum total of the damage.”

Carcasses are simply a poor method of assessing the impact of an incident like Deepwater Horizon, says co-author Scott Kraus, a whale expert at the New England Aquarium in Boston. “Our detection rate for mortality is very poor, and generally, surveys are not an effective way to pick up dead animals unless you have extremely high coverage.” Those surveying during the Gulf spill had vast areas of open water to cover, much of it far from shore.

The researcher’s findings have implications for understanding cetacean deaths from other causes, such as ship strikes and fishing gear entanglements. “The potential for underestimating total mortality is so high. We need a more sophisticated approach to the missing animals equation,” says Kraus.