The death toll of animals that perished as a result of the Deepwater Horizon/BP oil spill in the Gulf of Mexico may be 50 times higher than presently believed, according to a new study in the latest issue of Conservation Letters.
Until now, fatality figures have primarily been based on the number of recovered carcasses. Data on this varies depending on the source and the date of the count, but the authors report that as of Nov. 7, 2010, 101 whale, dolphin, and porpoise carcasses had been detected across the Northern Gulf of Mexico.
Past numbers of carcasses reflect just 2 percent of actual animal deaths, according to the study, so the true number of fatalities for cetaceans alone as a result of the spill could be in the thousands.
“Our calculations are rough, but are a good starting point, and are far better than assuming that the bodies on the beach represent the sum total of the damage,” lead author Rob Williams told Discovery News.
Williams, a member of the University of British Columbia’s Marine Mammal Research Unit, and colleagues focused their analysis on 14 cetaceans from the Gulf region. These included sperm whales, killer whales, Atlantic spotted dolphins, spinner dolphins, and others.
The 2 percent figure resulted from the team multiplying recent species abundance estimates for these marine mammals by the species mortality rate, and then dividing the mean number of documented strandings each year by the estimate of annual mortality.
“The 2 percent carcass recovery rate represents our best estimate of historic carcass recovery rates, based on the average of 14 species,” Williams said.
Very few remains are found because of decomposition, bodies lost at sea, predators consuming dead individuals, and for other reasons, according to the scientists. Since distance from shore and water depth can also impede finding remains, most Deepwater disaster victims may never be found. The oil spill took place 40 miles offshore and in close to 5,000 feet of water.
Despite disturbing images showing animals, like birds, dying under thick coatings of oil after the spill, there is still no evidence proving that cetaceans died as a direct result of the April 2010 Deepwater explosion and spill.
“Like many others we are waiting for the necropsies to be complete, and the results peer reviewed and reported,” Williams explained. “At that point, we can start a discussion about (precisely) how many animals in total were affected by the spill, and how large these mortalities were relative to population size.”
He and his colleagues believe that their calculations can be used to estimate marine mammal deaths due to other human-related causes, such as ship strikes and interactions with fishing gear. They might also be applied to past disasters, such as the Exxon Valdez oil spill.
Craig Matkin, director of the North Gulf Oceanic Society, has extensively studied marine mammals affected by that 1989 spill. He believes the new study is “welcome and important.”
Matkin told Discovery News that relatively few cetacean carcasses were found after the Exxon Valdez event. He said he and his team “would not have known that we lost 13 killer whales from the major resident pod in Prince William Sound if we were not able to determine the deaths because of individual tracking of each whale through photo identification.”
He added, that most whales and dolphins sink after dying, and don't end up stranded at shore.
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