Whale and dolphin deaths 50 times worse than admitted after Deepwater oil spill

The Deepwater Horizon oil spill of 2010 devastated the Gulf of Mexico ecologically and economically. However, a new study published in Conservation Letters reveals that the true impact of the disaster on wildlife may have been gravely underestimated. The study argues that fatality figures based on the number of recovered animal carcasses will not give a true death toll, which may be 50 times higher than believed.

Largest oil spill in US history
"The Deepwater oil spill was the largest in US history, however, the recorded impact on wildlife was relatively low, leading to suggestions that the environmental damage of the disaster was actually modest," said lead author Dr Rob Williams from the University of British Columbia. "This is because reports have implied that the number of carcasses recovered, 101, equals the number of animals killed by the spill."

The team focused their research on 14 species of cetacean, an order of mammals including whales and dolphins. While the number of recovered carcasses has been assumed to equal the number of deaths, the team argues that marine conditions and the fact that many deaths will have occurred far from shore mean recovered carcasses will only account for a small proportion of deaths.

Only 2% of deaths recorded every year
To illustrate their point, the team multiplied recent species abundance estimates by the species mortality rate. An annual carcass recovery rate was then estimated by dividing the mean number of observed strandings each year by the estimate of annual mortality.

The team's analysis suggests that only 2% of cetacean carcasses were ever historically recovered after their deaths in this region, meaning that the true death toll from the Deepwater Horizon disaster could be 50 times higher than the number of deaths currently estimated.

Carcass count highly misleading
"This figure illustrates that carcass counts are hugely misleading, if used to measure the disaster's death toll," said co-author Scott Kraus of the New England Aquarium "No study on carcass recovery from strandings has ever recovered anything close to 100% of the deaths occurring in any cetacean population. The highest rate we found was only 6.2%, which implied 16 deaths for every carcass recovered."