Bird mortality from the Deepwater Horizon oil spill. I. Exposure probability in the offshore Gulf of Mexico

Abstract

Following the 2010 Deepwater Horizon MC 252 blowout in the Gulf of Mexico, most surface oil remained more than 40 km offshore, precluding reliable estimation of offshore avian mortality based on shoreline counts. Using an exposure probability model as an alternative approach, we estimated that between 36 000 to 670 000 birds died in the offshore Gulf of Mexico as result of exposure to oil from the Deepwater Horizon, with the most likely number near 200 000. Our exposure probability model is a technique for estimating this offshore component of avian mortality as the product of the oil slick area, the density of the birds above the oil slick, and the proportionate mortality of birds that could be exposed to oil during an assumed exposure period. The duration of the exposure period is treated as an estimated parameter to account for oil slick movement, exposure of birds immigrating to the oil-contaminated area, and re-exposure of birds that survived prior vulnerability to exposure. Total avian mortality is determined as the sum of mortalities from each exposure period. Exposure probability may be the only method available to estimate bird mortality from large, remote oil spills in the open ocean where carcasses are unlikely to ever reach shore. In the case of the Deepwater Horizon, the uncertainty interval is quite large because several parameters could not be well estimated. Historically sparse survey coverage effectively led to an under-appreciation of the effects of this spill on marine birds.