Ongoing population-level impacts on killer whales Orcinus orca following the 'Exxon Valdez' oil spill in Prince William Sound, Alaska

Killer whales were photographed in oil after the 1989 'Exxon Valdez' oil spill, but preliminary damage assessments did not definitively link mortalities to the spill and could not evaluate recovery. In this study, photo-identification methods were used to monitor 2 killer whale populations 5 yr prior to and for 16 yr after the spill. One resident pod, the AB Pod, and one transient population, the AT1 Group, suffered losses of 33 and 41%, respectively, in the year following the spill. Sixteen years after 1989, AB Pod had not recovered to pre-spill numbers. Moreover, its rate of increase was significantly less than that of other resident pods that did not decline at the time of the spill. The AT1 Group, which lost 9 members following the spill, continued to decline and is now listed as depleted under the Marine Mammal Protection Act. Although there may be other contributing factors, the loss of AT1 individuals, including reproductive-age females, accelerated the population's trajectory toward extinction. The synchronous losses of unprecedented numbers of killer whales from 2 ecologically and genetically separate groups and the absence of other obvious perturbations strengthens the link between the mortalities and lack of recovery, and the 'Exxon Valdez' oil spill.