Data release for Investigation of a largescale common murre (Uria aalge) mortality event in California in 2015

During 2015, beachcast bird survey programs (Beach Watch and BeachCOMBERS) reported higher than average deposition of common murres (Uria aalge) on central and northern California beaches from August through December. Increased common murre mortality was not reported for southern California beaches. International Bird Rescue (IBR) located in Fairfield, CA and other coastal wildlife rehabilitation centers received more than 1,000 live, stranded and debilitated murres from Sonoma County to San Luis Obispo County during August - October. Approximately 2/3 of birds admitted to IBR were after-hatch-year birds in emaciated body condition and in various stages of molt, with extremely worn plumage. To determine the probable cause of death of beachcast carcasses, internal examinations were performed on a sample of birds at the California Department of Fish and Wildlife Marine Wildlife Veterinary Care and Research Center (MWVCRC; n=28) and the United States Geological Survey National Wildlife Health Center (NWHC, n=7). As observed by IBR, most birds examined during necropsy were emaciated, with starvation the most likely cause of death. Birds were also tested for underlying infectious diseases at NWHC and harmful algal bloom toxins at the University of California, Santa Cruz (UCSC) and the National Oceanographic and Atmospheric Administration's Northwest Fisheries Science Center (NWFSC). Twenty-four out of 29 tested birds had detectable levels of domoic acid, and no indication of infectious disease was found. Emaciation is thought to be the cause of death for these birds, with a large warm water anomaly and harmful algal bloom playing a secondary detrimental role. While our work exclusively assesses the die-off in California, there was a large, concurrent die-off of murres in Alaska, Oregon, and Washington.