Alaska and British Columbia 2015 Large Whale Unusual Mortality Event (UME) Summary Report

Between May 22 and June 17, 2015, 12 finback whales (Balaenoptera physalus) were observed stranded around Kodiak Island and the western Gulf of Alaska. The atypical nature of these strandings, i.e. high number of mortalities in a relatively short period of time and small space, was sufficient to characterize the cluster as an Unusual Mortality Event (UME). Demographic, epidemiologic, pathologic and laboratory findings from an increase in large whale strandings in British Columbia (BC) between April 2015 and April 2016 were added to the UME investigation to help understand possible stranding causes. Data were compiled from:

1. 2015 and 2016 stranding reports and photographs
2. a partial necropsy of one of the 2015 Alaskan fin whales and necropsies of eight of the BC whales

Necropsy results of 2016 Alaskan (AK) fin and humpback whales were also assessed relative to the UME. The total number of whale cases investigated was 52, including from AK 34 in 2015 (12 fin; 22 humpback); 6 humpback whales in 2016; and from BC 11 in 2015 (5 fin; 6 humpback) and 1 humpback whale in 2016. Differential stranding causes included:

• sonar/seismic testing
• radiation
• ship strike
• infectious disease
• predation
• oceanographic changes leading to algal toxin exposure or starvation due to shifts in prey species/distribution

Analysis of the data did not reveal a definitive cause of the UME but we determined that sonar/seismic testing, radiation, and predation likely did not contribute to the UME. Because the strandings were concurrent with anomalous physical and biological shifts in the 2015 Alaskan marine environment, and accompanying mass mortalities of avian species and northern sea otters, the UME was likely one of a many indicators of broader, complex and dynamic ecologic change and therefore these ecologic changes were most likely a contributory factor to the UME. Consequently, while the event was not repeated in 2016, post-UME monitoring over the next few years will include continued surveillance of both large whale health and ecology, including field observations of body condition, necropsy findings, population shifts, unusual strandings etc. in a larger environmental context, as well as finalizing results.