Post mortem findings of a 2019 gray whale Unusual Mortality Event in the Eastern North Pacific
S. Raverty, P. Duignan, D. Greig, J. Huggins, K. Burek, M. Garner, J. Calambokidis, P. Cottrell, K. Danil, D. D'Alessandro, D. Duffield, M. Flannery, F. Gulland, B. Halaska, et al.
International Whaling Commission
Examination of stranded animals offers insights into the health of a population. This is especially so for a coastal and near-shore migratory species such as the eastern North Pacific gray whale. The annual mean stranding number for this species along the West Coast of the United States between 1995 and 1998 was 41; however, for 1999 and 2000 the strandings increased to an unprecedented 283 and 368, respectively. As a result, an Unusual Mortality Event (UME) was declared by the Working Group on Marine Mammal Unusual Mortality Events. While few whales were necropsied, several proximate causes of death were identified, and the UME was assumed to be associated with suboptimal nutritional state. Annual strandings declined to 21 in 2001 and remained under 28 until 2019 in the United States. Between 1st January and 24th April 2019, increased numbers of gray whale strandings were reported along the west coast of Mexico, California and Washington State, prompting declaration of a second gray whale UME. In addition, between 50-55% of live gray whales in Mexico were observed as thin or in suboptimal nutritional condition in contrast to 10-12% in prior years (Christenson, F. pers. comm). In total, 215 animals stranded in 2019 from Mexico through Alaska with 81 in Mexico, 35 in California, 6 in Oregon, 34 in Washington, 11 in British Columbia and 48 in Alaska. Basic stranding logistics and demographic information (Level A data) and necropsy reports from these stranded animals are reviewed here. In the United States and Canada, there were 52 males, 96 females and 67 whales of undetermined gender. The nutritional status of animals was assessed. On site and field photographs were reviewed and the nutritional condition was scored by group consensus as emaciated, thin, average and fat. Nutritional status was coded 1-4 (poor to robust) for 89 whales: 29 were emaciated (code 1), 38 thin (code 2) and 22 were in average to good nutritional condition (codes 3 and 4). Suboptimal nutritional condition was noted in 2 calves, 5 yearlings, 9 subadults and 17 adults. Necropsy of 50 animals stranded in the United States and Canada disclosed 19 animals with evidence of mortality due to killer whale attack, an additional 8 whales with rake marks, and 10 individuals with gross lesions consistent with lethal vessel strike. There was a single animal stranded in California with evidence of past entanglement, but no residual gear was attached. Although multiple whales with evidence of incised wounds or blunt force trauma were malnourished, there did not appear to be a specific association with, or predisposition to, vessel strike by suboptimal nutritional condition. Six whales were tested for presence of biotoxins in feces and four had traces of domoic acid (1.3-47ng/g) and three had saxitoxin (3.1-9.7ng/g). Skin samples from one whales and lung from another were tested for cetacean morbillivirus by PCR and both were negative. Between 2017 and 2019, there were unprecedented loss of sea ice and substantial increases in sea surface temperatures in Bering and Chukchi Sea water suggesting transformative changes to the ecosystem. The contribution of environmental, traumatic, infectious, toxic and other processes to malnutrition and increased gray whale mortalities have not yet been resolved. Furthermore, the advanced state of decomposition of most examined carcasses precluded detection of potential infectious or toxic causes of morbidity or mortality. Nevertheless, the marked increase in strandings likely represents complex and dynamic ecologic processes.