Gray Whale Unusual Mortality Event, 2019-2023
Eastern Pacific Gray whales undertake the longest migrations of any cetacean species throughout the world. These animals range from summer feeding grounds in the Chukchi and Bering Seas to overwintering calving and maternity lagoons off the coast of Baja, Mexico. Long term census work indicates that the population numbers of this species significantly fluctuate. Genetic estimates of prehistoric populations range from 75,000 to 120,000 whales and pre-whaling population levels of 15,000 to 30,000.
Starting in December 2018, increased numbers of dead gray whales were initially reported in Mexico and subsequently documented throughout the migratory range along the western seaboard of North America. Between 2018 and 2023, the population declined from approximately 28,000 to less than 14,000 whales which prompted the declaration of an Unusual Mortality Event (UME) by the United States National Marine Fisheries Service. The UME designation afforded an opportunity to coordinate local and regional stranding response programs in Mexico, the United States and Canada to standardize necropsy and tissue sampling protocols, interpretation of nutritional condition and gross lesions, and share data for analyses.
From December 2018 to July 2022, 503 floating and beach cast dead gray whales were reported. There were 187 males, 167 females and 149 whales of undetermined sex with 193 adults, 194 subadults, 40 calves, 1 fetus and 75 whales of undetermined age class. Post mortem exams were performed in 61 animals (12% of reported carcasses) and a cause of death was assigned in 33 (54%) individuals. There were 16 whales (26%) that presented with severe emaciation, 11 whales (18%) with evidence of blunt force trauma (vessel or propeller strike) and 19 whales (31%) that featured rake marks consistent with killer whale predation. Two whales (3%) were entangled at the time of necropsy and 1 whale (1%) was entrapped in the pilings of a pier. Screening of ingesta for harmful algal bloom toxins revealed low levels in a few individuals and there was no indication of an infectious agent in tissues from the necropsied whales.
Although similar population declines have been reported in the Eastern Pacific gray whales in the early 1990s and 1998-2001, there was little documented information regarding cause of deaths. Anecdotal reports of emaciated animals were suggested, but not confirmed. This study is among the first large scale investigations into a long lived, migratory whale and revealed 3 major contributory factors in the population decline; starvation, vessel strike and killer whale predation. Without a historical context, it is difficult to infer changes in the incidence of these conditions from non-UME years or with past die offs. However, in this case series, the suboptimal body condition is likely related to unprecedented and transformative changes in the arctic associated with increasing water temperatures and reduced ice cover, which has impacted prey availability, composition and nutritive quality. Detailing the cause of death of stranded gray whales may better define impacts of climate change on benthic prey availability for gray whales and inform conservation and management measures to mitigate impacts on this species.
Stewart et al, 2023 Boom-bust cycles in gray whales associated with dynamic and changing Arctic conditions. Science. 382, 207-2111.