In March, the National Marine Fisheries Service publicly announced that the unusual mortality event (UME) it declared in 2019 involving eastern North Pacific gray whales is over. In consultation with the federal Working Group on Marine Mammal Unusual Mortality Events and outside experts, NMFS determined that the increased stranding rate that triggered the UME declaration is no longer occurring.
Under the Marine Mammal Protection Act, a UME is defined as a stranding event that is (1) unexpected, (2) involves a significant die-off of any marine mammal population, and (3) demands immediate response. If the UME working group determines that an event meets one or more of the seven UME criteria, it forwards a recommendation to the National Oceanic and Atmospheric Administration’s assistant administrator for fisheries to declare a UME, prompting protection efforts.
The die-off that triggered the UME determination began in December 2018 and continued until November 2023, with peak strandings taking place December 2018 through December 2020. It involved 690 known strandings throughout the eastern North Pacific gray whale migratory route and resulted in a
46 percent population decline.
Necropsies of dead whales identified malnutrition as a common cause of death and did not detect the presence of underlying infections. Leading up to and during the UME, gray whales in Mexico were also observed to be in poorer condition compared to prior years. A UME investigative team of scientists concluded that localized ecosystem changes—including both access to and quality of prey—in the northern Bering and Chukchi seas contributed to the poor nutritional condition observed in live and stranded gray whales. This malnutrition led to increased mortality during the whales’ annual northward migration from Mexico to Alaska and to decreased production of calves.
While fewer strandings is welcome news, AWI is concerned that the UME may have been declared over prematurely, since most gray whale strandings occur during the northbound migration in the spring. Indeed, in 2024 there have already been at least 15 stranding along the US coast, 19 in Mexico, and one in Canada. Moreover, what appears to be a sizeable decline in gray whale calves—based on data collected in Mexico—suggests that something remains amiss with this species and that ongoing annual counts of adults and calves are needed to monitor the population.