A nearly five-year “unusual mortality event” involving the deaths of hundreds of gray whales along the West Coast is officially over, federal investigators have determined.
During the event, which stretched from Dec. 17, 2018, through Nov. 9, 2023, about 690 gray whales died due to strandings, with peak strandings occurring from December 2018 to December 2020, according to the Fisheries branch of the National Oceanic and Atmospheric Administration (NOAA).
Some 347 whales perished along U.S. shores, while 316 died and in Mexico and 27 in Canada, per NOAA Fisheries.
Over the course of the event, scientists estimated that the eastern North Pacific gray whale population incurred a roughly 40 percent decline.
The strandings were linked to localized ecosystem changes in the animal’s Subarctic and Arctic feeding areas, which led to surges in malnutrition, decreased birth rates and heightened mortality, the agency noted.
“We know the population has demonstrated strong resilience in the past,” Deborah Fauquier, veterinary medical officer for the NOAA Fisheries Marine Mammal Health and Stranding Response Program, said in a statement.
“We will be watching to ensure we know how the whales recover,” added Fauquier, who also served as coordinator of the 2019-2023 Gray Whale Unusual Mortality Event.
The population decline over the past five years shared some similarities with a shorter event that harmed the species from about 1999 to 2000, according to NOAA Fisheries. In the aftermath of that episode, gray whale numbers ended up rebounding even higher than their previous baseline.
During the event, federal researchers found that the eastern North Pacific gray whale population plunged from 20,500 whales in 2019 to 14,526 in 2023. Calf production, meanwhile, slid from around 950 calves in 2019 to about 217 in 2022 — climbing once again to 412 in 2023.
Necropsy findings from a subset of the stranded whales identified malnutrition as the common cause of death and did not discover any underlying illness, according to the scientists.
Among the specific nutrition problems were curtailed access to and diminished quality of prey, particularly in the northern Bering and Chukchi seas, according to the findings. These circumstances led to a rise in deaths during the annual northward migration of the whales from Mexico to Alaska, the researchers added.