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Closure of 2019–2023 Eastern North Pacific Gray Whale Unusual Mortality Event

March 14, 2024

The Unusual Mortality Event involving stranded eastern North Pacific gray whales has ended.

People on a beach performing a necropsy on a stranded gray whale

NOAA Fisheries has determined the Unusual Mortality Event involving eastern North Pacific gray whales is over. We made this determination in consultation with the Working Group on Marine Mammal Unusual Mortality Events and outside experts. The increased stranding rate that triggered the Unusual Mortality Event UME declaration is no longer occurring. It was associated with localized ecosystem changes in the whale's Subarctic and Arctic feeding areas that led to changes in food, malnutrition, decreased birth rates, and increased mortality.

We declared the UME in 2019. It involved hundreds of dead gray whales that stranded along the Pacific coast from Mexico to Alaska, including in gray whale wintering, migratory, and feeding areas. Scientists estimate the UME led to a roughly 40 percent decline in the population of eastern North Pacific gray whales.

The UME occurred from December 17, 2018 through November 9, 2023, with peak strandings occurring between December 17, 2018, and December 31, 2020. It involved 690 gray whale strandings, including 347 in the United States, 316 in Mexico, and 27 in Canada.

While the number of strandings spiked at the start of the UME, they have since declined to annual numbers similar to those recorded before the UME began. The number of calves born to the population also appears to be improving, with other signs that the population may have begun to recover.

The population decline over 5 years resembled a similar but shorter UME that affected the same population of gray whales from about 1999 to 2000. The population rebounded in the years that followed. It eventually climbed higher than it had been before that UME, until the more recent decline occurred.

“We know the population has demonstrated strong resilience in the past, and we will be watching to ensure we know how the whales recover from this UME,” said Deborah Fauquier, Veterinary Medical Officer for the Marine Mammal Health and Stranding Response Program in the NOAA Fisheries Office of Protected Resources and Onsite Coordinator of the 2019-2023 Gray Whale UME.

Findings of the Investigation

Necropsy findings from a subset of dead whales supported malnutrition as a common cause of death and did not identify underlying infections. Additionally, the nutritional condition of live gray whales in Mexico was poorer leading into and during the UME compared to prior years. The UME Investigative Team 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 decreased production of calves. This resulted in an overall decline in population abundance.

The UME prompted numerous research papers published in science journals and presented in science forums such as the International Whaling Commission’s Scientific Committee. A recent paper by Oregon State University and NOAA Fisheries scientists identified factors that likely compounded this and previous UMEs. Many of the papers reflected the international collaboration between scientists in Mexico, Canada, and the United States.

“The work to understand what was affecting gray whales was truly a collaborative effort,” said Moe Flannery of the California Academy of Sciences, one of the organizations that helped respond to stranded whales over the course of the UME. “In my experience, it is rare that scientists and government agencies from three separate countries work together to solve a problem spanning the entire coast.”

Importance of the Marine Mammal Stranding Networks

Volunteers and staff from the organizations that make up the Alaska and West Coast Marine Mammal Stranding Networks responded to the gray whale strandings during the UME. Teams examined carcasses, often in remote locations and challenging weather, collecting details to support the investigation into the likely causes and extent of the UME. NOAA Fisheries coordinates the U.S. Stranding Networks. Our transboundary partners in the Canadian Marine Mammal Response Program and Mexican gray whale colleagues also supported the investigation.

“We could not have come close to collecting the important data that we needed to assess the UME without the Stranding Networks and our transboundary colleagues,” said Fauquier, who coordinated an international team of experts representing three countries and more than 35 organizations. “We owe great thanks to the many teams who responded to strandings rain or shine, often in remote areas, and even over the course of the pandemic. It was not an easy job, and we were fortunate that we could rely on the Stranding Networks and our partners for support.”

Post-Unusual Mortality Event Monitoring

The various marine mammal stranding networks from the three countries in the United States, Canada, and Mexico will continue to respond to fresh dead to moderately decomposed carcasses and fully sample the animals as is feasible. NOAA Fisheries Southwest Fisheries Science Center conducted a 2023–2024 survey to assess gray whale abundance. The SWFSC will conduct a 2024 survey to evaluate calf production.


Throughout this investigation, many partners supported NOAA Fisheries including:

United States

California Partners

Washington and Oregon Partners

Alaska Partners