Gray whales die-offs driven by food supply swings as a result of Arctic changing conditions

October 12, 2023

A long-term Arctic biological data set maintained by a University of Maryland Center for Environmental Science (UMCES) faculty member has been proven instrumental in determining the cause of a continuing series of die-offs of eastern Pacific gray whales. According to a new study published in the journal Science, fluctuating stocks of a shrimp-like creature living in the sediments of the Bering and Chukchi seas are likely to have caused three major die-offs in the eastern North Pacific gray whale population since the 1980s. During each of these die-offs, including one that began in 2019 and is ongoing, the gray whale population has been reduced by up to 25% over just a few years.

“This study shows the value of collecting data year after year in the Arctic to address the question of how climate change is impacting the ecological system. Physical changes such as the retreat of sea ice are well-documented, but it is much more challenging to understand how these changes affect the biological system,” said University of Maryland Center for Environmental Science Professor Jacqueline Grebmeier.

The study and its conclusions depend upon a long-term biological data set on gray whale prey in the Bering Strait region, which was initiated by Grebmeier in the 1980s and has now been incorporated into an Arctic monitoring network called the Distributed Biological Observatory (DBO). Lee Cooper, another UMCES faculty member, has also contributed to the development of the data set.

Long-term data collection in the Arctic is helping understand North Pacific gray whale die-offs. (Photo by Kathy Kuletz)

Eastern Pacific gray whales are one of the few populations of large whales that have recovered to what may be similar numbers that existed prior to commercial whaling. These small baleen whales, which currently number about 14,500, make a lengthy migration each year, with many transiting along the Pacific Coast of North America, from lagoons in Baja California, Mexico to the productive waters of the Arctic where they feed on tens of thousands of amphipods per square meter that live in the sediments and filter feed on rich and productive water column plants in the summer months. As the population has recovered from the historic whaling period and approached levels close to what their Arctic feeding areas can support, “they have likely become more sensitive to environmental conditions due to competition for limited resources,” according to lead author Joshua Stewart, an assistant professor with Oregon State University’s Marine Mammal Institute.

“This research demonstrates the value of long-term data in understanding not only the species under study but also the environment it depends on,” said Dave Weller, another co-author and the director of NOAA’s Southwest Fisheries Science Center’s Marine Mammal and Turtle Division. “When we began collecting data on gray whales in 1967, little did we realize the important role they would play in understanding the effects of climate change on an iconic sentinel species in the Pacific. This research would not have been possible without our reliable long-term record.”

Benthic amphipods, the calorie-rich prey that gray whales prefer, are sensitive to sea ice cover. Algae that grow underneath sea ice sink to the seafloor, enriching the amphipod population. Less ice leads to less algae reaching the seafloor, warmer water that favors smaller benthic crustaceans and faster currents that reduce habitat for gray whales’ preferred prey.

“I wouldn’t say there is a risk of losing gray whales due to climate change,” Stewart said. “But we need to think critically about what these changes might mean in the future. An Arctic Ocean that has warmed significantly may not be able to support 25,000 gray whales like it has in recent past.”

"Boom-bust cycles in gray whales associated with dynamic and changing Arctic conditions" was published in Science.

Besides, Stewart, Grebmeier, and Weller, other coauthors include Trevor W. Joyce of Ocean Associates; John Durban of the Marine Mammal Institute and Sealife Response, Rehabilitation and Research; John Calambokidis of Cascadia Research Collective; Deborah Fauquier of the NOAA Fisheries Office of Protected Resources; Holly Fearnbach of SR3; Morgan Lynn, Wayne Perriman of the Southwest Fisheries Science Center, NOAA Fisheries; Manfredi Manizza of the Scripps Institution of Oceanography, University of California, San Diego; and Tim Tinker of Nhydra Consulting and University of California, Santa Cruz.