Scientists summarize 2019–2023 grey whale unusual mortality event: 690 reported strandings and persistent nutritional stress

Mo Flanner of the California Academy of Sciences summarized the scope and findings of the recent grey whale unusual mortality event (UME), which began in late 2018 and produced a large spike in strandings along the Pacific coast. "These animals are really skinny coming into the lagoon. Something is happening. We need to be aware," Flanner said, describing observations from Mexico through Alaska.

Flanner told the workshop that the assembled tally for the event included 690 stranded whales reported across the range. Teams carried out 72 full necropsies and a larger set of partial examinations or sampled animals where decomposition or logistics prevented full dissection; another several hundred animals were recorded from photographs or lacked feasible response. Of a subset fully examined in a published synthesis, roughly 26% (16 of 61) were judged emaciated; other identified causes among examined carcasses included blunt‑force/vessel trauma (~18%), killer‑whale predation (~5%), and entanglement (~5%), while many animals (about 46% in one examination set) had no specific cause identified.

The panel linked the UME to broad ecosystem change and food‑availability problems on Arctic/subarctic feeding grounds rather than a single novel pathogen: teams ran viral discovery and did not find a clear new viral driver in available tissues. Flanner noted that NOAA Fisheries or the U.S. Fish and Wildlife Service makes official UME determinations, and the multi‑agency international response spanned Mexico, the United States and Canada.

Panelists emphasized limits of the available evidence: many strandings occur in remote or marine environments where the carcass cannot be recovered intact, and decomposition frequently prevents determining cause of death. Organizers said UME criteria have not been fully resolved and that the working group moved to post‑UME monitoring: they now meet quarterly and convene more frequently during heavy stranding seasons to continue surveillance and collaborative necropsy work.

Next steps cited at the workshop included continuing cross‑jurisdictional sampling to increase the fraction of carcasses with full internal exams, improving logistic capacity for remote responses, and integrating necropsy findings with live‑animal metrics (drone photogrammetry, fecal or baleen biomarkers, and stable isotopes) to better track population health.