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Surge in gray whale deaths sparks concern in San Francisco Bay
San Francisco Bay has become an unexpected stopover for Eastern North Pacific gray whales, but their presence is raising alarms as record numbers of emaciated and dead whales wash ashore. Researchers attribute the deaths to dwindling Arctic prey, climate change, and human activities, including vessel strikes.
Record mortality and shifting migration patterns
In 2025, 21 dead gray whales were found in the bay, the highest number on record. This year, seven have already died, a trend researchers link to food scarcity and habitat changes. The 4,140-square-kilometer bay, the largest estuary on the U.S. West Coast, was not a known seasonal stop for gray whales before 2018.
Josephine Slaathaug, lead author of a recent study on gray whale mortality in the bay, noted that the whales are now utilizing the bay as a new habitat due to steep declines in their Arctic prey. The species undertakes the longest annual migration of any mammal, covering 15,000 to 20,000 kilometers roundtrip between Baja California and the Arctic.
"They don't have the energy reserves necessary to complete the entire migration back to the Arctic, so they may be driven into the bay by hunger," Slaathaug said.
Josephine Slaathaug, Sonoma State University
Human activity and climate change exacerbate threats
While hunger may drive whales into the bay, vessel strikes are a leading cause of death. Nearly one-fifth of gray whales entering the bay die there, often after collisions with ships, according to Slaathaug's study published in Frontiers in Marine Science. The bay's crowded waters, shared by container ships, ferries, and marinas, heighten the risk of such incidents.
Researchers also report unusually low calf counts, signaling a declining birth rate. This, combined with human-caused mortality, has raised concerns about the species' ability to recover. The gray whale population along North America's west coast has dropped from 27,000 in 2016 to 12,500 in 2025, prompting the U.S. National Oceanic and Atmospheric Administration to declare an "unusual mortality event."
Efforts to mitigate risks and study the phenomenon
Scientists are racing to understand the whales' shifting behaviors and develop solutions. Kathi George, whose team assisted with necropsies, emphasized the urgency of addressing the crisis. "It's an immediate problem that needs to be addressed," said Moe Flannery, a co-author of the study, adding that the bay offers a rare opportunity to study how climate change is altering migration routes and food supplies.
The U.S. Coast Guard and local ferry companies have implemented safety measures, including training captains to slow down and report whale sightings. Gary Reed, director of vessel traffic for the Coast Guard in San Francisco, is testing an infrared camera on Angel Island to monitor whales in high-traffic areas at night.
"Anything we can throw against the wall and see what sticks, we're willing to give it a try," Reed said.
Gary Reed, U.S. Coast Guard
A glimmer of hope amid challenges
Despite the grim findings, researchers see potential for the bay to become a vital foraging stopover for gray whales. Michelle Barbieri Lino, a wildlife veterinarian with Washington state's SeaDoc Society, noted that gray whales have historically rebounded from stressors, such as after the U.S. banned commercial whaling in the 1970s.
"If they have the protections they need in San Francisco Bay, this could be a place where they can successfully create a new foraging stopover to help them complete their migration and come back again and thrive," Lino said.
Michelle Barbieri Lino, SeaDoc Society
For now, the early arrival of whales-with strandings beginning in January instead of the usual April peak-underscores the urgency of conservation efforts. As scientists continue to study the phenomenon, the hope is that the bay's waters can be made safer for these majestic migrants.
