Why are so many gray whales dying in the waters of San Francisco Bay?

The whales have raised eyebrows, as residents and researchers can now observe up close how they feed, reproduce and interact. They have also generated growing concern: why are so many of them malnourished and dying?

In 2025 there were a record number of 21 gray whales killed in San Francisco Bay. So far this year, seven have died.

The 4,140 km² bay is the largest estuary on the U.S. West Coast. Before 2018, this species of whale was not known to seasonally or regularly stop in the bay, but instead avoided it on its migratory route to Baja California, Mexico, and back to the Arctic, according to Josephine Slaathaug, who led a recent study on gray whale mortality in the bay.

The impressive gray whales have the longest annual migration of any mammal, traveling approximately 15,000 to 20,000 km round trip to breed.

“It is a new habitat that they have chosen to use,” the Sonoma State University graduate student and lead author of the article explains to the BBC, pointing out the years of sharp decline in their prey in the Arctic.

Many of those that have appeared in the bay are adult males and juveniles heading to the Arctic. It should be noted that the whales observed are thinner than would be normal at this time of year, as Slaathaug and other researchers explain to the BBC.

“They do not have the energy reserves necessary to complete the entire migration back to the Arctic, so hunger may push them into the bay,” he said.

Dead or dying gray whales have also turned up in Washington state and Oregon. Although they were not included in Slaathaug’s study, researchers believe changes in their behavior could be related.

While a lack of food may be driving the whales into the bay, it is not necessarily starvation that is killing them. In recent years, nearly a fifth of the gray whales that have swam into San Francisco Bay have died there, usually after being struck by ships, according to Slaathaug’s study published in the journal Frontiers in Marine Science.

Slaathaug’s study examined hundreds of photographs of whales and carcasses found in the bay since 2018. His team described “a very concerning mortality rate in San Francisco Bay” that continued to increase in 2025, with whales in the bay highly susceptible to ship strikes.

Still, researchers say the factors causing these deaths are worth exploring. The bay offers a unique opportunity to better understand migration patterns and how climate change is changing routes and food supplies.

“It’s sad to see a dead whale. It’s even sadder to see a dead whale that you may have recognized from studying it. But there’s also a lot we can learn,” said Kathi George, whose team helped Slaathaug with his research and several necropsies.

Whales, he said, may be harbingers of larger changes beneath the ocean’s surface.

The fact that whale sightings and strandings have started earlier than usual this season – starting with two in January, when the peak usually occurs in April – is a cause for concern, as it indicates that these creatures are in more trouble than initially thought.

Slaathaug and his colleagues have also observed very low numbers of offspring, indicating a low birth rate. That could mean that this population is not recovering or rebounding as it has in previous times of population decline.

“That, along with the high human-caused mortality rate in this area, is really leading scientists to be concerned and looking for ways to find solutions,” Slaathaug said.

Moe Flannery, co-author of the study, told the BBC that this is the first time in decades that the problem seems imminent. He says scientists are learning how to make waters from Alaska to Mexico safer for whales.