Mystery of Grey Whale Mortality in San Francisco Bay: Scientists Investigate

California's blustery San Francisco Bay is home to the Golden Gate Bridge, commercial ports, public marinas and ferry boats. But more recently, a new sight is attracting locals' attention: Eastern North Pacific grey whales.

The whales have brought wonder, as residents and researchers now get to closely observe how they feed, breed and socially engage. They've also brought growing unease: why are so many of them undernourished and dying?

In 2025, a record number of 21 dead grey whales were found in the broader San Francisco Bay. So far this year, seven have died due to a combination of dwindling prey availability, climate change, and human causes, researchers say.

The 4,140 square-km bay is the largest estuary on the west coast of the US. Before 2018, this species of whales wasn't known to stop seasonally or consistently in the bay, bypassing it on their migration route down to Baja California and back up to the Arctic, said Josephine Slaathaug, who led a recent study on grey whale mortality in the bay.

The impressive grey whales have the longest annual migration of any mammal, travelling about 15,000 to 20,000 km roundtrip to breed.

It's a new habitat that they've chosen to utilise, the graduate student at Sonoma State University and the lead author of the paper tells the BBC, noting years of steep declines in their prey in the arctic.

Many of those that turned up in the bay are adult and juvenile males heading to the Arctic. Notably, the whales observed are skinnier than they normally would be at this time of year, Slaathaug and several other researchers tell the BBC.

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, she said.

Dead or dying grey whales have also been spotted in Washington state and Oregon. Although they weren't included in Slaathaug's study, researchers believe changes in their behaviors could be related.

While a lack of food may be driving whales into the bay, it's not necessarily starvation that's killing them. In recent years, nearly one-fifth of the grey whales that have swum into the 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 this week.

Slaathaug's study examined hundreds of photographs of whales and carcasses found in the bay since 2018. Her team described a very concerning high rate of death in San Francisco Bay, that continued to grow in 2025, with whales in the bay highly susceptible to vessel strikes.

Still, researchers say the factors leading to these deaths are worth exploring. The bay is offering a rare opportunity to better understand migratory patterns and how climate change is shifting routes and food supplies.

It's sad to see a dead whale. It's sadder to see a dead whale that you may have recognized from studying that particular whale. But there's also a lot that we can learn, said Kathi George, whose team assisted Slaathaug with her research and conducted several necropsies.

Slaathaug and her colleagues have also seen very low calf counts, signaling a low birth rate. That could mean this population is neither recovering nor rebounding as it has in previous times of population decline.

That, in combination with the high rate of human-caused mortality in this area, really leads scientists to be concerned and look for ways to find solutions, Slaathaug said.

The population of grey whales that hug North America's west coast is not considered endangered. However, their numbers have dropped from 27,000 in 2016 to 12,500 in 2025, a decrease that the US National Oceanic and Atmospheric Administration has dubbed an unusual mortality event.

It's an immediate crisis that needs to be addressed and this paper is just the first step in gathering the science that's needed to help inform conservation and management of this species that's in trouble, said Moe Flannery, a co-author of the study.

That trouble becomes imminent for humans, too, when a 30-40 tonne grey whale begins floating in the bay, as happened earlier this year. The Army Corps of Engineers towed a body to a secluded beach for George and her team to perform a necropsy.

But in an unusual turn of events, George said her team hadn't yet made it to the first whale when they received a call about a second that needed immediate attention.

The bay area waters are somewhat closed off and relatively crowded with major container ships, ferries and public marinas, increasing the possibility of a whale—dead or alive—colliding with a vessel.

Gary Reed, the director of vessel traffic for the US Coast Guard in San Francisco, highlighted the urgency to get these whales out of traffic lanes as quickly as possible. The Coast Guard and ferry companies have implemented safety measures, and captains are being trained to give the whales space, slow down and call in sightings.

As researchers learn how to make the waters from Alaska to Mexico safer for the whales, there is hope that grey whales can bounce back, as they have before. They are a species who can give us a sense of awe of how these animals can recover from stressors and impact, said Michelle Barbieri Lino, a wildlife veterinarian with Washington state's SeaDoc Society.

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, she added.