For gray whales, San Francisco Bay is becoming a deadly pit stop

Spotting a gray whale in San Francisco Bay can be thrilling, but researchers now know it can be bad news for the marine mammals.

Nearly 1 in 5 gray whales that enter the bay die there, researchers report April 13 in Frontiers in Marine Science. For a population that has lost hundreds of individuals in recent years, the toll is another reminder of trouble along the whales’ 16,000-kilometer migration route.

Most gray whales (Eschrichtius robustus) migrate from the freezing Arctic waters, where they eat, all the way to Mexico, where they stop for some time to mate and give birth, before returning to the Arctic to eat again. The trip is usually nonstop. But in 2018, researchers noticed that some possibly hungry whales began making pit stops in San Francisco Bay to find food. The behavior coincided with the onset of a large die-off among whales, which experts attributed to decreased food availability in the Arctic. A similar trend happened in the late 1990s.

Josephine Slaathaug, a whale biologist at Sonoma State University in Rohnert Park, Calif., and her colleagues used 100,000 photos of gray whales taken from 2018 to 2025 to identify 114 individuals that visited the bay during that period. In the same region, 70 gray whale carcasses were documented. The team matched 21 photo-identified whales, roughly 18 percent, to the carcasses and concluded they died after entering the bay.

And that’s probably an underestimate, the scientists say.

Many of the carcasses were too decomposed to be identified from photos. But most of the remaining 49 carcasses were found in or near the bay, suggesting they too died after entering, possibly because of being hit by boats. Examination of the carcasses showed that nine of 21 identified individuals and 30 of the 70 for which a cause of death could be determined were caused by vessel strikes.

“If you’re desperate, and you go into San Francisco Bay, it looks like you’ve got a really, really high chance of not making it back out,” says Joshua Stewart, a marine ecologist at Oregon State University in Corvallis, who wasn’t involved in the study.

While the San Francisco Bay pit stops may be deadly, they may also signal a broader trend. Gray whales have been spotted possibly feeding in other unusual areas, such as off Florida, New England and Hawaii.

The whales’ exploration of new places in search of food could make the populations more resilient to warming seas, but only if we do our best to protect them in the bay and other areas where they go to find food, Slaathaug says. Even so, climate change is altering how whales have historically migrated, and feeding in different areas might not be so beneficial, Stewart say. “I kind of take the view that it’s more of a desperation option, and it’s really only necessary because they’re not getting what they need in the Arctic.”