Nearly one in five whales that enter the bay do not come out
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Spotting a gray whale in San Francisco Bay can be exciting, but researchers now know it can be bad news for marine mammals.
Nearly one in five gray whales entering the bay dies therethe researchers report on 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 the difficulties encountered along the whales’ 16,000-kilometer migration route.
Most gray whales (Robust Eschrichtius) migrate from the frigid waters of the Arctic, where they feed, to Mexico, where they stop for a time to mate and give birth, before returning to the Arctic to feed again. The trip is generally non-stop. But in 2018, researchers noticed that some whales, perhaps hungry, had begun making stops in San Francisco Bay to find food. The behavior coincided with the start of high mortality among whaleswhich experts attribute to declining food availability in the Arctic. A similar trend occurred in the late 1990s.
Josephine Slaathaug, a whale biologist at Sonoma State University in Rohnert Park, California, and colleagues used 100,000 photos of gray whales taken between 2018 and 2025 to identify 114 individuals that visited the bay during that period. In the same region, 70 gray whale carcasses have been documented. The team compared 21 photo-identified whales, or about 18 percent, to the carcasses and concluded that they had died after entering the bay.
And that’s probably an underestimate, scientists say.
Many carcasses were too decomposed to be identified in photos. But most of the remaining 49 carcasses were found in or near the bay, suggesting they also died after entering it, perhaps because they were hit by boats. Examination of the carcasses showed that nine of the 21 individuals identified 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 seems like you have a very good chance of not getting out,” says Joshua Stewart, a marine ecologist at Oregon State University in Corvallis, who was not involved in the study.
While pit stops in the San Francisco Bay Area can be deadly, they can also signal a larger trend. Gray whales have been sighted, possibly feeding, in other unusual areas, such as off Florida, New England and Hawaii.
Whales exploring new places in search of food could make 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 foraging, Slaathaug says. Even so, climate change is changing the way whales have historically migrated, and feeding in different areas might not be as beneficial, Stewart says. “I kind of think it’s more of a desperate option, and it’s really necessary because they’re not getting what they need in the Arctic.”
