Gray whales have one of the longest annual migrations of any mammal, traveling over 10,000 miles between summer Arctic feeding grounds and Mexico. But as the climate changes, more have been exploring the San Francisco Bay—a mistake that is often fatal.
A new study published in Frontiers in Marine Science found that at least 18% of all the gray whales (Eschrichtius robustus) that entered the San Francisco Bay between 2018 and 2025 died. Vessel strikes caused over 40% of the deaths.
Researchers who published the study used a combination of citizen science photos and surveys in San Francisco Bay as well as necropsy records of the 70 whales found dead in the bay in that period.
“It was historically very unusual for them to enter the bay, especially for longer amounts of time or consistently year after year,” one of the authors, Josephine Slaathaug of Sonoma State University, told The Guardian.
Scientists said that warming Arctic waters are making it harder for the whales to get their food, prompting them to forage elsewhere in places like San Francisco Bay.
The bay’s busy vessel traffic combines with environmental conditions and the whale’s own biology to increase the riskiness for the marine mammal. Gray whales are difficult for vessel operators to see because of their low profile when surfacing, especially in the foggy conditions that the area is known for.
