Why Are Gray Whales Dying in the San Francisco Bay?
Gray whales once had an easier life. They spent the summer in the Arctic, fattening their massive bodies with an abundance of krill that prefers deep, icy waters. As winter approached, they headed south, relying on their full bellies to travel thousands of miles to breed in the tropical lagoons of Baja Mexico, rarely feeding along the way.
However, lately gray whales in the eastern North Pacific (Robust Eschrichtius) are appearing in San Francisco Bay, a new outpost on their migratory routes. Since 2018, in fact, they have been observed every year. With prey densities decreasing due to climate change in the Arctic, they are likely looking for a refueling stop en route to Mexico.
In a recent study, biologists from the Marine Mammal Center and the California Academy of Sciences explored the fate of 114 whales observed in San Francisco Bay from 2018 to 2025. Using survey data, combined with citizen science photos, they cataloged whales entering and leaving the bay.
Read more: “Why ships kill thousands of whales every year”
Their conclusions were sobering. “At least 18 percent of individuals identified in San Francisco Bay later died in the region,” study co-author Bekah Lane of the Center for Coastal Studies said in a news release. “Our broader analysis of local strandings inside and outside San Francisco Bay found that more than 40 percent of these whales died from vessel-caused trauma.”
Seventy gray whales were found dead during the study period, 30 of which had suffered blunt or sharp injuries from boat strikes. Because gray whales barely breach the ocean surface when swimming, boaters may not notice them. Compounding the problem, San Francisco Bay receives significant shipping traffic, which passes through the Golden Gate Strait, an entry corridor shared with gray whales.
According to other studies, boat strikes with whales can be reduced by implementing speed restrictions and changing routes. Although ships can still collide with whales in the bottleneck of the Golden Gate Strait, slower speeds make collisions less likely to be fatal.
As the Arctic climate continues to warm, gray whales in the eastern North Pacific will continue to require dietary supplements during such arduous migrations. Some subgroups began feeding south of the Arctic, adapting to consume other prey like ghost shrimp and herring eggs. But many will continue to look for snacks in the San Francisco Bay Area.
The key is to find ways to make sure it’s not their last meal.
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Main image: Dary Maltseva / Adobe Stock
