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One in Five Gray Whales That Enter San Francisco Bay Die There

As oceans warm and whale prey becomes increasingly scarce, gray whales have begun venturing into San Francisco Bay in search of food. But nearly one in five gray whales who enter the bay die there, many of them killed by passing boats, new research shows.

“Gray whales have a low profile to the water when they surface, and this makes them difficult to see in conditions like fog, which are common to San Francisco Bay,” said lead author Josephine Slaathaug, a graduate student at Sonoma State University in California. “Additionally, San Francisco Bay is a highly trafficked waterway, and the Golden Gate Strait serves as a bottleneck through which all traffic and whales must enter and exit.”

Each year, Eastern North Pacific gray whales migrate more than 4,000 miles from their feeding grounds in the Arctic to their calving grounds in the lagoons of Baja California. Before setting out, the whales gorge themselves on shrimp and other crustaceans, storing up enough fat that they are able to complete their epic journey without stopping to feed.

But as the Arctic warms, sea ice is dwindling, imperiling the ice algae that are food for crustaceans and, in turn, the crustaceans that nourish gray whales. Over the last decade, the number of Eastern North Pacific gray whales has dropped in half. And many whales that remain are struggling to store up enough fat to swim all the way to Mexico, scientists believe, prompting many to make a pit stop in San Francisco Bay.

But that stopover has proved perilous. Since 2018, the first year gray whales were seen beneath the Golden Gate Bridge, until 2025, 70 were found dead in and around the bay. Of those, 30 were clearly hit by boats, while many others likely died of malnutrition, according to the study.

Scientists identified as many whales as they could, both living and dead, by analyzing thousands of photographs, including from amateur naturalists. Of the dozens of whales they were able to identify in San Francisco Bay, 18 percent died there. The findings were published in the journal Frontiers in Marine Science.

The study notes that most whales are not returning to the bay year after year, suggesting that it is serving only as an emergency food stop. Scientists say that more research is needed to understand where, exactly, whales are headed in search of food and how best to protect them.