"Gray whales have a low profile on the water when they surface, and this makes them difficult to see in conditions like fog, which are common in San Francisco Bay," explained Josephine Slaathaug of Sonoma State University, lead author of the study. "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."

Off the map

Because it's considered unusual for gray whales to eat during their migrations to and from tropical lagoons, they are heavily dependent on the prey in their Arctic feeding grounds. But the impact of the climate crisis on their preferred prey means that gray whales are increasingly vulnerable. According to the National Oceanic and Atmospheric Administration, their population has dropped by more than 50% since 2016. Very few calves are being spotted.

Formerly, gray whales' migration pathways didn't include San Francisco Bay, but since 2018, scientists have spotted gray whales appearing there—and dying there. The scientists set out to determine what was happening to these "Bay Grays," and whether they were regular visitors.

They developed a catalog of visiting whales using opportunistic surveys and citizen science photographs from 2018–2023, supplemented by systematic surveys from 2023–2025. This was compared to necropsy records. However, whales were identified using unique markings on their skin, which decays quickly, so strict identification criteria mean that some cataloged whales may have been among the unidentified necropsied whales.