One in five gray whales entering San Francisco Bay die there

Scientists tracking gray whales in San Francisco Bay have found that nearly one in five individuals identified there died in its waters after becoming victims of vessel strikes, starvation, and a migration route that was never meant to pass through one of the world’s busiest ports.

Gray whales have long made one of the animal kingdom’s great migrations – journeying between the rich feeding grounds of the Arctic and the warm lagoons of Baja California. San Francisco Bay was never part of that route. Until now.

Since 2018, scientists have been documenting a troubling new phenomenon: gray whales entering the busy, boat-filled waters of San Francisco Bay in search of food. What drew them there – hunger, desperation, a prey base collapsing under the weight of a warming ocean – is becoming clearer. But it’s what happens to them once they arrive that makes for worse reading.

A new study published in Frontiers in Marine Science, led by researchers at the Marine Mammal Center and the California Academy of Sciences, has pieced together seven years of survey data, citizen science photographs and necropsy records to build the most detailed picture yet of these so-called ‘Bay Grays’. Between 2018 and 2025, scientists identified 114 individual whales entering the bay. Of those, 21 were later found dead in the surrounding area – a minimum mortality rate of 18%. Just four individuals were recorded returning in more than one year.

The Golden Gate Strait, through which every whale must pass to enter or exit the bay, is also one of the busiest maritime corridors on the US Pacific coast. Ferries, container ships, and recreational vessels funnel through the same narrow bottleneck, in conditions that make spotting a surfacing whale far from straightforward.

“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,” 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.”