29-05-2025
- Science
- San Francisco Chronicle
An astounding number of whales are showing up in San Francisco Bay
A shocking number of gray whales have been seen congregating — and dying — in San Francisco Bay this year. Five perished in the past week, including three found floating in different parts of the bay on Memorial Day.
Gray whales travel close to the coast each spring, many with calves in tow, on their way to their main source of food in Alaska. Until fairly recently, stopping in San Francisco Bay and lingering there was considered unusual. But so far this year, 33 gray whales have been sighted in the bay compared to four last year, based on photo identification, according to the California Academy of Science in San Francisco and the Marine Mammal Center in Sausalito.
Some have spent as long as 20 days in the bay and many have been seen foraging near Tiburon. Though it can add a thrill to ferry commutes, the behavior concerns scientists because it puts the whales at risk of being hit by ships, and it may mean that they are not getting enough food due to global warming and changing conditions in the arctic. The whales will likely be around for only another week or two before the last of them heads farther north.
'It's unexpected,' said Bill Keener, a researcher at the Marine Mammal Center who spotted whale number 33. The total sightings are at least twice as many as ever before, though he and other observers, including members of the public, have also increased the amount of time spent looking for them in recent years. 'The numbers were so much lower last year. It was trending in the right direction.'
In addition, more gray whales are washing up dead on Bay Area beaches than in recent years, with a total of 13. That is close to the record-high numbers in 2019 and 2021, when 14 and 15 whales died respectively, the groups said. This difference this year is that almost all of the dead whales were found within the bay rather than on coastal beaches or farther away from San Francisco, said Sue Pemberton of the California Academy. The reasons for the changes are not yet clear, she said.
'It's very interesting and a little perplexing. We haven't really seen anything like this before,' she said. 'Having this many whales die inside the bay definitely feels like it's a new event.'
Pemberton said scientists are performing necropsies, or autopsies for animals, to the extent possible. That proved difficult this week, because many of the whales — found in the bay near Berkeley, Alcatraz and Point Bonita as well as in Bolinas and near Southeast Farallon Island — were in advanced stages of decomposition or hard to reach.
The dead whale spotted by researchers on Southeast Farallon Island was in an inaccessible area, so the scientists used a live webcam operated by the Academy to study it and to determine that it was likely a gray whale.
One theory for why gray whales are lingering in the bay and why they're struggling in general, is because they may not be getting enough to eat in the Arctic. The base of the food chain there is a type of algae that grows underneath sea ice, which is retreating with climate change, Pemberton said.
'Does this represent a blip? Something that's just weird and they decided to stop at the snack shack in SF Bay?' Pemberton said. 'Or, at the other end of the spectrum, is this an adaptation to conditions out in the ocean?'
From 2019 to 2021, the West Coast gray whale population experienced what scientists called an 'unusual mortality event,' meaning a high number of deaths. Federal scientists are monitoring this year's population from an observation station south of San Luis Obispo, counting the cetaceans as they swim from Baja California to Alaska, and will have an estimate in a few weeks, said Michael Milstein, a spokesperson for National Oceanic and Atmospheric Association.
So far the total number of gray whale deaths along the West Coast, including Alaska, is 38, higher than last year's 31, Milstein said. At the same time, scientists in Mexico and other observers in Southern California are reporting seeing fewer calves than usual, something that was seen during the mortality event, said Milstein.
'The thinking is that if the females are not getting as much food or having to work harder for the food, they don't have enough energy to give birth and nurse a calf to the point it will survive,' he said.
However, while some of the dead whales appear to be skinny, others are not. A few show signs of blunt force trauma from ship strikes, though the U.S. Coast Guard is working to make ships aware of the location of whales inside the bay or shipping lanes, and ferry boat operators inside the bay are slowing down or changing routes to avoid them.
'There's not one silver bullet explanation at this point,' Milstein said.
