A humpback whale first spotted in San Francisco Bay on Tuesday, September 15, continued traveling east through the Bay and into the Sacramento-San Joaquin Delta near Rio Vista. The following morning, the U.S. Coast Guard reported the whale – identified through Cascadia Research Collective’s photo-identification catalog as CRC-11620 and named “Rio” – was farther up the Delta.

The Marine Mammal Center's experts are planning a response for visual assessment and are in close coordination with partners observing the animal as the situation evolves. We are hopeful Rio will find their way back to open ocean, and kindly ask boaters, kayakers, paddleboarders, and other observers to give this animal at least 100 yards of space for safety.

This whale is in an out-of-normal habitat area. Our team is actively monitoring the situation, and we kindly ask the public to please avoid changing the whale's course and let the animal decide where it wants to go.

“While it’s unknown whether this whale is just passing through or if there is an underlying, unknown ailment, the best thing people can do is experience this adult whale’s presence from a respectful distance,” said Kathi George, Director of Cetacean Conservation Biology at The Marine Mammal Center. “This whale is in an out-of-normal habitat area. Our team is actively monitoring the situation, and we kindly ask the public to please avoid changing the whale's course and let the animal decide where it wants to go.”

Humpback whales frequent the Bay this time of year, and multiple other individuals have been sighted in these waters this week. Historically, a humpback known as Humphrey spent several weeks in the Rio Vista area before returning to the ocean through the Bay in 1985. Then, in 2007, a mother-calf pair known as Delta and Dawn spent approximately 20 days in the Sacramento River Delta before eventually making their way back toward the Bay and ocean. The Marine Mammal Center responded to both events with the whales eventually making their way back out to open ocean.

Though their sex is unknown, Rio is a well-known individual that has been sighted 15 times over the past 24 years in California and Mexico. Their first recorded sighting occurred off California in 2002, and the most recent documented sighting prior to this week occurred in Baja California in February 2026. Cascadia Research Collective’s photo-identification catalog is publicly available at happywhale.com.

Photo by Norely Faz © The Marine Mammal Center, NOAA Permit #26532