Wildlife mystery: Why are gray whales swimming into San Francisco Bay in increasing numbers?

Gray whales have been veering off their normal routes along the West Coast and swimming under the Golden Gate Bridge into San Francisco Bay in unprecedented numbers.

Using thousands of photographs of distinctive markings on the whales’ backs to identify them, marine scientists have confirmed that at least 71 different gray whales — and possibly 84 or more — swam into the bay between 2018 and 2023, with some staying for more than two months, raising their risk of being hit by cargo ships, oil tankers or other large vessels.

From 2010 to 2017, only about 1 or 2 of the giant marine mammals came into the bay every year. Last year, however, there were at least 16, and in 2019 there were at least 21.

“We think it has a lot to do with the fact that the whales haven’t been getting enough food,” said Bill Keener, a biologist with The Marine Mammal Center, a non-profit group in Sausalito. “They may be weak and resting for a while, or they looking for an alternative food source.”

Some are malnourished, he said.

From 2019 to 2023, 22 gray whales were found dead in or near San Francisco Bay, according to data from The Marine Mammal Center, the California Academy of Sciences and public agencies. Of those, 14 died from unknown causes. Researchers performed studies, called necropsies, on nine of the whales. Six died from malnourishment. Three died from a collision with a ship.

Over the past four years, dead gray whales have been found inside San Francisco Bay off Angel Island, and near Richmond, Rodeo, Hercules, San Leandro, Mountain View, the Port of Oakland, Tiburon, the Berkeley Marina and Martinez.

“They aren’t just near the Golden Gate,” said Keener, who said a slower speed limit for big ships in the bay may be needed. “They are way into the bay, past Angel Island, down to Treasure Island. There’s a lot of ship traffic there.”

Wayward whales have inspired public interest for years.

One lost humpback, nicknamed Humphrey, gained national attention in 1985 and became the subject of children’s books, songs and a movie — drawing crowds of onlookers with binoculars — when he swam into the bay. Humphrey meandered up the Delta to sloughs 25 miles south of Sacramento, staying 26 days before finally returning to the Pacific Ocean as researchers played whale songs from speakers off boats to lure him west.

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