Gray whales have historically been a rare sight in the San Francisco Bay. They trek from the warm lagoons of Mexico’s Baja California more than 10,000 miles (16,000km) north to the Arctic region to feast on shrimp-like animals during the summers, seldom stopping in the busy shipping corridor for prolonged periods.

But in recent years, that story has changed in a dire way. A new study, published this week in the journal Frontiers in Marine Science, has found that gray whales in the bay have been dying at alarming rates, largely due to collisions with vessels.

Eastern North Pacific (ENP) gray whales began to show up with more frequency in the well-trafficked maritime corridor around 2018. According to researchers, at least 18% of gray whales that entered the bay from 2018 to 2025 have died. They determined that for more than 40% of the whale carcasses, the cause of death was blunt force trauma consistent with vessel strikes, prompting calls for renewed efforts to help avoid more fatal collisions.

“It was historically very unusual for them to enter the bay, especially for longer amounts of time or consistently year after year,” said Josie Slaathaug, lead author of the study.

There are whale subgroups known to hunt for food south of the Arctic, but a majority of the recently spotted whales feeding in the bay were not a part of these foraging clusters.

A wave of new whale presence had not been observed in the waters since the late 1990s, according to Slaathaug. Then and now, researchers have theorized that Arctic warming is disrupting food availability for the whales, driving them to hunt in new places such as the bay, although it remains unclear what exactly they may be eating there.

Their potential new feeding corner, though, is a major shipping route.

The true mortality rate for whales in the bay may be higher, hovering somewhere from 40% to 50%, Slaathaug said. It was difficult to match photographs of living whales to dead animals, due to skin decay or lost carcasses, she said.

In recent years, there have been several reports of dead whales that wash up on Bay Area beaches. The ENP gray whale population has been in decline due to malnutrition and starvation from climate-driven prey shifts in the Arctic, according to Slaathaug. The Southwest Fisheries Science Center estimated a population total of about 13,000 whales, its lowest count since 1970.

“It’s not unique to their migratory corridor that a lot of whales are dying,” Slaathaug said. “What is unique about San Francisco Bay and this study was that there was such a clear emerging cause of death.”

Some local efforts are under way to reduce vessel collisions.

The Marine Mammal Center, which is affiliated with the study, has developed a program called Whale Smart, to educate vessel operators in the San Francisco Bay on how to interpret whale behavior to avoid close encounters.

In Alaska, where vessels also pose a threat to the whale population, one fleet company partnered with WhaleSpotter, a company that uses AI and thermal imaging to detect the presence of whales, so they can change course well in advance.

Last year, the Center for Biological Diversity, a conservation group, sued the US Coast Guard, which regulates vessel traffic off the California coast, for failing to analyze how vessel routes may harm whales and sea turtles.

“This most recent study about the gray whales reaffirms that we have way underestimated the problem and we are not managing human activities well enough to avoid the whales,” said Catherine Kilduff, senior attorney at the center.

Federal action is needed to reduce the fatal collisions, Kilduff said.

According to the Endangered Species Act, she said, the coast guard should be consulting with the National Marine Fisheries Service, the government agency tasked with protecting the nation’s ocean life and their habitats, when setting shipping lanes to assess impact to marine wildlife, said Kilduff.

The coast guard did not immediately respond to a request for comment on the study’s findings.

Kilduff also suggested mandatory speed limits for vessels. “There are voluntary speed reductions on the west coast, but there is evidence that those aren’t effective. The compliance rate isn’t high enough,” she said.

A 2022 study co-authored by the National Oceanic and Atmospheric Administration found that the average speeds of large vessels had decreased from 2010 to 2019 in voluntary speed reduction zones. But, researchers determined that the cooperation rate of roughly 50% was lower than the amount needed to reduce vessel strike-related mortality to a level that maintains a sustainable whale population.

“These whales are using the oceans in such a sophisticated way. We can learn so much from them, and if we can figure out ways to avoid killing them, I know that they’ll come back to healthy population levels,” Kilduff said.