San Francisco Bay Becomes a "Death Trap" – Nearly One-Fifth of Bay Gray Whales Perish Here
Gray whales are using San Francisco Bay as a temporary refuge for survival, but for many, it has become a fatal endpoint. As climate change reshapes marine ecosystems, these long-distance migratory marine mammals are exhibiting unusual behaviors, with some foraging in the busy and dangerous waters of San Francisco Bay, a choice that often ends in tragedy.
Gray whales typically undertake long-distance migrations between their Arctic feeding grounds and the warm breeding lagoons of Baja California, Mexico. However, climate change is disrupting their food supply in the Arctic, making these marine giants more vulnerable. According to the National Oceanic and Atmospheric Administration, the gray whale population has declined by more than 50% since 2016, and sightings of calves have become rare.
San Francisco Bay has historically not been part of the gray whale migration route. This began to change in 2018, when gray whales started appearing in the area more frequently, accompanied by a disturbing increase in mortality. To understand what was happening to these so-called “bay gray whales” and whether they would return year after year, researchers launched an in-depth investigation.
Josephine Slaght-Hogard, of Sonoma State University, and her research team explained in an article published in *Frontiers in Marine Science*: “When gray whales surface, their profile is relatively low, making them difficult to spot in conditions common in San Francisco Bay, such as foggy weather. Additionally, San Francisco Bay is a heavily trafficked waterway, and the Golden Gate Strait acts as a bottleneck through which all vessels and gray whales must pass.”
To conduct the investigation, scientists created an individual gray whale catalog using opportunistic sightings and photos collected by the public between 2018 and 2023, as well as more systematic surveys conducted between 2023 and 2025. They compared these records with necropsy data from stranded gray whales. Identification relied on unique markings on the skin of each whale, although these markings fade rapidly after death, meaning some individuals may not have been matched.
The results showed that a total of 114 individual gray whales were recorded in San Francisco Bay between 2018 and 2025. Only four gray whales were observed in more than one year, suggesting that most do not return. Researchers believe the bay may serve as an emergency foraging station for gray whales in poor condition, as very thin whales have also been observed foraging in other unusual habitats.
Despite potentially using the bay as a temporary refuge, many gray whales do not survive. From 2018 to 2025, a total of 70 dead gray whales were found in the area. Thirty were killed by ship strikes, and many other individuals with known causes of death showed signs of malnutrition. Of the 45 deceased gray whales that could be identified, 21 matched individuals in the catalog.
Becky Lane, of the Coastal Research Center, and a co-author, noted: “At least 18% of the individuals identified in San Francisco Bay later died in the area. Our broader analysis of local strandings both inside and outside of San Francisco Bay found that over 40% of these gray whales died from trauma caused by vessels.”
In 2025 alone, 36 gray whales entered the bay, sometimes in groups of more than 10. Researchers say more work is needed to understand how gray whales are utilizing the bay and how to reduce the risks they face. Further investigation and necropsies could help determine whether the lower number of returning gray whales is due to high mortality rates, and whether death is primarily caused by starvation, ship strikes, or a combination of both. Weakened gray whales may have more difficulty avoiding vessels.
Efforts to reduce these risks could include educational programs for commercial vessel operators or changes to ferry routes. Lane said: “The biggest threat to these gray whales in San Francisco Bay is vessel traffic. Continued monitoring will help elucidate their distribution patterns and behavior within the bay, which could inform risk. Route changes and speed restrictions have been shown to significantly reduce fatal ship strikes to large whales, and risk assessments can help determine the most effective strategies for protecting these animals.”
Scientists warn that significant gaps remain in understanding how individual whales move and behave on a day-to-day scale. Even so, these findings provide valuable insights into how gray whales are coping with rapidly changing environmental conditions.
Slaght-Hogard said: “This study is the best analysis we can do with the data we’ve collected, but it’s important to remember that we don’t have a complete picture of every gray whale’s movement on a day-to-day timescale. These results are an important piece of the larger puzzle of what’s happening as the entire population attempts to adapt to climate change in real time.”