Over seven years, researchers photographed and cataloged 114 individual gray whales inside San Francisco Bay. Then they checked those identification records against every carcass recovered in and around the bay. Twenty-one matches came back positive.
That is a minimum mortality rate of nearly 18%, according to a peer-reviewed study published in Frontiers in Marine Science in early 2026. Because not every dead whale is found, the actual death rate is almost certainly higher. The finding lands at a precarious moment for the species: NOAA’s most recent abundance data shows the eastern North Pacific gray whale population continues to decline and now sits at record lows not seen since systematic counting began in 1967.
For a population already under severe pressure, losing roughly one in five whales that enter a single coastal embayment is a number that demands scrutiny.
How the count was built
The research team used a combination of boat-based surveys, opportunistic sightings, and citizen-submitted photographs taken between 2018 and 2025 to build individual profiles of gray whales visiting the bay. Each whale was identified by unique markings, scarring patterns, and pigmentation visible in high-resolution images.
Those profiles were then cross-referenced with necropsy records from carcasses recovered in the bay and along nearby shorelines. Of the 114 whales with confirmed identifications, 21 were matched to dead animals. The methodology, detailed in the Frontiers paper, represents one of the most granular efforts to quantify mortality for gray whales in a specific geographic area rather than across the broader migration corridor.
Separate necropsy data compiled during what NOAA designated as an Unusual Mortality Event from 2019 to 2023 identified malnutrition as the most common cause of death among gray whales along the West Coast, with vessel strikes and entanglement documented as secondary factors. A peer-reviewed synthesis published in PLOS ONE detailed how blunt-force trauma from ship collisions is assessed during post-mortem examinations. NOAA’s investigation into the Unusual Mortality Event, which the agency has since closed, found no underlying infectious agent driving the die-off.
A population at historic lows
The bay-specific mortality numbers sit against a broader backdrop of decline. Shore-based counting stations off central California have tracked the eastern North Pacific gray whale population for decades, and the methodology is documented in a NOAA technical memorandum covering the 2023/2024 assessment cycle. NOAA’s winter 2025 survey confirmed that the downward trend has not reversed.
The population had rebounded strongly after commercial whaling ended, eventually being delisted under the Endangered Species Act in 1994. But the recent decline, accelerated by the Unusual Mortality Event years, has erased much of that recovery. The whales entering San Francisco Bay are not outliers from a thriving population. They belong to a species whose numbers are falling across its entire range.
What the study could not answer
Several gaps prevent a clean causal story from forming around the 18% figure.
The most fundamental limitation is decomposition. Skin decay degrades the markings used for photo identification, meaning a whale that dies and deteriorates quickly, or whose body sinks or drifts out to sea, would never appear in the matched-carcass count. The 18% is a confirmed floor, not a ceiling, and no upper bound has been established.
Movement data is also incomplete. Without satellite tags or continuous monitoring at the bay’s entrance, researchers could not track every whale’s arrivals and departures in real time. Some of the 114 identified animals may have visited the bay multiple times; others may have passed through briefly. That makes it difficult to determine whether extended stays increase mortality risk or whether the bay is dangerous even for whales transiting quickly.
Perhaps the most intriguing finding is that the bay’s gray whales showed limited overlap with known foraging groups along the Pacific coast. That raises a question researchers could not yet resolve: Are these animals already in poor condition when they arrive, driven into the bay by hunger? Or are otherwise healthy whales drawn in by prey and then exposed to lethal hazards like ship traffic?
If the former, the bay may simply be where weakened animals go to die. If the latter, it may function as a population-level sink, actively removing healthy individuals from a shrinking population. The distinction matters enormously for conservation strategy, and the current data cannot separate the two scenarios.
Ship traffic and the limits of voluntary slowdowns
San Francisco Bay is one of the busiest ports on the West Coast. Container ships, tankers, and bulk carriers transit the same waters where gray whales have been documented feeding and resting. The potential for collisions is not theoretical; vessel strikes have been confirmed as a cause of death in gray whale necropsies along the California coast.
A voluntary vessel speed reduction program, coordinated through the Greater Farallones National Marine Sanctuary, requests that ships of 300 gross tons or more slow to 10 knots in designated zones during peak whale season. Compliance is tracked using AIS transponder data from the U.S. Coast Guard. But the program targets offshore shipping lanes near the Farallon Islands, not the narrower interior channels of the bay where whales and large vessels share the tightest quarters.
The program also remains voluntary. Updated compliance figures beyond 2023 have not been published in available sources, and without them it is difficult to assess whether the slowdowns are making a measurable difference. Proposals to make speed limits mandatory, expand slow-speed zones into the bay itself, or redesign traffic separation schemes have been discussed in conservation circles, but none have advanced to formal rulemaking as of May 2026.
What would change the picture
The 18% mortality figure is striking enough to warrant immediate follow-up, but translating it into effective policy requires filling specific knowledge gaps.
First, systematic necropsies on every gray whale carcass recovered in the bay would help determine what proportion of deaths are caused by vessel strikes versus malnutrition versus entanglement. Right now, the 21 matched carcasses represent a confirmed death count, but the breakdown of causes has not been published at the individual level for the bay specifically.
Second, satellite tagging or acoustic monitoring at the Golden Gate and within the bay would provide real-time data on how long whales stay, where they concentrate, and how closely their movements overlap with shipping lanes. That information would allow managers to design targeted interventions rather than relying on broad-area speed reductions that may not cover the highest-risk zones.
Third, researchers need a clearer picture of what draws gray whales into the bay in the first place. If prey availability inside the bay has increased due to changes in sediment composition, water temperature, or nutrient runoff, that could explain why more whales are entering and staying longer. Conversely, if prey is declining elsewhere along the coast, the bay may be a refuge of last resort for animals with few other options.
Elevated risk in a waterway a declining species cannot afford to lose
For now, the most defensible reading of the evidence is straightforward: San Francisco Bay is a zone of elevated risk for a species that cannot afford the losses. A population at historic lows is sending a measurable fraction of its members into a confined, heavily trafficked waterway, and nearly one in five of those animals is not coming back out alive. Whether the bay is a cause of death or merely the place where struggling whales finally succumb, the outcome for the population is the same.
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*This article was researched with the help of AI, with human editors creating the final content.
