{"id":540,"date":"2026-04-13T06:24:01","date_gmt":"2026-04-13T13:24:01","guid":{"rendered":"https:\/\/scienceblog.com\/wildscience\/?p=540"},"modified":"2026-04-13T06:24:05","modified_gmt":"2026-04-13T13:24:05","slug":"starving-gray-whales-are-swimming-into-san-francisco-bay-and-nearly-one-in-five-dies-there","status":"publish","type":"post","link":"https:\/\/scienceblog.com\/wildscience\/2026\/04\/13\/starving-gray-whales-are-swimming-into-san-francisco-bay-and-nearly-one-in-five-dies-there\/","title":{"rendered":"Starving Gray Whales Are Swimming Into San Francisco Bay, and Nearly One in Five Dies There"},"content":{"rendered":"\n

Something odd started happening beneath the Golden Gate Bridge in 2018. Gray whales, those barnacle-crusted migrants that normally barrel past the California coast on their way between Arctic feeding grounds and the lagoons of Baja Mexico, began turning left. They swam into San Francisco Bay, lingered for weeks at a time, and then some of them stopped showing up entirely. A new study tracking these wayward visitors has now confirmed what marine biologists feared: roughly 18 percent of the gray whales photographed alive in the bay were later matched to carcasses. Most had been struck by boats.<\/p>\n\n\n\n

The findings, published in Frontiers in Marine Science, paint a grim portrait of a population caught between two crises. Climate change is hollowing out their Arctic food supply, and the places they’re turning to for relief happen to be some of the busiest waterways on the West Coast.<\/p>\n\n\n\n

Gray whales are marathon migrants. They travel somewhere between 15,000 and 20,000 kilometres round trip each year, one of the longest migrations of any mammal, fattening up on amphipods in the Bering and Chukchi Seas before heading south to breed. That journey is fuelled almost entirely by fat reserves; it is considered unusual for gray whales to feed along the way. But the Arctic isn’t what it used to be. Prolonged thermal anomalies in the North Pacific have favoured less nutritious prey, and the population has cratered. According to NOAA, numbers have dropped more than 50 percent since their peak in 2016, and calf counts in 2025 sit at some of the lowest levels ever recorded.<\/p>\n\n\n\n

So the whales are improvising. Small subgroups like the Pacific Coast Feeding Group and the Puget Sound “Sounders” have long foraged in habitats south of the Arctic, snacking on ghost shrimp and herring roe. But the animals appearing in San Francisco Bay are, for the most part, not those seasoned opportunists.<\/p>\n\n\n\n

A team led by Josephine Slaathaug of Sonoma State University and researchers at the Marine Mammal Center built a photo-identification catalog of every gray whale they could document in the bay from 2018 to 2025. Using scars, mottling patterns, and the distinctive knobby ridges along each whale’s tail stock, they identified 114 unique individuals. When they compared these to catalogs of known foraging subgroups (some 503 whales in total), only eight matched. The researchers have taken to calling these newcomers “Bay Grays,” and their lack of connection to established groups suggests this isn’t simply a range extension by whales that already knew how to forage in busy coastal waters. It looks, perhaps, more like a conditional strategy: hungry animals trying something new.<\/p>\n\n\n\n

The trouble is that San Francisco Bay is a terrible place to be a whale. All vessel traffic accessing the ports of Oakland, Richmond, and San Francisco funnels through the Golden Gate Strait, and high-speed ferries, tour boats, and recreational watercraft criss-cross the central bay. “Gray whales have a low profile to the water when they surface, and this makes them difficult to see in conditions like fog which are common to San Francisco Bay,” Slaathaug explained. “Additionally, San Francisco Bay is a highly trafficked waterway, and the Golden Gate Strait serves as a bottleneck through which all traffic and whales must enter and exit.”<\/p>\n\n\n\n

Between 2018 and 2025, researchers examined images from 70 gray whale carcasses found in and around the bay. Of the 45 with enough skin remaining for comparison, they successfully matched 21 to their live catalog, a process that required identifying at least three unique marks on the same body area (whale skin sloughs off quickly after death, which makes the whole exercise rather painstaking). Of those 21 matched dead whales where a cause of death could be determined, nine out of 11 had died from blunt or sharp force trauma consistent with vessel strikes. Across the broader dataset of 70 carcasses, 30 showed similar evidence of being hit by boats. Malnutrition accounted for most of the remaining determined deaths. One whale, known as TMMC-1-06, survived an initial vessel strike that left a visible wound across its back but apparently couldn’t leave the bay afterwards, it stayed for 75 days before a second collision killed it, fracturing its ribs, skull, and vertebrae.<\/p>\n\n\n\n

The interannual resighting rate tells its own story. Only four of the 114 cataloged whales were spotted in the bay across multiple years. That is 3.5 percent, compared with resighting rates above 50 percent for established foraging subgroups elsewhere on the coast. “At least 18% of the individuals identified in San Francisco Bay later died in the area,” said Bekah Lane of the Center for Coastal Studies, a co-author. “Our broader analysis of local strandings both inside and outside San Francisco Bay found that over 40% of these whales died of trauma from vessels.”<\/p>\n\n\n\n

Intriguingly, whales belonging to established subgroups were significantly more likely to survive their time in the bay than the unaffiliated Bay Grays. The researchers reckon this could reflect learned behaviour; animals with prior experience navigating busy shipping lanes might be better at dodging boats. But the sample size is small (only eight subgroup members) and detection bias could play a role, since those whales are also monitored by other research teams and therefore less likely to have their fates classified as “unknown.”<\/p>\n\n\n\n

What didn’t predict mortality was how long a whale stayed. You might assume that spending more time in a vessel-clogged estuary would raise your odds of getting hit, but the data showed no significant relationship. Some whales entered and exited the bay multiple times within a season, two were documented foraging on northern anchovies off Pacifica before returning, which perhaps reduces their cumulative exposure. Still, every transit through the Golden Gate means running the gauntlet again.<\/p>\n\n\n\n

The situation is getting worse, not better. In 2025, 36 gray whales entered the bay, the highest count in the study period, with sometimes more than 10 present at once. Humpback whales have also begun using the bay in recent years. As Arctic conditions continue to deteriorate, more whales could be pushed into these improvised feeding grounds. Slaathaug said these findings are just one piece of a larger puzzle, as the overall population attempts to adapt to climate change in real time.<\/p>\n\n\n\n

Dynamic speed restrictions, onboard observers, and rerouted ferry lines could all help reduce the kill rate, and Lane has argued that continued monitoring is essential for figuring out where exactly the whales go once they’re inside the bay. For now, though, the Bay Grays remain caught in an ugly bind: too hungry to stay in the open ocean, too conspicuous to survive the alternative.<\/p>\n\n\n\n

DOI: 10.3389\/fmars.2026.1775666<\/a><\/p>\n\n\n\n

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