Bird Flu Breaches California's Coast: Inside the H5N1 Outbreak Killing Elephant Seals at Año Nuevo

The first-ever detection of highly pathogenic avian influenza in northern elephant seals has researchers racing against time at a beloved Bay Area state park — and watching nervously as the virus jumps to new species along the San Mateo coastline.

A Breeding Colony Under Siege

On the fog-shrouded beaches of Año Nuevo State Park, roughly 55 miles south of San Francisco, a crisis is unfolding among one of the Pacific coast's most iconic marine mammal populations. In mid-February 2026, researchers from the University of California, Santa Cruz noticed something alarming: elephant seal pups — freshly weaned and preparing for their first ocean journey — were dying at an unusual rate [1].

A healthy pup, weaned on February 18, was dead within two days. Others displayed neurological symptoms never before seen in this colony: weakness, tremors, convulsions, and respiratory distress. By February 19 and 20, the pattern was unmistakable. Scientists from UC Santa Cruz's Beltran and Costa labs collected mucus and respiratory specimens and rushed them to UC Davis for testing. The results came back with alarming speed [2].

The samples were positive for highly pathogenic avian influenza (HPAI) H5N1 — the first confirmed detection of the virus in any northern elephant seal, and the first case of a marine mammal contracting H5N1 in California [3].

"This is exceptionally rapid detection of an outbreak in free-ranging marine mammals," said Christine Johnson, director of the UC Davis Center for Pandemic Insights [4].

The Death Toll Mounts

In the weeks since that initial detection, the outbreak has steadily expanded. As of March 12, the U.S. Department of Agriculture's National Veterinary Services Laboratory has confirmed H5N1 in 18 marine mammals at or near Año Nuevo: 16 northern elephant seals, one southern sea otter, and one California sea lion [5][6].

But confirmed cases likely represent only a fraction of the true toll. Patrick Robinson, UC Santa Cruz biologist and Año Nuevo Reserve director, reported that at least 47 seals have died on the mainland beaches during the outbreak period — a mortality rate four times higher than usual for this time of year. On nearby Año Nuevo Island, death rates are running three times higher than the mainland, with several adult males among the dead — a rare and concerning development [7].

"This count reflects only the animals that have gone through sampling and confirmatory testing in multiple labs," Johnson cautioned. "So there are likely more animals that we will be updating on in the coming weeks" [5].

The virus is spreading at a steady pace. Researchers report discovering an average of approximately two newly dead animals and two newly symptomatic animals each day [5].

Jumping Species: Sea Otters and Sea Lions

The most alarming development came on March 12, when researchers confirmed the virus had jumped beyond elephant seals to other marine mammal species along the San Mateo coast [6].

The discovery of an infected sea otter caught scientists off guard. "We were certainly not expecting to see a sea otter," Robinson told the East Bay Times [6]. He noted that while five to ten dead sea otters appear annually along the coast — typically victims of shark bites — this death was different. A "handful" of California sea lions are also awaiting test results.

Johnson described the species jump as concerning but not unprecedented. "It's of concern but hasn't spread much so far. It has been a very limited outbreak," she said [6]. Similar spillover events occurred during previous H5N1 outbreaks in South America, where occasional infections in non-primary host species were typical.

Despite the expanding species range, Robinson emphasized the broader picture: "There are still thousands of apparently healthy animals. We are only observing a small number of impacted animals every day" [6].

How Did the Virus Get Here?

The H5N1 strain circulating at Año Nuevo did not appear out of nowhere. This particular variant emerged in 2022 among bird populations migrating through the Pacific Flyway — one of the four major north-south flyways used by migratory birds in the Americas. It had previously caused a mass mortality event among northern fur seals on an island in eastern Russia [5].

Scientists believe the virus reached the elephant seals through contaminated bird waste. Año Nuevo Island, located just offshore, is popular with cormorants, Western gulls, and other seabirds that share habitat with sea lions, elephant seals, and other marine mammals. The seals likely contracted the virus from droppings deposited on shoreline rocks or the island itself [8].

UC Davis researchers have identified a specific genotype in this strain showing "adaptation to infect mammalian hosts" — a finding that underscores the virus's continued evolution [7]. A full necropsy performed on one deceased pup by UC Santa Cruz wildlife veterinarian Megan Moriarty revealed swollen lungs, enlarged lymph nodes, and brain inflammation — hallmarks of severe H5N1 infection [2].

Samples from 60 additional elephant seals are currently being tested for the virus [9].

Source: GDELT Project

Data as of Mar 13, 2026CSV

The Ghost of South America

What makes the Año Nuevo outbreak especially nerve-wracking for scientists is the precedent set by H5N1's devastating march through South American marine mammal populations.

In 2022 and 2023, the virus tore through coastal ecosystems along the Pacific coast of South America. More than 30,000 South American sea lions died as H5N1 spread along the coasts of Peru and Chile. Porpoises, dolphins, and otters were also killed in smaller numbers [10].

The worst catastrophe came in late 2023 at Península Valdés, Argentina, home to the world's largest breeding colony of southern elephant seals. There, the virus killed an estimated 17,400 pups — more than 95 percent of that season's newborns. Epidemiological data published in Nature Communications confirmed mammal-to-mammal transmission was driving the outbreak, not just repeated bird-to-mammal spillover [11].

The carnage didn't stop there. At South Georgia Island in the South Atlantic — home to the world's largest southern elephant seal population overall — drone surveys revealed a 47 percent decline in breeding females between 2022 and 2024. Researchers estimate roughly 53,000 females across the island may have missed the 2024 breeding season entirely [12]. The virus has since been detected in Antarctic wildlife as well, killing more than 50 skuas during the 2023–2024 austral summer [13].

"A change in the number of pups that survive in a given year has a really, really, really long-lasting consequence on the population," said Roxanne Beltran, who leads UC Santa Cruz's elephant seal research program at Año Nuevo [2].

A Silver Lining — For Now

Amid the grim numbers, there is cautious optimism about the Año Nuevo colony's long-term prospects. Roughly 80 percent of adult female elephant seals had already migrated away from Año Nuevo before the outbreak began, and none have been found dead or showing flu-like symptoms [5]. The breeding season at Año Nuevo runs from December through March, with pregnant females arriving from late December through early February to give birth. By early March, most adults have departed for the open ocean [14].

Approximately 5,000 seals use Año Nuevo State Park during the winter breeding season. When the outbreak was first detected, about 1,350 seals remained on the beach — primarily weaned pups preparing for their first ocean journeys [7]. The fact that the outbreak arrived late in the season, after most adults had left, may have prevented a far worse disaster.

The outbreak also remains geographically contained. No infections have been detected at the other major northern elephant seal colonies along the California coast: Point Reyes National Seashore in Marin County, where the peak single-day count this season was 2,233 seals on January 30, or Piedras Blancas in San Luis Obispo County [6][7].

Researchers are monitoring those colonies closely. The concern is that young seals from Año Nuevo, some potentially carrying the virus, will disperse along the coast in the coming weeks as they begin foraging.

Public Health and the Human Question

California health authorities have moved swiftly to address both wildlife and human safety concerns. The California Department of Public Health (CDPH) issued a public advisory urging people to avoid contact with sick or dead marine mammals and birds along the California coast [15].

San Mateo County infectious disease physician Catherine Sallenave emphasized that no human infections have been confirmed in connection with the Año Nuevo outbreak. The risk to the general public remains very low, and there is no evidence of seal-to-human transmission [6].

Still, the advisory comes against a broader backdrop of growing H5N1 concern nationwide. The virus has been confirmed in dairy cattle herds across 19 states since 2024, and at least 70 people in the United States have tested positive for avian influenza since the dairy cattle outbreak began — mostly farm workers exposed to infected cows or poultry. Symptoms have generally been mild, ranging from conjunctivitis to typical flu symptoms [16].

Researchers working directly with infected animals at Año Nuevo are taking no chances. All scientists wear full personal protective equipment — body-protective coveralls, splash goggles, N95 masks, and gloves [9]. The Marine Mammal Center has paused rescue operations in the area pending further understanding of transmission risks [2].

The Economic and Cultural Impact

California State Parks has temporarily closed public access to the elephant seal viewing areas at Año Nuevo and cancelled its popular guided elephant seal tours for the remainder of the 2025–2026 season [3]. These tours, which typically run from December through March, are among the most popular wildlife experiences in the Bay Area, drawing tens of thousands of visitors each year to observe the massive animals during their dramatic breeding season.

The closure represents a significant loss for both the park system and the coastal communities that benefit from visitor traffic. It also means the public will miss witnessing the final weeks of a breeding season that, for many of the colony's youngest members, has ended in tragedy.

What Comes Next

The multi-agency response now involves an unprecedented coalition: UC Santa Cruz, the UC Davis Center for Pandemic Insights, NOAA Fisheries, the California Department of Fish and Wildlife, The Marine Mammal Center, the West Coast Marine Mammal Stranding Network, and the U.S. Department of Agriculture are all coordinating monitoring and response efforts [15].

The critical weeks lie ahead. As weaned pups disperse from Año Nuevo in March and April, researchers will be watching for signs of the virus appearing at other colonies and along new stretches of coastline. The question is not just whether more animals will die at Año Nuevo, but whether this outbreak will remain a localized tragedy or become the first chapter of a larger crisis for northern elephant seals — a species that was hunted to near-extinction in the 19th century and has been celebrated as one of conservation's great comeback stories.

"We know these individuals," Beltran said of the seals she has studied for years. "This makes it especially difficult for our team" [2].

The virus, indifferent to such attachments, continues its steady arithmetic: two new deaths, two new cases, each day.