HPAI H5N1 found in California seals

California State Parks closes beach to visitors

Seven weaned elephant seal pups that looked sick on February 19 and 20 in California’s Año Nuevo State Park tested positive for highly pathogenic avian influenza (HPAI) H5N1, the U.S. Department of Agriculture’s National Veterinary Services Laboratory confirmed late Tuesday, February 24. Thirty seals have died, but test results have not yet confirmed that they died of H5N1

The outbreak marks the first cases of HPAI H5N1 in marine mammals in California and the first detection in northern elephant seals. 

“This is exceptionally rapid detection of an outbreak in free-ranging marine mammals,” said Professor Christine Johnson, director of the Institute for Pandemic Insights at the University of California, Davis’ Weill School of Veterinary Medicine. “We have most likely identified the very first cases here because of coordinated teams that have been on high alert with active surveillance for this disease for some time.” 

On Monday, February 23, California State Parks closed Año Nuevo State Park to public access for the elephant seal viewing area of the Año Nuevo Coast Natural Preserve. With the confirmation that H5N1 was responsible, the tours have been canceled for the rest of the season.  

State and federal agencies work together

UC Davis scientists have been testing samples from marine birds and mammals at the park and other regions along the coast since 2024. The NSF Center for Pandemic Insights at UC Davis and UC Santa Cruz have been working together to increase surveillance at this site over the past two months in anticipation of a possible outbreak in seals.

The scientific team is now working closely with NOAA Fisheries, the California Department of Fish and Wildlife and the West Coast Marine Mammal Stranding Network to continue to monitor marine mammals along the coast.

On Feb. 19 and 20, Beltran’s team noticed seals at Año Nuevo Reserve with abnormal respiratory and neurological signs, including weakness and tremors. The Reserve’s elephant seals are intensively studied, with 380,000 observations collected over the years. One of the dead weaners was the pup of a marked mother who was herself born on that beach. The pup was entered into the research program when she was 15 days old. She was weaned when her mother left the beach, and two days later the team observed her convulsing on the beach at 9:30 am. By afternoon, she was dead.

“It’s tough to watch animals we have followed and watched for years get sick,” Beltran said. “We know their family lineages." She praised her team’s professionalism in continuing to collect information.

That individual information will help to assess the long-term effects of how many pups survive, whether females are affected, and future births, on the population.

They collected samples from sick and dead elephant seals and took them to UC Davis for testing at the California Animal Health and Food Safety Laboratory System. Initial screening revealed the samples were positive for avian influenza, which the USDA’s NVSL lab now confirms is HPAI H5N1.

“Given the catastrophic impacts observed in related species, we were concerned about the possibility of the virus infecting northern elephant seals for the first time, so we ramped up monitoring to detect any early signs of abnormalities,” said Roxanne Beltran, a professor in Ecology and Evolutionary Biology at UC Santa Cruz. Beltran’s lab leads UC Santa Cruz’s northern elephant seal research program at Año Nuevo.

“We are cautiously optimistic, as most of the adult females had already departed the beach for their routine migrations before the outbreak began, and most seals on the colony seem healthy,” Beltran said. 

Virus threatens wildlife worldwide

As reported by Sharon Guynup in Mongabay in 2024, H5N1 is a highly contagious, deadly strain of avian influenza. It evolved from a mild form of avian influenza in the 1990s into a deadly pathogen from infected industrial poultry operations, breaking out in Europe in 2020.

Since then, it has leapt the species barrier, affecting at least 485 bird species and 50 mammal species, killing seals, sea otters, dolphins, foxes, California condors, albatrosses, bald eagles, cougars, polar bears and a zoo tiger. Animals as diverse as rats, elephant seals, bears, house cats, and humans — with an unprecedented outbreak in U.S.dairy cattle having affected 17 states and more than 1,000 herds. 

It is carried by migratory birds, and has been found on every continent except Australia, including Antarctica. In some bird and mammal species, the toll has been devastating. Some imperiled species could be pushed to extinction, with wildlife already fighting to survive against a changing climate, disappearing habitat and other stressors.

Since 2024, 71 human cases have been diagnosed in the U.S., with two deaths. Current public health risk is rated Low, with no person-to-person transmission reported.

“The more a virus like this is able to mutate and find its way into a wide range of species, especially farmed species that live in close contact with people like poultry and now cattle, the more the odds go up that a viral strain will more easily make that leap to people,” wildlife veterinarian Steve Osofsky, D.V.M., the Jay Hyman professor of wildlife health and health policy at Cornell University’s College of Veterinary Medicine, wrote in Stat in June 2025.

Rapid transmission

The virus’ ability to change rapidly, and its record of infecting other species makes it a cause of intense concern. 

Pinnipeds such as elephant seals, which congregate in large groups, are among the vulnerable species. The virus swept along South America’s Atlantic and Pacific coastlines, slaying more than 30,000 sea lions in 2022-23, when the H5N1 virus moved down from North and Central America to invade both the Atlantic and Pacific coasts of South America. In 2023, it killed an estimated 17,000 Southern elephant seal pups  (Mirounga leonina) on Argentina’s Península Valdés — the species’ largest die-off ever.

No more tours this season

At Año Nuevo, a California State Parks spokesperson said that 134 tours totaling 1,203 tickets were canceled through March 1, including 22 school tours. Thousands ofvisitors pay $11 per person to hike out a mile and a half to two miles with a guide to view the elephant seals during the mid-December through March pupping and mating season.

The park’s Mainland Camera remains active to watch the seals on the beach.

#añonuevo #CaliforniaStateParks #sanmateocoaststateparks #ThatsMyPark

The Marine Mammal Center has suspended its rescue response to elephant seals and harbor seals, until further information indicates whether this is an isolated cluster or the beginning of a wider outbreak.

“If it’s a cluster, we will figure out,” Dominic Travis, Chief Programs Officer, The Marine Mammal Center, said. “If it’s perpetuated, it will be really tricky. We will assess it day by day with NOAA.”

 

Most of the adult seals have left the beach on their next migration, escaping from what may be only a cluster of infection. As of February 25, only a small number of seals have been affected.

Traveling with an elephant seal bull

 J912’s tracking device reveals his journey

An adult male elephant seal, marked J912, with a tracking device on his head and an antenna on his back, hauled out February 5 at Piedras Blancas.

J912 ar Piedras Blancas. David Lawrence. 

Friends of the Elephant Seal docent Barrett Stuart was the first to notice that the seal, among many other males on the beach, had a device on his head. In February, breeding season, male seals are on the beach, moving around, seeking advantage with females and threatening each other.  He reported the sighting. FES docents Rian Dom and Dave Lawrence decided to revise their plans for the following day and go look for him.

 “Rian and I were going to tag hunt at VP2, but she suggested we start at VP3 to try to photo this seal,” Lawrence said.  “Rian is responsible for getting me there that following morning. I’m glad we did!”

VP2 is the elephant seal site south of the main viewpoint. “Tag hunting” is looking for seals with flipper tags, to photograph and report to researchers. Resighting tagged seals is important, to document where seals go and when.

Elephant seal J912’s tracking device, glued to mesh that is glued to the seal’s head. Dave Lawrence

Tracking male seals

It’s unusual, and exciting, to see any seal with a tracking device at the viewpoint. It’s always a female. Males are hardly ever tracked.

They, like much wildlife, migrate across international boundaries, making protection complicated. Hoary bats have flown from California to Hawaii. In January, a waved albatross, a critically endangered species from the Galapagos Islands and South America, was seen off Cambria’s coast. The bird had traveled about 3,300 miles from its typical range, making it an avian “vagrant,” the term used for birds outside their usual range.

Wide-ranging wildlife requires a global perspective. In J912’s case, it led to international cooperation to test a tracking device.

Dan Costa, Distinguished Professor of Ecology and Evolutionary Biology at UCSC, and head of the Costa Lab, said “J912 is a male that we tagged at Ano Nuevo last summer-fall after he molted. He is carrying two tags. One of the tags is a device we were testing from King Abdul University of Science and Technology, or KAUST. KAUST is in Jeddah, Saudi Arabia, and has an excellent technology development team. The device had GPS tracking capability and recorded dive behavior, and it used the Iridium Satellite system to transmit data. The other tag is one of our more standard tags built by Wildlife Computers in Redmond, Washington. It recorded diving behavior and animal position; position data were transmitted via the ARGOS satellite system.”

The KAUST tag has a much higher data bandwidth than the standard Argos tags, but is more challenging to establish communications with, especially on diving animals. It transmits GPS locations and has an accelerometer for measuring fine-scale head movement. Those head movements can be interpreted as feeding, when the seal catches prey.

The standard tag collects basic oceanographic data, depth, and temperature, the standard set of measurements for understanding the ocean environment.

“The rationale for the study is to learn more about male elephant seals.,” Dr. Costa said. “We have over 1000 records from individual female elephant seals, but only a few tracks of males (fewer than 50). Males are less reliable than females; that is, they aren’t as likely to return to Año Nuevo, as this individual shows, so there is a greater chance of not recovering the tag and thus not retrieving as much data.”

During the summer, J912 was on the beach to molt his skin. The device was attached after he had molted, to the new skin that would last for a year. He then left to go feed along the continental shelf, gaining blubber to sustain him though the breeding season.

 

In J912’s case, the tracking record shows that he left Año Nuevo on August 30 and arrived in the Aleutian Island foraging site, 2,573 miles away, on October 11, a 42-day migration, averaging 61 miles/day.

At the urge of some unknown signal, after feeding for two months, on December 4 he turned around to return to California from the Aleutians. He arrived on land near Point Buchon, 2,718 miles, on January 17, a 51-day migration, averaging 53 miles per day.

 

In both directions, he travelled in more or less a straight line. He stopped in places, perhaps to eat along the way, but his main goals were the rich foraging grounds in the Aleutians and then to return to California.

 

J912’s tracking records show that he had been around Piedras Blancas for a few weeks, before hauling out on the beach at Piedras Blancas February 5. He visited at least four birthing beaches in the rookery, but it isn’t clear whether he stayed to breed.

He was seen at Año Nuevo a few days earlier, but he was in the water then, and the team couldn’t get to him. “Apparently he stayed in the water and swam south as fast as he could!” FES member Kathy Curtis posted on GroupMe.

I didn’t see J912 while he was at Piedras Blancas. He stayed only two days, then moved on north to the Lighthouse Beach on February 8. He may find a breeding beach where he stays for a while, or move on. The device will record where he goes, and for how long.

The team may leave the device on him until he returns to Año Nuevo to molt, in July. The data from that, and the other four seals’ devices, will add to the understanding of these seals’ lives.

“Our goal is to better understand large male foraging behavior and ecology, energy gain rates, and diving physiology,” another member of J912’s research team said. “The other four males all returned to Año and we were able to recover all of their tags over the last month, which is really exciting.”

The complex tracking equipment on elephant seal JP12’s head allows researchers to collect data on the ocean environment as well as the seal’s dives and migration. Dave Lawrence

Webinar on elephant seals

Dr. Roxanne Beltran and research technician Conner Hale of the Beltran Lab at UC Santa Cruz will discuss how tracking individual variations can contribute to understanding the species in a free webinar "Unraveling individual and environmental drivers of variation among elephant seals," Tuesday, February 24, 7 pm. Register here.

American Cetacean Society - San Francisco Bay Chapter — Dr. Roxanne Beltran, "Unraveling individual and environmental drivers of variation among elephant seals"

Christine Heinrichs is the Monterey Bay National Marine Sanctuary Advisory Council SLO At-Large member. Her elephant seal column won first place from the National Society of Newspaper Columnists in 2024. Follow her on Facebook, Bluesky, LinkedIn, and Substack.