An outbreak of a highly pathogenic strain of bird flu has killed more than two dozen elephant seal pups in California, leading to the temporary closure of seal-viewing areas at a popular Bay Area park.

California’s Año Nuevo state park is home to an elephant seal colony with about 5,000 seals during the marine mammals’ breeding season, which runs from mid-December through March. Researchers said about 30 seals had died as of Thursday, nearly all of them weaned pups, amid the rise of avian influenza.

A team of researchers from UC Davis and UC Santa Cruz has intensified its monitoring efforts since detecting the outbreak last week, which marks the first cases of HPAI H5N1 in marine mammals in California. This strain was catastrophic farther south, in Argentina, where the outbreak led to a 70% mortality rate among pups born during the 2023 breeding season.

“Our team has been so dedicated working around the clock, over time, whatever it takes to continue monitoring this outbreak,” Roxanne Beltran, a professor and leader of Beltran’s lab, UC Santa Cruz’s northern elephant seal research program at Año Nuevo, told the Guardian. “It’s a really hard, emotional thing to go through as undergraduate, graduate students and young scientists to watch the seals that you’ve gotten to know over the years get sick.”

Beltran noted that, as of Thursday, deaths had remained at a steady, relatively low number rather than increasing exponentially.

The elephant seal colony at Año Nuevo state park is one of the most intensively studied in the world, according to Beltran. For more than six decades, scientists have monitored the colony through flipper tagging and long-term tracking of individual seals’ survival, reproduction and diving behavior.

“We’ve gathered more than 380,000 observations of 55,000 seals, so we know an extraordinary amount about what’s typical for those seals,” Beltran said.

She called the outbreak “unique” in disease ecology research because the team appears to have detected it at a very early stage. Precisely because the population is so well documented, with individual seals identified and tracked since birth, researchers can match infection outcomes to specific animals and better understand the outbreak’s consequences.

It remains unclear how the seals contracted the virus or whether it will spread beyond California.

“We don’t yet know where the seals got the virus, and we don’t know if, for example, they got it from birds, if the birds move in a way that could transmit the virus outside of California,” she said.

It also remains uncertain why weaned pups seem more susceptible to contracting the strain.

Researchers are also working with Noaa fisheries, the California department of fish and wildlife and the West Coast Marine Mammal Stranding Network to monitor marine mammals on the coast.

Año Nuevo state park is currently open with restrictions, according to its website. Cancellations for the seal-viewing tours, which are mandatory during the seals’ breeding season, have been issued through 1 March.

“Out of an abundance of caution, access is being paused to give wildlife space and allow for ongoing monitoring,” reads a notification on the state park’s website.

The park receives more than 60,000 visitors each year.