Australia's already fragile sea lion population is facing a troubling new threat after H5N1 bird flu was detected in a long-nosed fur seal in South Australia on Saturday.
Researchers say the virus could move rapidly through crowded seal and sea lion colonies, raising the stakes for one of the country's most vulnerable marine mammals.
The first confirmed mainland mammal case of H5N1 in Australia followed testing on a dead long-nosed fur seal found at Beachport on the Limestone Coast, according to The Guardian.
Authorities were also investigating a second dead long-nosed fur seal from the Pages, off Kangaroo Island.
At Charles Darwin University, mammal scientist John Woinarski said the outbreak could have major consequences.
"The carnage may be substantial and represent a serious conservation impact," he told The Guardian. "I fear for Australian sea lions in particular because of their small population size."
Australian sea lions number only about 12,000, and roughly three-quarters of them are in South Australia. Some assessments place the total below 10,000.
One of the country's largest Australian sea lion breeding colonies is on the Pages, where mass deaths among greater crested terns have been recorded, fueling concern that the virus may be present in the marine environment.
What has happened elsewhere to marine mammals is a major reason scientists worry about H5N1.
The scale of loss overseas has been enormous. More than 17,000 southern elephant seal pups died in Argentina in 2023, and drone surveys later found upward of 13,000 dead southern elephant seal pups on Australia's Heard Island in 2025 and 2026.
Jane Younger, who studies Antarctic disease ecology at the University of Tasmania, said the pattern is difficult to ignore.
"In South America, we've seen huge infection rates in sea lions and fur seals," she said, per The Guardian. "In some places, the numbers are just astronomical."
Sea lions are part of ocean systems that coastal communities depend on, and major losses among them can signal deep ecological stress.
South Australia is contending with environmental strain from an algal bloom, and conservationists say animals weakened by food shortages may be less able to survive another blow.
Officials are exploring possible responses even though no registered mammal vaccine exists for H5 bird flu.
The South Australian government has said it is looking into possible vaccination steps for high-priority at-risk mammals, including Australian sea lions at Seal Bay on Kangaroo Island.
"Once you get mammals infected, it's likely you will see mammal-to-mammal transmission, which is obviously of concern because that facilitates more rapid spread between [members of] a species," said Rachael Gray, who specializes in pinnipeds at the University of Sydney's School of Veterinary Science, per The Guardian.
The Australian government has classified the long-nosed fur seal, Australian fur seal, and endangered Australian sea lion as facing a very high risk from H5 bird flu.
Surveillance, testing, and rapid responses around breeding colonies could be critical.
Alexia Wellbelove, campaigns director at the Australian Marine Conservation Society, said the group has been pushing for more funding to reduce other pressures on sea lions, including fishing impacts, so the animals are not facing multiple threats at once.
"There's only 12,000 Australian sea lions in existence," Younger said. "For some of these other species like elephant seals, we've seen many more than that killed by this virus. Hopefully it doesn't wipe out the species, but that's a possibility."
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