More than 13,000 seal pups die on Heard Island amid H5N1 outbreak

More than 13,000 southern elephant seal pups have died on Heard Island, an Australian external territory in the subantarctic Indian Ocean, after testing confirmed the spread of H5N1 bird flu among wildlife on the island and nearby McDonald Island. Government scientists observed the mass mortality during drone and ground surveys carried out in October 2025 and January 2026. The deaths were recorded on Heard Island, about 4,000km south-west of Perth and 1,700km north of Antarctica.

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The findings, which have been submitted as a preprint research paper, say death rates among seal pups averaged 76% across the island and reached as high as 97% in one location. Researchers also reported evidence of high seal pup mortality at McDonald Island. Dr Jarrod Hodgson, a senior research scientist and co-lead author, said the figures may be an underestimate because mortality was still ongoing when the team left the island.

He said typical pup mortality in a normal year would generally be below 5%. Testing has also confirmed H5N1 in six out of nine species on Heard Island, including southern elephant seals, king penguins, gentoo penguins, Antarctic fur seals and South Georgia diving petrels. Several hundred dead adult king penguins were also observed.

Wildlife veterinarian and epidemiologist Dr Tristan Burgess, who coordinates bird flu response for the Australian Antarctic Program, said the impact on elephant seals was consistent with other outbreaks in the southern hemisphere. The islands are home to more than a million breeding seals and seabirds, including many species listed nationally and internationally. The outbreak matters because Heard Island and McDonald Island are among Australia's most remote territories and support large, vulnerable wildlife populations with limited human presence.

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The detection is also significant because it is the first confirmed H5 bird flu detection in an Australian external territory, according to the research paper. Genetic analysis of samples suggests the virus was probably introduced via wildlife from the French subantarctic Crozet Islands, about 1,800km away, with an estimated arrival in August last year. The broader concern is the scale of the mortality and the number of species affected across the subantarctic islands.

Southern elephant seals and several seabird species breed in dense colonies, which can allow disease to spread quickly once introduced. The paper says the outbreak reflects the continued eastward spread of H5N1 in the southern hemisphere, adding to concern among scientists monitoring wildlife populations in remote regions. What remains unclear is how far the virus has spread beyond the islands and whether mortality will continue to rise in the coming surveys.

The researchers said the latest counts may still be incomplete because deaths were ongoing during their departure. Further monitoring will be needed to establish the full impact on seal and bird populations and to track whether additional species test positive.

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