The islands, which sit about 2,485 miles (4,000 kilometers) southwest of mainland Australia have long been an isolated sanctuary for breeding birds and marine mammals.<\/p>\n
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Wildlife biologists collected drone imagery and conducted field work at Heard Island in October 2025 and January 2026, to check for signs of bird flu. – Courtesy Australian Antarctic Division\/Jarrod Hodgson<\/p>\n
The southern elephant seal pup mortality was estimated to be 76% across a population of 17,000 seal pups born on the islands, the program said. One area had a concentrated death rate of 97%.<\/p>\n
\u201cThe thing we don\u2019t know from our surveys so far is what the impact was on the breeding adult population of southern elephant seals,\u201d Hodgson said.<\/p>\n
Data collected in January also revealed several hundred adult king penguins across Heard Island have died, with scientists noting mortality was above normal levels.<\/p>\n
\u201cThese observations of H5 bird flu at Heard Island and McDonald Island are the first detection in an Australian external territory and show the continued eastward movement of the virus around the sub-Antarctic,\u201d wildlife biologist Julie McInnes said.<\/p>\n
\u201cOur results show a similar pattern to other sub-Antarctic islands, such as South Georgia, where elephant seals have been hardest hit,\u201d added McInnes, who is also lead author of the group\u2019s study.<\/p>\n
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Wildlife experts assessed a H5 bird flu mass mortality event among southern elephant seals at Capsize Beach, Paddick area of Heard Island, Australia on October 2025. – Courtesy Australian Antarctic Division<\/p>\n
As of February, the Australian mainland<\/a> and New Zealand<\/a> didn\u2019t have any cases of the H5N1 strain, which has spread among birds worldwide and affected some mammals.<\/p>\n Analysis of genetic data suggested the H5 bird flu was likely introduced to the islands through wildlife from the French sub-Antarctic Crozet Islands, 1,800 km away, likely arriving around August 2025.<\/p>\n The findings have been published<\/a> in the scientific journal BioRxiv but have not yet been peer reviewed.<\/p>\n