The grave of the explorer Ernest Shackleton on South Georgia island has become inaccessible to visitors due to bodies of “dead seals blocking the way”, as increasing numbers of animals are killed by bird flu’s spread through the Antarctic.

The H5N1 virus has spread to 10 species of birds and mammals since it arrived in the region last October, with five king penguins and five gentoo penguins the latest to test positive on the sub-Antarctic islands. Those confirmations follow reports of mass die-offs of elephant seals at the end of last year.

Astrid Saunders, a racing journalist who was on a cruise around South Georgia island and the Antarctic peninsula on 15-30 January, saw the devastation first-hand, as the ship was repeatedly prevented from docking. “Every day they would say we can’t land here because there are too many dead animals,” said Saunders.

The only place the ship could anchor was in Grytviken, home to Shackleton’s grave, a British post office and a museum – but no one was allowed to disembark. “We weren’t allowed to go up to the grave of Ernest Shackleton because there were so many dead seals blocking the way,” she said. “My impression from what I could see was that there were hundreds of dead seals.”

Saunders said calculating the number of mortalities was challenging, as it is difficult to distinguish between dead, ill and resting seals, but added: “There seemed to be an awful lot of bodies on the beach … a lot of seal pups with no mum. They were desperately trying to run around.”

Shackleton was buried on the island in 1922 after he had a heart attack while exploring the Antarctic continent, and his grave has become a pilgrimage site for enthusiasts. One man on the cruise wanted to read a poem at Shackleton’s grave, Saunders said, and another had brought a special whiskey to propose a toast to the explorer.

Colette Engstrom, a travel consultant who was on the same cruise, said: “There did definitely appear to be a lot of dead seals on South Georgia, in particular around Shackleton’s grave.” Visibility was poor, she said, but “we were being assured that the reason we couldn’t go [to the grave] was because all the seals – or many of the seals – were dead, and bird flu was being blamed”.

Saunders says many of those on her cruise had not known about bird flu, and were not informed of its impact before the trip. “My dream was to walk among the king penguins – and I couldn’t,” said Saunders. “But no one wants to contaminate the environment.”

A spokesperson for Silversea said: “The government of South Georgia and the South Sandwich Islands temporarily restricted access to various sites as part of an effort to protect certain species of wildlife within its delicate ecosystem. Our Antarctica, Falklands, and South Georgia cruises will continue sailing as planned, and will call on alternative sites or add additional Zodiac cruising or sea-kayaking opportunities, based on the conditions. The safety of our guests, crew, and the destinations we visit – including their wildlife – is our top priority.”

Dr Norman Ratcliffe, a seabird ecologist at the British Antarctic Survey (BAS) who has worked on penguins and seabirds on South Georgia, said these observations of seal deaths were in line with what researchers have seen in the region. “There has been widespread mortality along the north coast of South Georgia,” he said.

On South Georgia, H5N1 has been confirmed in elephant seals, fur seals, Antarctic terns, wandering albatrosses and kelp gulls, according to the Scientific Committee on Antarctic Research (Scar).

On the Falkland Islands – 900 miles (1,500km) west of South Georgia – deaths of gentoo penguins have been confirmed at three separate sites. Other confirmed cases have been reported in birds such as southern fulmar, variable hawk, brown skua and black-brown albatross.

While Scar’s database does not provide data on the number of seals that have died in the region, H5N1 has caused mass die-offs in marine mammals before: in 2023, scientists estimated the strain had killed off more than 17,000 southern elephant seal pups in Argentina, with the mortality rate in some breeding seasons reaching 96%.

With the breeding season drawing to a close on the island, experts hope that the spread be limited. Ratcliffe said: “Macaroni penguins will remain vulnerable as they gather in large concentrations to moult but subsequently will disperse to sea where transmission will be low. Gentoo and king penguins, however, continue to form communal roosts or to breed, respectively, throughout the winter and so may remain at risk.”

Researchers have previously raised the alarm about “one of the largest ecological disasters of modern times” if bird flu reached remote Antarctic penguin populations.

The current outbreak of the highly infectious variant of H5N1 – which started in 2021 – is estimated to have killed millions of wild birds. The strain, 2.3.4.4b, has decimated bird populations across the UK, continental Europe, South Africa and the Americas.

Find more age of extinction coverage here, and follow biodiversity reporters Phoebe Weston and Patrick Greenfield on X for all the latest news and features