The Southern Elephant Seal, Mirounga leonina, is the largest species of Pinniped (the group which includes Seals, Sea Lions, and Walruses) and an important predator in the circumpolar Southern Ocean. They breed annually, coming ashore on Sub-Antarctic islands in the southern summer. The males emerge first, carving out territories in bouts of competitive fighting, which they then defend for the rest of the season. The pregnant females emerge later, settling in the territories of one of the males, to form harems, groups of females defended by a male. Here they give birth, wean their cub, and finally, at the end of the season, come into oestrus, allowing the male to mate with them before they return to the sea.
There are four genetically distinct populations of Southern Elephant Seals, the Peninsula Valdés Population, which breeds on the Argentinian coast, the South Georgia Population, which breeds on the islands of the South Atlantic, including South Georgia, South Sandwich Island, and the Falkland Islands, the Macquarie Population, which breeds on the Islands of the South Pacific, and the Heard and Kerguelen Population, which breeds on the islands of the southern Indian Ocean, including the Crozet and Prince Edward archipelagos. Of these populations, the South Georgia Population is thought to be the largest, containing about 54% of the global population.
Because Southern Elephant Seals live in remote locations, data on their populations is difficult to gather, making it difficult to compare populations directly. The Peninsula Valdés Population, possibly the easiest to monitor, is known to have grown by between 1 and 3.4% each year for the past five decades. The South Georgia Population is thought to be stable. The Macquarie Population is thought to have shrunk throughout the twentieth century, going through a slight recovery in the early twenty first century, before starting to shrink again more recently. In the Indian Ocean, the sub-population on Marion Island, in the Prince Edward Archipelago, has declined by 83% since 1950, which represents an annual decline of 5.8%, the Îles Crozet sub-population shrank by 5.4% per year between 1970 and 1990, but more recently have been growing at about 5.1, while the Îles Kerguelen sub-population shrank by 47% between 1952 and 1987, then rose by almost 1% per year between 1987 and 2009, and has been rising at about 1.6% annually more recently.
Influenza A Viruses are Negative-strand RNA Viruses which circulate naturally in both Animal and Human populations, causing seasonal flu. Occasionally a particularly more virulent strains of Influenza A appear, causing large mortality rates among affected species, such as the notorious Spanish Flu, which may have killed over 100 million people between 1918 and 1920. Avian Influenza is a form of Influenza A Virus which first appeared on poultry farms in China in late 2003, rapidly spreading to other farms across East Asia during 2004, causing mass deaths of poultry everywhere it reached. In 2005 it caused a mass death of wild Birds on Qinghai Lake in central China, a lake which is used as a stopover for many migratory Bird species. Later that year cases began to appear in Europe and Africa. While mainly affecting Birds, Avian Influenza can infect Mammals, including Humans, however, although it can be fatal, it seldom spreads between members of most Mammal species, limiting the size of any outbreaks. However, some Mammals appear to be more vulnerable than others with the Virus spreading without the need for further contact with Birds. Such vulnerable Mammals include Cows and Seals.
Since this time several strains of Avian Influenza have appeared, which have been loosely divided into Highly Pathogenic Avian Influenza Viruses, which typically kill more than 75% of the Birds in any infected population, and Low Pathogenic Avian Influenza Viruses, which typically kill less than 75%. The 2.3.4.4b clade is a Highly Pathogenic Avian Influenza Virus which first appeared in Europe in 2020, and has subsequently spread to North and South America, where it caused mass deaths among Seabirds and Marine Mammals in 2022. In September 2023, a Brown Skua, Stercorarius antarcticus, on Bird Island, South Georgia, was found to have died as a result of infection with Highly Pathogenic Avian Influenza Virus, with the infection subsequently spreading to Gentoo Penguins, Pygoscelis papua, Snowy Albatrosses, Diomedea exulans,Antarctic Fur Seals, Arctocephalus gazella, and Southern Elephant Seals. In 2024, the Virus was also confirmed on Îles Crozet and Îles Kerguelen in the southern Indian Ocean.
During the 2023/4 breeding season, Southern Elephant Seals on South Georgia were monitored for Highly Pathogenic Avian Influenza Virus via sample collecting at sites which could be accessed from the sea, in combination with observations made from research vessels and cruise ships, suggested that the colony could have lost as many as 97% of its pups.
In a paper published in the journal Communications Biology on 13 November 2025, Connor. Bamford, Nathan Fenney, Jamie Coleman, Cameron Fox-Clarke, and John Dickens of the British Antarctic Survey, Mike Fedak of the Sea Mammal Research Unit at University of St Andrews, Peter Fretwell, also of the British Antarctic Survey, Luis Hückstädt of the Centre for Ecology and Conservation at the University of Exeter, and Phil Hollyman, again of the British Antarctic Survey, and of the School of Ocean Sciences at Bangor University, present a study of the three largest Southern Elephant Seal numbers on South Georgia, based upon photographic data collected by an uncrewed aerial vehicle.
In October 2024, 4373 female Southern Elephant Seals were observed at St Andrews Bay, whereas 6305 were recorded in October 2022. At Hound Bay, 1154 females were observed in 2024, compared to 1901 in 2022. This represents a 47% decline in the number of breeding females at these two sights between 2022 and 2024, and a 33.7% decline compared the long term population average (taken between 1958 and 2022). Bamford et al. extrapolate for that this would represent in a total fall in the number of breeding females across South Georgia of about 55 000.
Bamford et al. note that there are gaps in our knowledge of Elephant Seal breeding, which affect how their data should be interpreted. It is possible that the reduction in the number of breeding females represents a direct measure of the rate of adult mortality suffered by Southern Elephant Seals on South Georgia as a result of Highly Pathogenic Avian Influenza Virus infection. However, female Seals whose pups die tend to leave the breeding beaches fairly quickly, and it is therefore possible that many Seals left beaches in 2023 before coming into oestrus and mating, and therefore did not return in 2024 to raise pups. It is unclear how female Elephant Seals return to the breeding population after losing a pup, although it seems unlikely that they simply stop breeding altogether.
Another potential disruption to Elephant Seal breeding behaviour comes from sea ice. In 2023 sea ice in the Southern Atlantic cover was the lowest ever recorded at that time (2024 subsequently produced even lower ice levels). This potentially had an impact on the Elephant Seals, as sea ice has a stabilising effect on the Southern Ocean, tending to suppress the formation of storms, as well has having an impact on the ecology of many prey species. However, Bamford et al. do not believe that this is likely to have caused a sufficient disruption to the Seals to explain the observed population drop, given the wide area over which they typically forage.
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