H5N1 Devastates Southern Hemisphere Seals and Sea Lions — North Pacific Coasts Remain Unaffected for Now

H5N1 Hits Southern Hemisphere Pinnipeds Hard; Pacific Coast So Far Spared

Over the past 18 months, the H5N1 bird flu has made headlines for outbreaks in poultry and some livestock. Less widely reported, however, is the virus's catastrophic toll on wild animals worldwide. In the Southern Hemisphere, elephant seals, sea lions and fur seals have suffered mass die-offs that in some places left beaches strewn with carcasses and orphaned pups.

Major breeding colonies hit

A research team led by marine ecologist Connor Bamford of the British Antarctic Survey reported a 47% decline in breeding females between 2022 and 2024 across the three largest elephant seal colonies on South Georgia Island. Bamford said his team saw the virus arrive in 2023, but their 2024 visit made the impact unmistakable: vast stretches of normally crowded beach were empty.

Normally there's about 6,000 seals on St. Andrews Bay — by 2024 the colony had large, empty stretches and far fewer animals than expected.
— Connor Bamford

Other major breeding sites have also been struck, including colonies along Argentina's coast and several islands north of the Antarctic Circle. In 2023, UC Davis researchers documented nearly 97% pup mortality for elephant seals at Peninsula Valdés in Argentina — the highest recorded for this species.

Scope of the losses

Across the region, mass die-offs have been reported among southern sea lions, fur seals and crabeater seals. Between 2022 and 2024, more than 30,000 sea lions died in Peru and Chile, and Argentina reported roughly 1,300 deaths among sea lions and fur seals. Many remote species may have suffered additional losses that went unobserved because of limited human presence.

Virus evolution and geographic differences

Genetic analyses by researchers including Ralph Vanstreels of UC Davis indicate the lineage circulating in Argentina has acquired mutations that may enable easier mammal-to-mammal transmission. It remains unclear whether all affected pinniped populations in the region carry those same mutations.

Northern elephant seals and other North Pacific marine mammals, including populations that breed along California's coast, have not shown widespread infection to date. One likely reason is that the viral lineage currently detected off the North American Pacific coast does not carry the same mammal-adapting mutations identified in South America. Other possible explanations include differences in host density, local ecosystems, or the presence (or absence) of an effective transmitter species.

We think the South American sea lion played a big role in transmission, carrying the virus along the coast and perhaps introducing it to the elephant seal population.
— Ralph Vanstreels

Ecological consequences and uncertainty

Researchers warn that the loss of large numbers of pinnipeds will ripple through coastal ecosystems. Elephant seal placentas are an important food source for coastal scavengers such as birds and crabs. The seals' deep foraging also helps recycle nutrients toward the surface, supporting fish, kelp, shrimp and other nearshore life. Removing a large fraction of a population can therefore have measurable ecological impacts.

Key unknowns remain: whether the South American-adapted strains will spread farther north, whether other pinniped species might become reservoirs, and whether any changes could increase the risk of transmission back into humans. Scientists emphasize the need for continued surveillance of marine mammals, genetic sequencing of viral samples, and coordination between wildlife and public health agencies.

What to watch for: expanded surveillance reports from the North Pacific, new genetic data showing mammal-adaptive mutations, and increases in strandings or unexplained deaths among pinnipeds along coastal regions.

This article is based on reporting from researchers at the British Antarctic Survey and UC Davis and was adapted from an original Los Angeles Times story.