Antarctic Fur Seal

Three categories in a single day

On 9 April 2026 the IUCN Red List moved the Antarctic fur seal from Least Concern to Endangered. Least Concern means nobody is especially worried about you. Endangered means you are now considered to be at serious risk of dying out altogether. There are categories in between, and this seal went straight past all of them in a single day.

The assessment behind that jump was led by Jaume Forcada of the British Antarctic Survey, and the finding underneath it is blunt. Count the adults old enough to breed in 1999, count them again in 2025, and more than half of them are gone.

Picture a hundred breeding seals lying on a South Georgia beach at the turn of the century. Today about forty three of them are left. That is what the Endangered category is for: a loss that big, inside a single human lifetime.

Look this animal up almost anywhere else and you will still be told it is Least Concern and that there are four to six million of them. That was the official line for thirty years. Both halves of it are wrong now, and the next section explains why the second half was never really right.

Why the old number was too big

You cannot count seals in the open ocean. Nobody ever has. What surveyors actually count is the animals sitting on breeding beaches, and then they multiply that by a correction factor to allow for all the ones who are somewhere else. Get the multiplier wrong and you do not make a small mistake. You make a mistake the size of a whole population.

Which is what happened. Most male Antarctic fur seals at South Georgia do not breed until they are about ten years old, and even then only briefly, so on any given survey day roughly eight of every ten males are simply not on the beach. The multiplier used to cover those absent males was too generous, and the headline total had been puffed up for years.

Reworked properly, the true peak came in the late 2000s, and it was about three and a half million animals rather than six million. That is still a great deal of seal. But it means the species never climbed as high as anyone believed, and the fall started from lower down than the record showed.

So part of the drop you see on paper comes from better counting rather than from a sudden crash. That does not make the decline imaginary. It makes it worse in one particular way. If it took better counting to notice, the seals had probably been slipping for a while before anybody saw it.

An animal made of krill

Antarctic krill is a shrimp like creature about the length of your little finger. It swarms in numbers you can see from space, and almost everything in the Southern Ocean either eats it or eats something that does. At South Georgia, krill is at least three quarters of everything an Antarctic fur seal eats. The seals breeding there get through close to four million tonnes of it a year, roughly ten Empire State Buildings' worth of shrimp.

Being a specialist works beautifully right up until the specialty runs out. Warming water and shrinking sea ice are cutting the krill around South Georgia, and the IUCN assessment says plainly that the trend is unlikely to turn around unless greenhouse warming is seriously reduced.

The shortage lands hardest on the youngest. Out of every hundred pups born at South Georgia, about forty five used to live through their first year. Now about fifteen do. Adults are long lived, so the beaches still look occupied, but they are occupied by ageing animals with almost nothing coming up behind them.

Bird Island shows what that does over time. The population there fell off a cliff in 2009 and has shrunk every year since, back down to where it stood in the 1970s.

On the beaches researchers watch most closely, the number of pups born each year used to average around seven hundred. Now it averages around four hundred. Same beaches, same people counting, three hundred fewer pups.

Not every colony is failing, and the ones that are fine tell you exactly what the problem is. Where krill is scarce, at Heard, Macquarie, Crozet and the Prince Edward Islands, these seals eat squid, lanternfish and other fish instead, and those colonies are steady or growing. Marion Island is climbing while South Georgia falls, and the South Shetlands have lost more than nine seals out of every ten over the same stretch of years. Same species, same ocean, different dinner.

Built for cold water, and a four month deadline

An Antarctic fur seal survives in near freezing water on a coat, not on fat. Its skin grows hair in bundles: one stiff waterproof guard hair with dozens of fine underfur hairs packed in underneath it. When the seal dives, the outside gets wet and a layer of air stays locked against the skin. The pale tips on the guard hairs are what give a dry adult that frosted, grizzled look.

Behind the eyes sit the small pointed ear flaps that mark every eared seal, bare at the tip on this species. The long fore flippers do the swimming, flapping like wings. On land the hind flippers swing forward underneath the body, so this seal can walk, climb, and over broken rock move faster than you would like.

The breeding season is short and everybody runs it off the same clock. Bulls haul out in late October to claim their patch of beach. The pregnant females follow a couple of weeks later, from mid November onwards, and most give birth within a day or two of coming ashore.

A mother stays with her newborn for about a week, mates again during that window, and then settles into a rhythm she keeps all summer: four or five days out at sea hunting, then two or three days back on the beach feeding the pup. It is a commute nobody would sign up for.

Hunting, she usually works the top forty metres or so, chasing prey that drifts up toward the surface after dark. That is about the depth of a ten storey building turned upside down.

She can go much further when she has to. The deepest dive recorded for a female reached roughly two hundred and ten metres and lasted five minutes on one breath. Males go deeper again, down to around three hundred and fifty.

Then the clock runs out. A mother nurses her pup for about four months, and most are weaned by the end of March. Four months is not a guideline, it is a wall. The pup has to be feeding itself before the southern autumn shuts the whole season down, which is why a bad krill year turns so directly into dead pups.

One letter of DNA redrew the map

Every so often, a South Georgia fur seal is born the wrong colour. Somewhere between one pup in six hundred and one in fourteen hundred arrives pale cream instead of dark brown, a blond seal in a crowd of chocolate ones.

The cause is one letter. A seal's DNA is a set of instructions written in billions of letters, and in a blond seal a single one of them has been swapped in the gene that tells the body how to make dark pigment. That is the whole difference. The gene is called MC1R, and it is the same gene that gives some people red hair. The blond version is recessive, which means a cream pup had to inherit the changed letter from its mother and from its father.

Then somebody realised that one letter was a tracking device.

The question was how the species came back after the sealers. Everyone had assumed that survivors from South Georgia spread out and repopulated the whole Southern Ocean. To test it, Hoffman and colleagues read the DNA of nearly fifteen hundred pups on eight breeding islands, from South Georgia right out to Macquarie, and looked for the blond letter in every one of them.

At South Georgia, about three seals in every hundred carry it. Everywhere else Hoffman and colleagues looked, it was not there at all: not at Marion, not at Crozet, not at Heard, not at Kerguelen, not at Macquarie.

Which means the old story was wrong. South Georgia's survivors filled in the islands nearby and essentially nowhere else. The far off colonies rebuilt themselves, from their own hidden survivors that nobody knew had made it through. One changed letter, read on eight islands, rewrote the history of where an entire species came from.