Antarctic fur seal numbers recovered after hunting – but now they face a new threat, say Cambridge scientists

The number of Antarctic fur seals has recovered since they were hunted to near extinction - but now they face another major threat as they struggle to find the food they need.

The Madingley-based British Antarctic Survey has found the modern-day population peaked in 2009 at about 3.5 million.

This was a healthy number, although down on previous and less accurate estimates.

But it is feared that rising sea temperatures amid our changing climate are affecting the numbers of krill - their main source of food - in the Southern Ocean, causing their numbers to crash again.

Jaume Forcada, a BAS scientist who led the new study and is lead author on the paper, says: “We found both good and bad news about the fur seals. The population has recovered very impressively throughout the 20th century when seal hunting was banned. But 21st-century changes to the abundance of krill in the Southern Ocean are now threatening these iconic animals all over again.”

But with stronger conservation protections, plenty of food and fast breeding, the population recovered much more quickly than other previously hunted species in the region, such as humpback whales

Atlantic fur seals, almost all of which live on the sub-Antarctic islands of South Georgia, were prized and hunted for their pelts in the 1700s and 1800s. By the early 1900s, there were so few left they could not be hunted commercially.

It was thought that by 2000, South Georgia was home to between 4.5 million and 6.2 million fur seals, but a re-examination of the data and methods used suggest that was not accurate.

Jaume said: “Our new results show this was a massive overestimation. That matters because the fur seal population size is used to judge the overall health of the species and the wider Antarctic ecosystems. And it turns out that neither were as robust as people thought.”

The business of counting seals is not straightforward, as surveys typically assess numbers at seal breeding beaches, but most males do not usually breed until they are 10 years old, and then only for two to three years, meaning 80 per cent of the population is missing.

Accounting for this can overstate the overall number.

The new study, published in the journal Global Change Biology, used several week-long helicopter surveys of South Georgia from 2007 to 2009 and better population assessment methods.

The fur seal populations at Bird Island, in the north-west of South Georgia, showed one of the fastest rebounds over the last century, but a new survey found that after the 2009 population peak, numbers have fallen there by seven per cent each year since - taking the population down to 1970s levels.

There was no significant evidence that krill fishing had impacted the population.

But initial analysis of climate data shows rapidly rising sea temperatures correlate with the seal population decline, suggesting the loss of krill as the most likely cause.

“Krill can make up to 80 per cent or more of the diet of fur seals at South Georgia, so they experience catastrophic declines in the number of pups produced and survival of individuals when environmental conditions remove the krill from their immediate foraging areas,” said Jaume.

This sensitivity to the availability of krill makes fur seals a useful indicator for the ecosystem.

But scientists say more detailed research is needed to understand the drop in availability of krill around Bird Island and the wider Southern Ocean.

“If the pressure on the fur seals at Bird Island also applies to the greater South Georgia population, there could be an ongoing decline there as well. So even though there were three and a half million of them there, the fast decline at Bird Island tells us they could be in trouble,” warned Jaume.