A ban on hunting fur seals has allowed the species to recover from near extinction – but now numbers are dwindling due to lack of food, scientists claim.
A study of Antarctic fur seals shows that numbers peaked in 2009, but have been declining since.
Scientists from the British Antarctic Survey (BAS) think that global warming is raising ocean temperatures and killing their main food source, krill.
The species lives almost exclusively on the sub-Antarctic islands of South Georgia in the south Atlantic.
Prized and hunted for their pelts in the 1700s and 1800s, by the early 1900s there were too few animals left to hunt commercially. However, bans on hunting the animals in the 20th Century enabled their numbers to recover.
The team from the BAS said that in 2009 there were about 3.5 million – a healthy number, although significantly less than previously estimated.
However, a recent, more detailed count of animals living on a particular South Georgia island called Bird Island shows the seals are finding it harder to find krill, a small shrimp-like crustacean, with numbers crashing in the last decade.
According to the study, published in the journal Global Change Biology, numbers on Bird Island have been dropping 7% a year since 2009, bringing them back to 1970s levels when the species was still recovering.
‘We found both good and bad news about the fur seals,’ said lead author Dr Jaume Forcada.
‘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.’
The species had done well, with strong conservation protections, plenty of food and fast breeding making it recover much more quickly than other previously hunted species in the region such as humpback whales.
However, re-examination of the data shows their numbers had been massively overestimated at up to 6.2 million.
‘Our new results show this was a massive overestimation,’ said Dr Forcad.
‘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 new estimate of 3.5 million animals comes from several week-long helicopter surveys of South Georgia from 2007 to 2009 and improved population assessment methods.
Scientists looked for evidence of krill fishing pressure on the fur seal population, but this was not found to be significant.
However, initial analysis of climate data shows that that rapidly rising sea temperatures in the region correlate with the seal population decline, pointing to a loss of krill as the most likely cause.
‘Krill can make up to 80% 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 Dr Forcada.
The team said that more detailed research is needed to establish why krill around Bird Island is now less available and how widespread the change could be across the rest of the 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,’ added Dr Forcada.
‘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.’
MORE : Generation of penguins drowns as Antarctic ice melts beneath their feet
MORE : Antarctica’s sea ice reaches a record low, second year in a row
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