Antarctic fur seals, once hunted to the brink of extinction, have made an impressive comeback in their population.
But now, research led by the British Antarctic Survey (BAS) has exposed a looming threat due to a decline in the primary food source for these seals.
The study was conducted primarily on the sub-Antarctic islands of South Georgia, where most fur seals reside. The experts found that the fur seal population peaked in 2009 with around 3.5 million seals. Although this is a substantial figure, it’s much lower than past estimations.
A detailed count on Bird Island indicated a sharp decrease in the availability of krill, a shrimp-like crustacean essential to the fur seal diet, over the last decade.
“We found both good and bad news about the fur seals. The population has recovered very impressively throughout the twentieth century when seal hunting was banned. But twenty-first century changes to the abundance of krill in the Southern Ocean are now threatening these iconic animals all over again,” explained study lead author Dr. Jaume Forcada.
Historically, fur seals were hunted for their pelts during the 1700s and 1800s. By the early 20th century, their numbers dropped so drastically that commercial hunting became unviable.
Benefiting from strict conservation measures, ample food, and rapid reproduction rates, the fur seal population surged, outpacing other hunted species in the region like the humpback whale.
It was estimated that South Georgia harbored between 4.5 to 6.2 million fur seals by 2000. Nevertheless, a closer look at this data suggests that these figures were likely inflated.
“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,” said Dr. Forcada.
Contrary to what might be assumed, accurately counting seals is a complex endeavor. Standard surveys usually focus on seal breeding beaches, but these counts are often biased.
The majority of male fur seals in South Georgia typically breed only when they’re about ten years old and even then, just for a few years.
Consequently, almost 80 percent of the male population is excluded from these counts, leading to potential exaggerations.
The recent estimate of 3.5 million is a product of intensive week-long helicopter surveys between 2007 to 2009, coupled with advanced assessment techniques.
Interestingly, Bird Island’s fur seal population showcased one of the swiftest recoveries over the last century. Yet, this trend has taken a concerning turn. Since peaking in 2009, there has been an annual decline of seven percent.
This means that the island’s current population has dwindled to a level not seen since the 1970s, when the population was still recovering.
Upon investigating the role of krill fishing on fur seal populations, the researchers found that it has been insignificant.
However, preliminary climate data analysis has linked escalating sea temperatures in the area with the decline in seal numbers, suggesting that a krill shortage is the probable culprit.
“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,” explained Dr. Forcada.
Given the fur seals’ pronounced dependency on krill, and the availability of extensive population data, they serve as a pivotal indicator for the ecosystems surrounding Bird Island.
There is an urgent need for comprehensive research to investigate why Bird Island’s krill availability has dwindled and the potential extent of this trend across 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,” said 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.”
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