Ringed seals (Phoca (pusa) hispida), which are the most abundant seals in the Arctic, rely on stable sea ice to survive. The ice is where they rest, molt and reproduce. But that ice is disappearing.
The most recent NOAA Arctic Report Card revealed that, due to persistent warming, the Arctic has now lost 95% of its sturdier ‘old ice’ (ice that is at least 5 years old). Consequently, only 1% of the Arctic ice pack is old ice, and the remaining ice is younger, and weaker.
As the ice rapidly disappears, some ringed seals have been observed adapting as best they can by seeking out land, and using rocks and mudflats as places to rest.
Nevertheless, they still need stable ice because this is primarily where they birth their pups and raise them. Moreover, it’s not just ice they need for this, but plenty of snow, as well. This is because sufficient snow accumulation leads to the formation of deep snow drifts where the seals can dig their lairs.
These little snow caves provide insulation and protection from predators, keeping pups safe from polar bears, as well as killer whales, walruses, wolves, wolverines, sharks and even seagulls. The seals also claw ‘breathing holes’ into the bases of their lairs in order to gain access to the water below, enabling them to more easily hunt for their own food.
When snow cover is insufficient, unweaned pups are exposed to predation. Indeed, studies have revealed a link between decreased snow depth and seal pup mortality.
According to NOAA’s Arctic Report Card, there is a long-term trend in declining snow cover in the Arctic. Satellite data and snow models over the past 15 years have shown that, compared to past decades, the Arctic snow-pack has been shallower and has been melting from land surfaces earlier in the spring.
As Arctic temperatures continue to rise at more than twice the rate of global average temperatures, Arctic snowfall is predicted to continue its decline and, among other things, this doesn’t bode well for ringed seals. But the extent of the effect on future populations wasn’t entirely clear. While estimates of survival and reproductive rates had been collected in the past, they had not been combined with climate forecasts to predict future seal populations.
Now, researchers at the University of Alberta in Canada and the University of Amsterdam in the Netherlands have developed a mathematical model that provides a clearer picture of how changing conditions in the Arctic will affect the long-term fertility and survival of ringed seals.
"It was surprising to see just how little snow is forecast to be available for the construction of lairs in the future," says lead-author Jody Reimer at the University of Alberta.
The model predicts that diminishing snow, even more so than early ice breakup, will lead to a smaller number of juvenile ringed seals relative to adults and pups over time. Ringed seals are a relatively long-lived species with lifespans of up to 43 years, but as snowfall decreases in the Arctic, fewer pups will reach their teenage years. This will lead to smaller populations in each successive generation.
Thus, according to the model, this could ultimately lead to a 50 to 99% decline in the Arctic ringed seal population by the year 2100.
The impact, of course, extends far beyond ringed seals.
“As an indicator species, ringed seals provide information on the health of Arctic marine ecosystems,” explains Reimer and her colleagues in their new paper. “But this information relies on our ability to detect the large scale changes resulting from climatic changes.”
“Mathematical population models such as these can contribute both to understanding past population trends as well as predicting future ones.”
The study has been published in the Ecological Society of America’s journal, Ecological Applications.
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