Populations of Antarctic seal species halve over half a century as sea ice shrinks, Cambridge researchers find
Populations of Antarctic seals have halved over 50 years amid a warming climate and shrinking sea ice, a new study has shown.
Researchers from Madingley Road-based British Antarctic Survey (BAS) have monitored seal numbers in the sub-Antarctic on Signy Island, in the South Orkney Island, since the 1970s.
Five decades of consistent seal counts and satellite sea ice data has enabled them to validate predictions of how seals’ habitat has been affected by conditions.
They found that numbers of Weddell seals, which depend heavily on sea ice to breed and feed, have dropped by 54 per cent over almost half a century. Their global numbers are now at around 800,000.
Antarctic fur seals, which breed on land but are affected by environmental impacts on the food chain, have declined by 47 per cent in the same period. Their global population is believed to be about 3.5 million.
Michael Dunn, lead author of the study from the BAS said: “We’ve seen fur seal populations bounce back in the sub-Antarctic after hunting was banned in the 1960s. But this study offers a rare glimpse into the changes now unfolding in one small part of Antarctica. For once, we’re not just predicting how wildlife might respond to shrinking sea ice and environmental shifts, we’ve had the rare opportunity to confirm it, using solid, long-term data. The emerging picture is deeply concerning.”
The findings challenge earlier assumptions that Antarctic fur seal numbers had stabilised in the South Orkneys and show that in fact, since around 2015, the population has fallen sharply and continues to fall.
Numbers of southern elephant seals - the largest seal species at up to six metres long and four tonnes in weight - are also In decline, but to a lesser extent. There are about 650,000 of them worldwide. They breed on land but hunts in icy seas, which are affected by food chain shifts
Population changes in all three species were strongly linked to shifts in sea ice - when it forms and melts each year and how long it lasts.
Satellite records of sea ice concentration dating back to 1982 were used to track annual changes in sea ice timing, extent and duration and this was compared to seal counts carried out every year since 1977.
The dataset spanned a period of long-term warming, punctuated by a temporary cooling phase from around 1998 to 2014 - a natural fluctuation that gave the team a ‘before and after’ scenario. This allowed them to observe how seal populations reacted to changing ice conditions over time, which shorter-term studies could only speculate about.
BAS says the results, published on 18 June in the journal Global Change Biology, underline the importance of long-term ecological monitoring, since many environmental effects on wildlife can only be properly understood and validated over decades, not years.
The study also demonstrated the significance of broader ecosystem risks, with climate change disturbing the fragile Antarctic food web upon which all three seal species rely.