We tend to think of hibernation as response to the cold; but for some fish it’s a response to the dark. Hamish Campbell, lead author of a paper on the new discovery in PLOS One (coverage in Daily Telegraph, Reuters) says that hibernation in the Antarctic cod (Notothenia coriiceps) is due to low light not low temperatures – which are pretty steady all year round in the Antarctic.

“It seems unlikely that the small winter reductions in water temperature that do occur are causing the measured decrease in metabolism,” says Campbell, formerly of the UK’s University of Birmingham and now at Australia’s University of Queensland (press release). “However, there are big seasonal changes in light levels, with 24 hour light during summer followed by months of winter darkness – so the decrease in light during winter may be driving the reduction in metabolic rates.”

By monitoring the cod’s swimming, growth, feeding and heart rate Campbell and colleagues found the fish were dormant for much of winter, only returning to summer levels of activity for a few hours every 4-12 days. This, they can’t resist suggesting, is equivalent to “‘putting themselves on ice’ during winter months until food resources improved”.

However, as fellow author Keiron Fraser of the British Antarctic Survey points out, “The hibernation-like state they enter in winter is presumably a mechanism for reducing their energy requirements to the bare minimum. The interesting question we still have to answer is why these fish greatly reduce feeding in winter when food is still available.”

Watch the BAS chainsawing through the ice and collecting fish on this video.

Image: British Antarctic Survey