As with hibernating mammals, the fish rouse themselves now and again from their dormant state for short periods. Researchers suspect the "hibernation" is triggered by changes in sunlight. The sea temperature varies by only about 2C between summer and winter, which is probably too small a difference to induce such a significant change in behaviour. "It appears the fish utilise the short Antarctic summers to gain sufficient energy from feeding to tide them over in winter," said Keiron Fraser from Bas.