Many terrestrial animals save valuable energy reserves during icy winters by going into a state of dormancy or hibernation. While some fish show signs of reduced activity, feeding and growth during winter (all characteristics suggestive of hibernation), fish are thought to differ fundamentally from terrestrial hibernators. This is largely because metabolism is directly coupled to environmental temperature: any dip in metabolism is usually explained by a drop in temperature. But two key facts suggest that Antarctic fish might not have read the textbook. First, large variations in annual growth patterns occur in these fish, suggesting that they reduce their food intake; even when food is abundant, dramatic changes in growth are visible during the winter. Second, sea temperatures in the Antarctic, while chilly,are not particularly cooler in the winter than in the summer; these marine environments have relatively constant temperature. This indicates that potential dips in metabolism are probably not strictly temperature driven. So is it possible that Antarctic fish undergo hibernation in the wild?
Hamish Campbell and Stuart Egginton, of the University of Birmingham, and their collaborators Keiron Fraser, Lloyd Peck and Charles Bishop at the British Antarctic Survey and the University of Bangor in Wales joined up at Rothera Research Station in Antarctica to tackle this intriguing question. Working on the notothenoid fish, Notothenia coriiceps, Campbell and the team investigated the fish's behavioural and metabolic strategies over a complete annual cycle using three approaches. First, they monitored growth and feeding in immature adult fish by capture–recapture techniques and found that whilst growth was higher than in temperate fish during summer months it was negative during winter. Second, the activity of fish was monitored remotely by implanting acoustic radio-transmitters. These data clearly showed a 20-fold reduction in activity and a 6-fold reduction in home range during winter compared with summer. Third, they estimated metabolic rate in the field and in the laboratory to establish whether metabolism dropped in winter and,if so, whether this change is controlled by environmental temperature variation.
To estimate metabolic rate in the wild, the team surgically implanted individual fish with a tiny data logger. However, because these loggers only record heart beats, the team needed to calibrate heart rate frequency with metabolic rate. After laboratory trials on a number of fish, and with the relationship between heart rate and metabolic rate established, the team then used fish with implanted loggers to quantify typical summer and winter energy consumption.
The total cost of living in the wild (field metabolic rate) showed a clear 58% drop from 3.59 to 1.48 mg O2 100 g–1h–1 between summer and winter. In the laboratory,respirometry experiments showed a seasonal 29% dip in standard metabolic rate(the resting cost of physiological processes), and reversing summer and winter water temperatures produced no significant change in standard metabolic rate. This indicates hibernation, rather than temperature-dependent metabolic rate variation. But the final proof that hibernation occurred in these fish was that SCUBA divers visiting the fish during the middle of winter found them unresponsive to handling. By contrast, simply catching these fish by hand in summer was nearly impossible.
Therefore, Campbell and co-workers have neatly demonstrated that N. coriiceps can in fact enter a hibernation-like state in Antarctic winters. Furthermore, the team propose that this state might be induced by changes in photoperiod characteristic of the onset of winter in polar environments. Finally, they suggest this sort of energy-saving metabolic strategy may have contributed to the remarkable success of these fish in the Southern Ocean.
