Fishes are ectothermic – commonly referred to as "cold-blooded", although their blood is not really "cold" – so their inner body temperature is dictated by the temperature of their surroundings. As a result, fishes living in Arctic waters are cold, both inside and out. Being cold affects metabolism – the total of all the chemical reactions that occur in a living organism to maintain its body. Much like butter, the fats that comprise animal cell walls become more solid in cold temperatures, slowing down the transfer of molecules between cells. Also, enzymes – the molecules that drive chemical reactions and exert close control over cellular metabolism – operate most effectively at higher temperatures. All in all, low temperatures result in slower metabolism.

Arctic fishes have many physiological adaptations to help them cope with life in the cold. The fats in their cell membranes are less saturated than those of southern fishes, a chemical exchange that is equivalent to replacing butter with olive oil – the latter remains more liquid when put in the fridge! The enzymes of arctic fishes also tend to be better able to carry out chemical reactions at low temperatures. However, these cellular compensations are only partially effective. Arctic fishes have much slower metabolisms at 2ºC than southern fishes do at 20ºC. This is evident in the slow movements of many polar fishes, like sculpins, snailfishes, and eelpouts, which can be caught in the hand, unlike their temperate zone counterparts. However, southern fish exposed to low temperatures have much lower metabolic rates than Arctic fish. Moreover, slow metabolic rates are not necessarily a disadvantage in cold environments. The slower you use energy means that less energy is required. Thus, arctic fish may benefit from their lethargic lifestyle – their lower energy requirements mean that they do not need as much food to survive!