Water freezes at 0’C, however this is simplistic. If it is moving then it doesn’t freeze immediately, it has kinetic energy associated with the movement, this overcomes the strength of the bonds as ice crystals form. Static water on surfaces ie. boundary layers freeze, then this ice slowly extends into the body of moving water, reducing the volume (and potentially increasing the level of movement in the reduced volume). This ultimately means that the moving water can get to temperatures below 0oC. Ice is less dense than water so it floats, and gradually thickens at its boundary layer in sub-zero temperatures. Salt is often used in ponds and this lowers the freezing point by interfering with ice crystal formation this means water can be even colder.
Fish, like all animals are mainly water (50-75%). Fishes own tissues will begin to freeze. As their tissue is cooled water molecules in the tissue between cells begin to clump and freeze, the ice crystals damage cells. The science of cryopreservation uses cryoprotectants to minimise freezing and so avoid damage.
Carp are by nature a warmwater fish. However they are extremely tolerant and will overwinter perfectly well in unheated ponds. EFSA (2008) suggests that carp will tolerate down to 2oC - actually they will go lower but some will die through stress or superchill.
Normal behaviour in water at freezing temperatures was said to be that fish became dormant and simply sat there on the bottom, waiting for Spring. More recent observations have show this to be untrue, they continue to be relatively active, if available they do eat some bottom living invertebrates, but generally they are not eating since their digestive processes don’t operate well at such low temperatures.
Fish have a body temperature which essentially mirrors that of the environment and the temperature at which they would be expected to freeze and die would be the freezing point of their blood plasma. They have been shown to be able to resist this by 1oC in ways not really understood except in the presence of ice formation. The presence of ice appears to encourage the formation of ice crystals in the skin and circulation, resulting in death.
Freezing temperatures in warm blooded animals lead to frostbite, here blood flow to the extremities has been reduced and then goes through an on-off cycling before stopping altogether. At that point frostbite begins with ice crystal formation in tissues, whilst at the same time those tissues are dying of lack of oxygen normally brought by the blood flow.
‘Superchill’ is a phenomenon which kills atlantic salmon in Canada, due to exposure to ice formation. The fish, like the koi in this case can tolerate the temperatures but close proximity to ice formation in the environment causes crystal formation in the skin and ensuing damage similar to frostbite.
Ice crystal formation in tissues rips cells apart, this will be especially acute in gills which have extremely cold water possibly with forming ice crystals in it being pumped across them all the time.