how do fish handle cold water

 winter can be hard on fishes.                                                                                                                             firstly, freshwater fisher are rarely threatened by freezing to death because of one of  water,s most unique  property. like most substance, water,s density increase as temperature drop. however, it's most dense just above its freezing  point ; density increase between 4 degree to 0 degree , causing ice to float. were it not for this property lakes and river would freeze from the bottom-up and fish at more northern latitudes would be seasonally extirpated. But not all fishes are not equally thermally adapted, and many cannot tolerate very cold or very warm temperature. Thermal tolerance is an important controller of fish distributions, but within their range, fishes have adapted several ways to deal with cold temperatures:                                             MOVEMENT.                                                                                                                     when temperature drop, many fish make seasonal movement to thermal refugia.                 thermal refugia can be particularly i climes        Main reasons which they adopt to the water;                               enzymes are proteins that catalyze metabolic reaction, but their efficiency is temperature-depended; enzymes perform best at an optimal temperature . to maximize their efficiency across a range of temperature, some fishes have evolved alternative enzyme systems called isozymes to perform the same function at different optimal temperature other fishes utilize allozymes-different form of the same enzyme that are controlled by different alleles that allow reactions to function at different temperature.    Antifreeze.                                                                                                                           in polar marine systems, high salt concentration reduce water,s freezing point to around -2degree-colder than the freezing point of most of the species, bodily fluids.in these habits, several species (most noteably the Notothenoidei   but also cods, sea ravens, and some herrings) have antifreeze proteins that prevent blood from crystalizing. This allows them to survive in the harsh, ice-covered polar region.