Question

why the fishes living in cold regions do not feel cold?

Answer 1

freshwater fishes are rarely threatened by freezing to death because
of one of water's most unique properties.Like most substances,water's density increases as temperatures drop. However, it's most dense just above its freezing point; density increases between 4° and 0°, causing ice to float. Were it not for this property, lakes and rivers 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 temperatures. 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 temperatures drop,
many fish make seasonal movements to thermal refugia. Thermal refugia can be particularly important in northern
climes, where warmer groundwater helps
keep aquatic habitats ice-free.

Alternative enzymes. Enzymes are proteins that catalyze metabolic reactions, but their efficiency is temperature-dependent; enzymes
perform best at an optimal temperature.To maximize their efficiency across a
range of temperatures, some fishes have
evolved alternative enzyme systems
called isozymes to perform the same
function at different optimal
temperatures. Other fishes utilize
allozymes- different forms of the same
enzyme that are controlled by different
alleles that allow reactions to function at
different temperatures.

Antifreeze. In polar marine systems,
high salt concentrations reduce water's
freezing point to around -2°-colder than
the freezing point of most species' bodily
fluids. In these habitats, 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 regions.