If memory serves me right I'm pretty sure the same thing that happens to us (hypothermia) happens to fish on a smaller level but slightly different as we are endothermic and they are ectothermic. Core temperature drops below desired levels, blood starts leaving extremities( how frostbite happens) to the try to protect core, depression, confusion, extreme pain, the feeling of burning from expanding cells(which is why most people found dead from freezing are naked), then unconscious, then death. It is not a good way to go for any creature and it is not humane.
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