Surprisingly, there really are some species of fish in polar regions that can survive sub-freezing water temperatures. But even these arctic residents can't survive being frozen solid. Instead, they've evolved to produce a kind of glycoprotein that circulates in their blood and prevents ice crystals from forming - essentially a natural fishy antifreeze. While the water temperature where they live may dip to 28 degrees Farenheit, which is below the freezing point for fresh water, they live in salt water, which remains liquid.
Rumors of fish emerging unharmed from a winter spent encased in a block of ice may arise from sightings of fish emerging from an ice-covered pond or lake. The truth is, these fish were never actually frozen. Cold-weather species like koi and non-fancy goldfish experience a significant metabolism slow down and retreat to the bottom of the pond where they enter a state of semi-hibernation called torpor. Since ice is less dense than liquid water, ice floats at the surface and insulates the depths. This means that the deepest levels are actually warmer. Ponds and lakes that are 3 feet deep or greater rarely freeze solid. In these ponds, your cold weather fish can happily last through the winter.
