It’s not the freezing that adversely affects fish, it’s actually the thawing process. The structure of any fish is a multitude of small balloon-like structures, filled with watery protein. When frozen, these balloons and their contents turn to solid ice crystals. Upon thawing, the sharp ice crystals move around puncturing the walls of the balloons. As a result the structure is changed and much of the internal moisture drains out.

 

Freezing and thawing is most damaging to fish filets and resistance to damage varies among differing types of seafood. In every case if you are forced to use frozen fish thaw it slowly in the refrigerator over a twenty four hour period to limit the damage.