The preservation of fish is a difficult challenge because of three main factors:
- Psychrophilic bacteria may be present
- Many fish oils are unsaturated and are easily oxidized
- Typical fish proteins are not as stable as red meat proteins

Chilling does not affect the activity of Psychrophilic bacteria to the extent it does mesophilic types, so the "keeping quality" of fresh fish is limited. Frozen fish is typically kept at much lower temperatures (-30^oC) tha other frozen foods to ensure the control of Psychrophilic bacteria.
Reference information from many sources