What nutritional differences are there between fresh, tinned, smoked and frozen fish?

The omega-3 fatty acids in frozen fish are found to degrade over time, with the extent of which depends upon the initial quality of the product, the conditions for freezing and the storage temperature (the best results are obtained with a storage temperature at or below –30°C, for a storage time of between 6 months and 1 year). The fatty acids in tinned fish degrade far more slowly.

In tinned foods, some loss of vitamins is observed, due to their diffusion in the liquid, and to the sterilisation temperature. But overall, their nutritional quality remains very high.

As regards smoked fish, particularly those of the salmonid family (e.g. salmon), the smoking process does not incur any degradation of the fatty acids. Degradation, if it occurs, would be attributable to the quality of the raw material, the preservation method and problems in keeping the food properly refrigerated and frozen. Some smoked fish, such as kippers, have a high salt content (necessary to the process and for preserving the fish).

Source: IFN