The only real difference in types of fish, as related to fish cookery, is variation in fat content. As a rule fat fish, such as salmon, herring, or shad, are preferred for baking, broiling and grilling, because their fat content will keep them from becoming dry.
Lean fish, such as cod and haddock, are preferred by some for boiling and steaming, as their flesh is firm, and will not fall apart easily while cooking. Both fat and lean fish are suitable for frying.
There are, however, so many exceptions to these rules, that actually all fish may be cooked by any of the basic methods, with excellent results, if allowances are made for the fat content. For example, lean fish such as halibut, when broiled or baked, will have a tendency to become dry, unless basted frequently with melted fat.
Fresh Fish may be recognised by its bright, bulging eyes, bright red gills, a firm flesh of elastic touch which holds no imprint when pressed with the thumb, and its characteristic fresh, saline, seaboard odour. Fish smelling even slightly of alkali or ammonia should be rejected.
Frozen Fish which are cleaned and frozen as quickly as possible (often caught and frozen in a day), are generally sold as fillets. This avoids all mess and fuss and is the most hygienic method of marketing fish. Such fish can hardly be distinguished from fresh fish, if they have been kept properly frozen and are cooked as soon as, or before, they have been thawed. Fish once thawed should never be re-frozen. Frozen fish fillets are particularly good buys for those living at a considerable distance from sources of supply of fresh fish, for, with modern transportation, they arrive at their destination in prime condition, with all their fresh qualities sealed from dust, dirt and deterioration.
Salted Fish are prepared either by pickling in brine or by dry salting. Fish such as cod, hake and haddock are dry salted, by packing in dry salt after gutting. Fat and oily fish are salted in brine, then frequently smoked. Findon (or Finnan) Haddie is prepared in this way.
Smoked Fish such as salmon, sprats, eel and sturgeon, are generally eaten without further cooking.