Fish differ considerably in their digestibility and nutritive value, in consequence of differences in the quantity of fat and the mode in which it is distributed; and in the colour of their blood and the consequent quality of their flesh. Salmon is a red-blooded fish, its fat is distributed much as in red-blooded mammalia, and it approaches in nutritive value and digestibility more nearly to meat than most fish.

It is a very wholesome form of food, and when it disagrees it is usually either from its being eaten too freely, forgetting how nearly it represents an equal weight of meat; or from neglect in cutting it up in thin slices across the grain while eating, a neglect which is promoted by the readiness with which the flesh separates into dense flakes convenient for putting into the mouth, and by its being eaten either with a blunt silver knife or with a fork only; or from an inordinate quantity of fat being swallowed, in consequence of the large proportion of fat contained in those parts of the fish usually thought to have the most delicate flavour.

If these points are borne in mind, salmon, in proper season, may be eaten with advantage by many delicate persons who otherwise would be obliged to refrain from it.

The herring, pilchard, sprat, mackerel, and eel, like salmon, have their fat distributed through the tissues, as in the mammalia; but they differ much in the proportion of fat which they contain, and must be selected for different stomachs very much in proportion to the power of digesting and assimilating fat. Of these, eels are the richest in fat, containing from 14 to 24 per cent., while mackerel only contains about 7, and salmon and trout from 5 to 6 per cent.

Most other fish, as whiting, flounder, brill, turbot, haddock, cod, sole, plaice, are very deficient in fat - soles, for example, contain only 0.248 per cent. - although it is often stored in large quantities in their livers, as in the cod. But in addition to this difference in the proportion of fat contained in different fish, great varieties exist in the closeness and toughness of their tissues. In cod, for example, the flesh is very dense and often tough and "woolly," when it is very difficult of digestion and should not be touched by those with weak stomachs. At all times cod, and other hard, dense-fibred fish, require careful cooking, and should be cut up in thin slices across the grain (as directed for salmon), not eaten in flakes with a fork only. The fat of fish is of an oily quality (see Solid and Liquid Fats, Chap. VII.), and cannot, even when abundant, take the place in nutrition of the solid fat of meat; and this, and the great deficiency of fat in many fish, make a fish diet unwholesome unless properly admixed with other foods.

But, as an adjunct to other forms of animal food in a mixed diet, fish occupies an important place; and the lighter sorts of fish, such as whiting and sole, if judiciously cooked, are valuable in the sick room. For this purpose boiling is the safest mode of cooking, but frying may be permitted if the outside is rejected. If boiled fish is thought too insipid, an excellent plan is to finely mince the boiled fish with a little suet, bread crumbs, boiled potato, salt and pepper, make it into cakes, and brown them in the oven.