One safe general direction for choosing fish may be given in few words, viz., choose the plump ones.
A short fish, thick about the shoulders, is much to be preferred to a long thin one. Thick soles, or thick turbots, are far preferable to thin ones. The same with codfish.
Lobsters and crabs should be chosen by weight, and those of medium size are best in flavour.
There are one or two kinds of fish which are positively improved by being kept a day or two, notably skate and red mullet.
Mackerel, on the contrary, is a fish than which none spoils more rapidly.
The sole holds a first position among flat fish, and is deservedly esteemed, as its flesh is firm and delicate and very easily digested, hence its great popularity with the sick. It has also the advantage of being obtainable all the year round in good condition. The skin of the back is sometimes dark, sometimes white, varying with the nature of the ground on which the fish feeds. Soles vary in size from quite little slips, called "tongues," to large fish weighing eight or nine pounds per pair. Those in roe are rather insipid in flavour, and are best for filleting. They vary in price, but are never a cheap fish.
Halibut is an excellent substitute for turbot, which it rather resembles in flavour, and is a comparatively cheap fish. It is abundant in spring and summertime, and always a favourite with Jewish people. Being a very large fish, it is rarely sold entire. The choice bits are the flackers over the fins and the pickings about the head.
A fillet or "steak" is the most profitable portion for general eating.
Cod is at its best about Christmas time. From the end of January to March it is less good and not abundant; in May again it is generally very fine. The best are those which are plump and round at the tail, the sides having a slightly ribbed appearance, with yellow spots on a clear skin.
Large cod are not generally cooked whole, being so much thicker at the head than at the tail. The head and shoulders, usually sold apart, form a handsome dish.