With few exceptions, the different species of fishes that are caught industrially are important because of their food value.

Some fishes are unsuitable for food because they have an unattractive taste; others are directly poisonous. Thus, in the Japanese fish of the genus tetrodon, the roe is poisonous, although the remainder of the fish is edible. Some fishes are poisonous during the spawning season. Others are provided with a special poison gland connected with special spines or barbs. In edible fishes, given the suitable conditions, poisons may be formed by bacterial activity in the flesh of the fish. Poisons so formed give rise to the kind of fish poisoning known as botulism. Cases of botulism have resulted from eating canned salmon and sardines that have become spoiled. In some cases, bacteria present in a diseased fish may produce poisonous substances in the body of the fish. Bacillus paratyphosus has been isolated from some poisonous fish, and certain poison-producing bacteria have been found in others.2

Certain shellfish are notoriously liable to be poisonous. The exact nature of the microbes concerned in the production of poisonous substances in shellfish is at present unknown; it is clear, however, that such poisonous substances may be produced in shellfish in three ways—

(1) Microbes of various infectious diseases, such as typhoid fever, may be absorbed by the shellfish from sewage.

(2) The shellfish may be diseased, or be seriously contaminated, by living in dirty water.

(3) Decomposition may set in after the shellfish have been removed from the water—particularly if they have been kept too long in a warm place.

It has been found recently that shellfish that have been deliberately fattened on sewage can be effectively cleansed in such a way as to get rid of ingested sewage bacteria. This process has been carried out successfully on a commercial scale at Conway by the Ministry of Agriculture and Fisheries. Danger from infected shellfish may also be safely avoided by boiling them. When shellfish are gathered at the right season of the year and from suitable localities, they are a perfectly safe and wholesome food.

Of the many species of edible fishes that are known and used, the number is by no means complete, and new species are added from time to time. Thus, in 1916, the United States Bureau of Fisheries introduced a new edible fish (Lopholatilus chamaeleonticeps), which they christened the tile fish. After this fishery had been in existence for twelve months, the known catch of tile fish amounted to over 10,000,000 lbs., valued at more than $400,000. In 1917, the same Bureau introduced the dog-fish under a new name. As people were prejudiced against the name “dog fish,” the Bureau altered it to “gray fish,” “which is descriptive, not preoccupied, and altogether unobjectionable.” The fish is now caught in large numbers, and forms the basis of a very flourishing canning industry. Attempts have been made recently to utilize as food the edible portions of the shark (which is closely related to the dog fish) and the porpoise.

The food value of most fishes varies very much according to the condition of the fish when it is caught—that is whether it is spawning or not. Further, it may be considerably modified by the changes that take place subsequently in the composition of the flesh during the processes of curing, cooking or preserving.

Generally speaking, all marine fish annually pass through a well-marked series of seasonal changes, the stages of which appear to depend upon changes in the temperature, salinity and alkalinity of the sea. These changes are directly connected with the development of roe and milt, with the fluctuation in the percentage of oil and fat in the liver and body tissues, and also with the rate of growth. Thus the chemical composition of the fish, and hence its food value, varies greatly according to the season at which it is caught.

Norwegian brisling (“Skipper Sardines”) are caught in the summer just before spawning time. At this time the fat content is high; in winter the fat content is low, and the fish possesses small commercial value.

The gradual change in the composition and food value (in calories per pound) of the herring as spawning time approaches is well shown in Table III. (Prof. J. Johnstone, Trans., Liverpool Biolog. Soc., Vol. xxxiii (1919), p. 106.)

The herrings are caught in September when they assemble in shoals for the purpose of spawning. They are thus most easily caught at the time when their food value is at a maximum.

The flesh of clupeoid fish—herrings, sprats, pilchards, sardines—contains a quantity of oil disseminated throughout the flesh in the form of fine globules. From the above table it will be seen that the percentage of oil in the flesh of the herring may be as low as 2·5 per cent in May, and as much as 31·5 per cent in August. In summer the adipose tissue forms two distinct layers, one situated just below the skin, the other being parallel to the first, but separated from it by a layer of muscular tissue. In winter the oil content becomes so small that these layers of adipose tissue disappear. A comparatively small amount of oil is contained in the liver of the fish.

In gadoid fishes, e.g. cod, as well as in skates and rays, the oil is almost entirely confined to the liver. During the summer the liver grows larger and richer in oil, until sometimes the oil amounts to more than half the total weight of the liver. (When cod are caught the livers are removed and kept apart, to be treated subsequently for their oil.) The percentage of oil in the flesh of the cod varies from 0·1 per cent to 1·0 per cent. Unlike that of the herring, therefore, the food value of the flesh of the cod does not fluctuate markedly according to the season.

When fish are dry-salted a certain proportion of the proteins and mineral salts in the flesh is extracted by the brine pickle that is formed. In Russia and Poland, where the greater proportion of salted herrings are consumed, the peasants eat them without further cooking, and also consume the pickle.

A great gain in food value per pound results from the removal of so much water from the flesh of the fish. Freshly caught cod flesh contains about 80 per cent water and 17 per cent protein; after being dry-salted for export it contains about 25 per cent of water and 55 per cent protein.

Thus, 1 lb. of dry cod is equal in food value to about 3 lbs. of fresh cod. The increased food value of salted fish will be seen from the following analyses—

Thus, the food value of salted sprats or herrings per pound is 50 per cent more than that of the same fish when fresh.

The original food value of a fish is generally diminished by the cooking process. The fish may be boiled or broiled for direct consumption, or it may be steam cooked in cans and sealed up for future consumption, as in the canning industry. When oily fishes, such as herrings, are cooked, the oil globules burst and some of the oil is lost, and the food value of the fish becomes correspondingly less. When salted fish is soaked in fresh water before being cooked, some of the gelatin and other coagulable proteins are extracted from the flesh. This loss of protein can be checked either by broiling the fish, when the protein near the surface becomes coagulated and so prevents the loss of protein from the interior of the fish, or by placing the fish that is to be boiled direct into boiling water, and not into the cold water before the heating has begun.

In addition to this diminution of the food content of the fish, the process of cooking, contrary to general expectation, also diminishes slightly its digestibility.

In the canning process the fish to be canned are cleaned (gutted) and boned, and packed into tins, together with the necessary sauce or seasoning. The tins are then closed, a small hole being left temporarily in the lid. The tins are placed on steam-heated racks, and the contents thoroughly cooked. In this way the contents are sterilized as well as cooked, and the air originally present in the tin is all driven out by the steam through the small hole in the lid. This hole is sealed with a spot of solder while the contents of the tin are still at boiling point. The tin and its contents are allowed to cool down, and are dispatched to the store-room. During storage the contents of the sealed tin gradually “mature.” This maturing process may last from six months to ten years. During this period the bones soften, the flesh becomes soft and pasty, and the taste becomes richer. The precise nature of the changes that take place during this maturing process is not fully understood; probably maturing is partly due to the action of certain enzymes in the flesh of the fish, and partly to the slow but continuous chemical action of the various juices present in the tin. Attempts to pickle herrings from the Zuyder Zee have been unsuccessful owing to a lack of the enzyme action that makes other herrings tender when pickled. The enzyme, although present, is apparently rendered inactive by the presence of an anti-enzyme.

The last, but by no means the least, important factor to be considered in estimating the food value of any particular fish is its retail price. The price of the different kinds of fishes is by no means proportional to their individual food values. It is determined primarily by the abundance or otherwise of the available supply of each individual species. Thus, the various pelagic fish—mackerel, herring, sprat—that are easily caught in enormous quantities at certain seasons of the year are by far the most valuable. Of trawl-caught fish, cod and whiting are more plentiful and are, therefore, cheaper than hake, although, again, the cheaper fish has the greater food value.

In some cases certain fish, although fairly abundant, are in poor demand owing to some prejudice on the part of the public, and are generally sold in poorer districts, or to the fried fish trade, at a disproportionately low price, for example skate, dog-fish, angler fish, john dory.

Taste and appearance also contribute to the popularity and, therefore, indirectly to the retail price of fish, such as the sole and the salmon.

In Table IV the present retail prices (Sept., 1921) and the food values of a number of different fishes are compared. From these figures, the actual food value per shillingsworth of each fish has been calculated.

The cheapest fish, therefore, are also those possessing the greatest food value, e.g. the herring in all its forms, dried cod and ling, and mackerel. These compare favourably both in cost and food value with meat, such as beef and mutton.

Finally, the popularity or otherwise of any foodstuff necessarily depends upon its flavour. Fishes differ greatly in this respect. In many cases the flavour of a fish can be seriously impaired by an unsuitable method of cooking. A full-flavoured fish like the mackerel lends itself to a variety of methods of cooking, equally good results being obtained by baking, grilling, frying in fillets or boiling. The plaice, sole, ling, hake, mullet, and turbot are essentially fish for frying, while cod, haddock and whiting are best boiled. To prepare a fish for the table requires considerable skill, but it is an art that, once acquired, can be used to render even what are regarded as inferior varieties both wholesome and palatable. In this country, fishes have long been a neglected form of food. They have a high food value, they are easily digestible, and are cheap and plentiful.

It has been shown recently that edible fish contain vitamins. Vitamins are complex chemical compounds of hitherto unknown composition, and of little understood properties, that occur in minute quantities in a great variety of natural food stuffs. These vitamins appear to be essential to healthy animal existence. Without them, the body rapidly becomes attacked by certain diseases, e.g. rickets, beri-beri, scurvy, and unless this deficiency of the diet is corrected, death soon follows. Three different vitamins have been discovered, known as vitamins A, B, and C. Vitamin A is contained in the oily part of most fish, while Vitamin B is present in certain fish roes.