The industrial value and importance of fishes is by no means limited to their use as food. They yield large quantities of valuable oil. The fish waste, or offal, chiefly heads, skins, bones and viscera—that is discarded by the fish curer, is worked up to yield fish glue, fertilizers and cattle food. The skins of certain large fishes, for example the shark, are tanned and manufactured into a valuable leather.

The story of the fishing industry would not be complete without a brief description of the methods by which these products are manufactured.

Fish Oils. The various kinds of oil that are obtained from different species of fish and other marine animals, such as whales and seals, may be divided into three classes, according to the part of the fish from which they are extracted.

(1) Fish oils proper are disseminated throughout the flesh of the fish in the form of fine globules. They are extracted from the entire fish, e.g. herring, sardine, sprat, menhaden.

(2) Liver oils are located in the fish liver, e.g. cod, shark.

(3) Blubber oils constitute a thick layer of adipose tissue just under the skin of the marine mammalia, e.g. whale, seal, dolphin, porpoise.

In oily fish, such as herrings and sprats, each minute globule of oil is enclosed within a thin skin. It is practically impossible to rupture this skin and liberate the oil simply by the application of pressure. When, however, these globules are heated the skin shrivels, the oil globules expand and burst the skin, and the liquid oil is liberated and can then be extracted from the flesh by pressure. To obtain the oil, therefore, the fish are boiled or steam heated in large vats until the oil is set free. The hot mass is then placed in a press and the oil squeezed out. The residue is made into cattle food and fertilizer.

In obtaining the best sorts of liver oils, e.g. codliver oil, the livers are taken from the fish as soon as they are caught, and are heated in steam-jacketed vessels until the cell membranes burst and the oil exudes. The oil is then separated by pressure.

Inferior qualities of oil are obtained by treating putrid livers in the same way at the end of the voyage. These tainted liver oils are unfit for medicinal purposes, but are used in large quantities in the leather industry.

Blubber (which is from 8 to 20 ins. thick) is stripped from the whale as soon after capture as possible. Generally the dead whale is made fast alongside the whaler, a deep, spiral cut is made round its body, and the blubber is stripped off and hauled aboard. This is then cut into pieces, chopped up in mincing machines and fed into melting pans and heated with steam, often under pressure. The oil gradually exudes and collects upon the water, the cell membranes, etc.—the greaves—settling to the bottom. At the conclusion of the boil, the oil is drawn off from above the aqueous (gluey) layer, and is clarified by straining through sieves or filters. The “greaves” is placed in hair or woollen bags and submitted to hydraulic pressure, by which means a further quantity of oil is obtained.

Fish oils, unless specially purified for medicinal purposes, are dark-coloured liquids, with a characteristic, unpleasant, fishy smell, due to the presence of small quantities of fishy decomposition products, for example trimethylamine.

When cooled, many samples of fish oil deposit solid masses of fish tallow (fish stearine).

Fish oils, and, to a less extent, the marine animal oils, e.g. whale, seal, porpoise, are drying oils like linseed oil, that is they possess to a very marked degree a capacity for absorbing oxygen from the air, and so become thickened and viscous. This thickening is generally induced by blowing air through the warm oil. Oils that have been thickened in this way are known as “blown” oils.

Blown fish oils are mixed with mineral oils for use as lubricants for heavy machinery. They have been used as vehicles for paints in place of linseed oil, but with somewhat disappointing results. They are used successfully in place of linseed oil in the manufacture of printers’ ink, and in making paints for painting smoke stacks. Such paints resist successfully the action of heat and light.

More particularly, they are used in the leather industry. Fish oils are used chiefly in the manufacture of chamois leather. Ordinary chamois or wash-leather is made from the flesh-splits of sheep skins. The skin is well washed and softened, and freed from hair by treatment with lime. It is then split, and the loose and fatty middle layer removed by a sharp knife. The lime is removed by a short bran-drench and the superfluous moisture is pressed out. The skin is thus rendered porous and easily able to absorb the oil. It is stretched on a table and oiled with fish or whale oil. The oiled skin is folded up and worked for two or three hours in the faller stocks and then shaken out and hung up for a short time to cool and partially dry. The process is repeated a number of times, until all the water originally present in the skin has been replaced by oil. The oiled skins are then piled in a warm place. The oil gradually oxidizes—probably owing to some fermentation process—and the skins become yellow and very hot. From time to time the skins are strewn on the floor to cool and then re-piled, the process being repeated until the oxidation of the oil is complete. In France the freshly-oiled skins are hung in hot stoves, and the oxidation of the oil is completed in one operation.

The skins are then dipped in water and passed through hydraulic presses, by which the surplus oil is removed. This surplus thick, oxidized oil is known as “degras” or “moellon,” and is used for stuffing leathers that have already been tanned. Stuffed leathers are supple and impervious to water, and are used for harness, belting, etc. A further quantity of oil may be removed from the “chamoised” leather by treating it with potash or carbonate of soda, “sod” oil being recovered from the extract by neutralization with sulphuric acid. The value of sod oil for oiling dressed leather is due to a resinous acid of unknown composition, that is soluble in alkali but insoluble in petroleum ether.

Enamel or patent leather is generally coated, after tanning, with a linseed oil varnish, boiled with prussian blue, and dried in a steam heated chest at 70° to 80°C., the process being repeated until a sufficiently thick coat is produced. Fish oils are now used successfully in place of linseed oil. The enamel leather produced, although not quite so glossy as that made with linseed oil, is said to be more pliable.

Fish oils are also employed in the manufacture of such closely-related, although happily diverse, substances as soap and margarine. All animal and vegetable fats and oils are essentially compounds of glycerine, with one or other of three acids: palmitic, stearic and oleic. Palmitic and stearic acids and their compounds are solids at the ordinary temperature, whereas oleic acid and its compounds are liquid. This difference appears to be connected in some way with the molecular structure of these substances. When oleic acid is heated with hydrogen gas under pressure, in the presence of finely-divided nickel, it absorbs hydrogen and is transformed into stearic acid. Oleic acid, therefore, is said to be unsaturated with respect to hydrogen, whereas stearic acid is called a saturated acid. This process, whereby a liquid oil is transformed into a solid fat, is called hydrogenation, or hardening.

Both the margarine industry and the soap industry require large quantities of hard fats. Originally the soap industry absorbed the available supplies of hard animal fats such as beef suet, hog’s lard, and mutton suet. The margarine industry depended upon these same supplies of animal fats, and the rapid growth in the production of margarine during recent years has seriously diminished the supply of hard fats necessary for the manufacture of soap.

The hydrogenation of whale oil and various fish oils has now made it possible to supply this demand, and has also made possible the industrial utilization of substances, such as fish oils, for which formerly comparatively little use could be found.

Hardened whale oil melts at 40° to 50°C., and is a white solid entirely devoid of taste or smell. It is used for making soap, and as a lard substitute for cooking purposes.

Fish Glue. Fish glue is the most important liquid glue on the market. The bulk of the fish glue manufactured to-day is made from the waste and offal that are discarded by the curers. This waste consists of heads, bones, viscera and skins. The best glue is obtained from the skins of non-oily, demersal fish, for example cod, haddock, soles, plaice and hake.

The waste is washed in running water to free it from salt. Sometimes the waste—particularly the heads—is decomposed with hydrochloric acid and afterwards neutralized with lime. It is then charged into a cooker provided with a perforated, false bottom. The stock is covered with water and heated with steam. The glue is extracted and gradually concentrates in the water. When this glue liquor is sufficiently concentrated (from 5 to 6 per cent), it is run off (the first run) and more water is added to the waste and the cooking continued. After about 10 hours cooking, nearly all the glue has been extracted and the liquor is again run off (the second run). The cooked waste is then withdrawn, and any remaining glue liquor is pressed out of it and added to the second run. From 2 to 4 per cent of phenol or boric acid are added to prevent decomposition by bacteria.

The glue liquor is evaporated down to a concentration of 32 per cent in open vats or closed evaporators, and is bleached with sulphurous acid. A small amount of some essential oil, e.g. cassia, clove, wintergreen, is added to check mould growth and mask the fishy odour. Glue is also made in a similar way from the “greaves” obtained from whale blubber.

Fish glue is manufactured in three grades.

Grade I is made from skins, only the first run being used. It is used for photo-engraving work, for the production of half-tone plates.

Grade II is made from second run skin liquors and fish waste. It is sold in small cans and bottles for general repair work.

Grade III is prepared from fish heads, and is sold in large cans and barrels for sizing, box making, cabinet making, and general joiner work.

The glue is sometimes made more flexible by the addition of glycerine and glucose. The flexibility of fish glue makes it useful for the manufacture of court plaster, labels, stamps, and in book-binding.

The residue from the press is dried and sold as chicken feed or fertilizer. For the latter purpose it is frequently mixed with Carnallite.

Fish Gelatine. Fish gelatine or isinglass is obtained from the swimming bladder of the sturgeon and also of the cod. The bladders are exported, either opened (pipe isinglass) or washed, split open and dried (purse, lump or leaf isinglass).

Isinglass is the purified and dried inner skin of the bladder. It has but feeble adhesive power. It is used for clarifying wines, ciders and beers, and for making jellies and plasters.

Fertilizers. In many places near the sea, fish are employed whole as manure. Sprats particularly are caught in large numbers and distributed over the fields, and left to decompose. Fresh sprats contain 63·7 per cent of water, 1·94 per cent nitrogen, 2·1 per cent ash (0·43 potash and 0·90 phosphoric acid).

Fish guano or fish manure is generally prepared from the fish waste discarded by the curer. An average sample of this manufactured fish manure will contain 12 per cent water, 60 per cent organic matter, yielding 10 per cent ammonia, 16 per cent of calcium phosphate, and a residue of salt, sand, magnesia and potash, the amount of potash being inconsiderable. Fish guano is mainly valuable as a source of ammonia, the ammonia content ranging from 6 to 11 per cent, according to the kind of fish used and its previous history, e.g. whether fresh or salted.

In many places, such as London, the fish offal from the shops and restaurants is collected, dried and ground up for use as manure. In Germany in 1918 herrings’ heads were removed by the curers to be utilized for the production of oil, albumen, and phosphate of lime. The herring meal contained up to 50 per cent of albumen and calcium phosphate, the latter being obtained from the bones and heads. The albumen was extracted chemically and prepared for human consumption. The oil was extracted with benzol or other solvents, and, after hardening, was used in the manufacture of butter substitutes. Fish waste or offal is fed into a continuous cooker. This cooker consists essentially of a long, cylindrical vessel, through which runs a hollow steel shaft on which are mounted perforated radial vanes in such a way that the whole arrangement forms a spiral conveyor. By means of the hollow shaft and vanes, steam is blown into the mass of fish waste as it travels slowly through the vessel, so that it is completely cooked and disintegrated by the time that it emerges at the other end.

The cooked mass is then fed into a press in which a screw conveyor urges it through a gradually tapering cylinder with perforated sides. In this way the oil is extracted from it, and it is then dried and disintegrated by a rotary drier.

There is always a little residual oil in fish manure that tends to delay its decomposition in the soil. It is important, therefore, that the oil be removed as completely as possible.

Dry fish manure requires careful storing, as the presence of this small amount of oxidizable oil tends to promote spontaneous combustion.

In addition to its value as a fertilizer, the high content of protein (albumen)—namely, 50 per cent—makes fish meal a suitable food for live-stock and poultry.

The commercial importance of this industry will be realized when we remember that practically half of the total catch of fish in the world is discarded by the curers as waste.

Fish Leather. The hides of such marine mammals as the walrus and the seal have long formed the basis of a regular tanning industry.

Of recent years, however, particularly in America, successful attempts have been made to tan the skins of certain fish, notably the shark. The skins are treated with alkali to remove fat and oil, the alkali is then neutralized with acid, after which the skins are washed and tanned. The leather is said to be soft and pliable, and well adapted for many uses.

Shark skins are also tanned hard, and used to print a grain on imitation pigskin.

Shark fishing was commenced off the American coast in October, 1918. The fish are hunted from fast, powerful motor boats, with specially constructed nets. A small shark 5 ft. long will yield a hide 10 sq. ft. in area.

Shark skin is naturally very tough and durable, and in its untanned condition is used by jewellers as a natural emery paper for grinding and polishing metal surfaces. It is also used as an abrasive in working hard woods and ivory.

A method has been devised by which a shark skin can be split into three. The first split, after tanning, is strong and thick, and suitable for high grade, heavy shoes. The second furnishes leather suitable for second grade foot wear, and the third resembles suede and is used in making fancy articles. In addition to the shark’s skin, the fins, blood, teeth, flesh, and oil of the fish are also utilized commercially and yield a satisfactory profit.