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.