Elasmobranch fish

The Elasmobranch fishery includes those of sharks, rays and skates which are characterized by the possession of cartilaginous skeletons, distinct placoid scales and separate gill slits without operculum. The most important contribution of this class of fish is the shark liver oil, which is a rich source of vitamin A, so essential for stimulating proper protection of the moist surface tissues of eye, throat and bronchial tubes. The oil also contains vitamin D, which is very essential for proper growth of bones. This class of fish is also a source of insulin and other valuable biochemicals like squalene and paucreatin, besides protein rich food material for man and animals.

Some special features

Elasmobranch fish are essentially marine in nature, but some varieties are encountered in freshwater also. These fishes are also unique in that flesh contains urea in larger proportions than other fishes. The trimethylamineoxide contents of the muscle of this class of fish are also high compared to those in other fishes. These non-protein nitrogen compounds raise the osmotic pressure of body fluids to the level of that of the seawater surrounding the fish. This process is known as "Osmoregulation", upsetting of which incidentally is the cause for the death of marine fish suddenly transferred to freshwater and vice versa.

Fishery

The Elasmobranch fishery is distinct from others by the following features.

The fishing is not restricted to any particular zone.
It is distributed both in inshore and offshore areas.
The fishery is operative almost throughout the year.
The main fishing centers are:
Bombay – Cambay area
Mangalore – Calicut zone
Tuticorin – Madras, Vishakhapatnam and West Bengal area.
Of these, Bombay, Mangalore, Tuticorin and Tamil Nadu are the most important landing centers.

The fact that the elasmobranch fishes constitute an important marine fishery of India is borne out by the following landing figures of this class of fish in out country during the past few years.

Both the east and west coasts of India contribute equally towards the landings of these fishes and the fishery is independent of seasons. Even though the important gear used for shark fishing in our country is the hook and line considerable quantities are caught in nets also.

Important varieties

The sharks of commercial importance are given below:

Scolicodon (Grey dog shark)

Carcharinus (Grey dog shark)

Sphyrna (Hammer head shark)

Stegostoma

Rhincodon (Whale shark)

Nedrius

Rhincodon typus is the largest living fish growing over 50ft. Feeding on planktonic algae.

Scoliodon palassorah is one of the few sharks regarded in fresh state as food.

Carcharinus is widely distributed on both coasts of India: its fins are more important than the flesh, which is of poor quality.

Hammer headed sharks are ferocious and carnivorous. Certain species grow upto 20 feet and their large livers yield highly valuable liver oil.

Saw fishes (Pristidae) Grow up to 35 feet and ascend the estuaries and are common in both the coasts.

The species of Rhinobatus and Rhyncobatus contribute to the fishery of skates. The rays are disc shaped and the tail is whip like in most of the species. The commercially important species belong to the genera. Dasyatis, (Sting ray), Acctobatus (Eagle ray), Mobula (Dent rays). The electric ray represented by genera Narcine are not commercially important though common in waters, as they do not grow to large size.

Industry

Even though the peculiar flavour of the fish is not relished by many fish eaters, the fresh meat is sold to some extent in internal markets. At present in the country, the salting and drying is done and are marketed as "Wet – cured" or dry- cured.

A method has been recently reported to eliminate urea almost completely from the flesh. The fish salted in the ratio of 1:4 (common salt: fish) is desalted for 24 hours in 5% brine, resulted in the ratio of 1:6 (salt to fish) for another 24 hours and dried in sun, when the urea is reduced to negligible quantities.

Dry – cured cartilaginous fish finds good markets in Sri Lanka, Burma, Singapore etc. Dried shark fins are exported in large quantities to Singapore, Malaya, Hongkong etc.

The most valuable product is the shark liver oil, which is a very rich source of vitamin A. The average annual production of shark liver oil in India was 113 tonnes in the year 1951-59.

A high quality of leather from sharkskin is also obtained for manufacture of shoes, suitcases etc.

The shark liver oil, could become an important resource in the fight against cancer. Shark liver oil contains alkysoksy-lglycerol, which is said to strengthen the shark’s immune defense system. The shark is the only living creature, which believed never to contract cancer – ever after laboratory attempts to give the species the disease. Shark liver oil also strengthens tissues and has a favorable effect on patient undergoing radiation treatment.