New evidences on the distribution of predatory pelagic sharks in the tuna grounds of the Indian Ocean

Since the commencement of the exploitation of oceanic tuna resources of the Indian Ocean seventeen years ago, the hooked rates for the tuna species have declined in many areas of the Ocean but there are no evidences of such a trend in the case of the sharks. As a result, the percentage composition of sharks in the longline catches and the percentage of the tuna catch damaged by sharks show an increase. Hence there is an urgent need for innovation of the existing longline gear in order to increase the fishing efficiency for hooking the tuna species with a corresponding reduction in its efficiency for hooking sharks. At the beginning of this fishery, hooked sharks were discarded at sea, at a later stage the liver and fins were taken and the carcass discarded and presently the sharks are also brought along with the tuna catch. Though the shark meat has a very low market value it is brought in order to cover up for the declining tuna catches. Thus it has become very necessary to increase the demand for shark meat by developing products or by-products utilizing shark meat and ensuring the successful continuity of the tuna longline fishery. The pattern of distribution of shark species in the time grounds of the Pacific, Indian and Atlantic Oceans and also the predation of hooked tunas by sharks were discussed earlier (Sivasubranianiam 1963, 1964 and 1966). Some contribution to these studies is made in this paper based on new data become available.

Torpedo sinuspersici Olfers, 1831

Electric rays, thought to be the most primitive of the skates and rays, have stout tails but have rather expansive disc. This group is distinguished by the presence of powerful electric organs, derived from branchial muscles in head region. Torpedo sinuspersici found inshore in sandy bottoms, and well offshore from the surf zone down to 200 m. Also on or near coral reefs (like Kish Island in Persian Gulf). Common in shallow sandy areas. Occasionally hooked by anglers, more often seen by divers; can deliver a strong shock. Flesh is edible. T. sinuspersici can survive for hours after being stranded on the beach. Little is known of the life history of the Gulf torpedo. It is a sluggish predator of bony fishes. At night it actively hunts for food, sculling slowly through the water about a meter above the bottom; during the day it usually rests on the bottom and opportunistically ambushes unwary prey. It uses its broad pectoral fins to envelop the target fish before delivering an electric shock to stun it. Usually solitary, they may form groups during the mating season. Reproduction is a placental viviparous, with the developing embryos initially surviving on their yolk sacs, and then on enriched uterine fluid produced by the mother. Litters of 9-22 young are birthed in the summer. Newborns measure about 10 cm wide; males mature at a disc width of 39 cm and females at 45 cm.