Hilsa, Tenualosa ilisha, is commercial fish in the Indo-Pacific region supporting the livelihood of millions of people. The most productive fishing ground for hilsa is the Padma-Meghana-Jamuna delta which drains in the Bay of Bengal—the highest landing of hilsa in the world being from Bangladesh. Like Atlantic Salmon, hilsa is an anadromous fish migrating over 1000 km into the freshwater zone, where they are born, for breeding. The young ones migrate back to the sea and remain there till they attain maturity. In India, hilsa has a discontinuous distribution along the north-east and north-west coast. It is caught from the inland and marine waters in West Bengal, Odisha, Andhra Pradesh, Assam and Gujarat. Hooghly, Narmada, Brahmaputa, Godavari, Mahanadi; Chilika lake and Ukai reservoir are famous for hilsa fishery. About 90 per cent of the hilsa catch in India is from the Hooghly-Bhagirathi river systems.
The landing of hilsa along its conventional fishing ground in India has drastically declined due to various reasons including recruitment failure and overexploitation of breeding stock and juveniles. The total landings of hilsa from the Hooghly systems were 43985 tonnes in 2003-04, which declined to 14005 tonnes during 2009-10. As it is the most expensive finfish in India, the associated fishing pressure is very high. Thousands of stationary bag nets with small mesh size (including mosquito net type) cod end are in operation along the estuarine stretch of all the major river systems. Bag nets are usually operated to catch shrimps. About 170 species of finfish and shell fish species have been identified from the bag net landings in the country. More than 80 per cent of catch in stationary bag nets is constituted by juveniles of fish and shell fish and they are sold in fresh or dried condition. Tonnes of juveniles of hilsa are being caught by these non-selective nets. The average catch of juveniles in Hooghly system alone is estimated at 85 tonnes per year. Gill net is the most important fishing gear used for the capture of hilsa in the country. Nylon monofilament gill net with mesh size above 120 mm is used for catching mature hilsa in the marine and inland waters. There is a targeted fishery for sub-adults of hilsa migrating back to the sea, by gill netters in the estuarine systems with mesh size less than 90 mm. This adversely affects the hilsa breeding stock and recruitment.
Drag and seine nets are long rectangular nets made of small mesh size netting. They are popular in several places and major share of the catch of these nets are constituted by juveniles. It is reported that a large quantity of hilsa juveniles are caught by the drag nets (mahajal) in the Ukai reservoir and are exported to north-east hill region after drying.
Additionally, there is a practice of collecting wild prawn seeds from natural water bodies for aquaculture purposes. Small mesh scoop nets, bag nets and skimming nets are used for seed collection. Fishermen discard and inadvertently destroy the fish component (bycatch) after sorting the prawn seeds. Similarly exploitation of the stock by the mechanised vessels in marine waters is highest at present. About 11,213 mechanised boats are in operation in West Bengal which includes trawlers, gill netters, bag netters, etc. Breeding migration of hilsa is affected in many river systems due to the construction of dams across the rivers and the fishery is collapsing in several places.
Technological intervention: In view of this declining catch of hilsa and huge quantity of hilsa juvenile bycatch Central Institute of Fisheries Technology (CIFT), Cochin, in collaboration with Central Inland Fisheries Research Institute (CIFRI), Barrackpore has taken up a project to reduce the capture of juveniles of hilsa and other commercially important fishes in stationary bag nets. A Bycatch Reduction Device (BRD) consisting of a square mesh window of size 1m x 0.75 m made of 50 mm mesh has been fixed near the cod end of bag nets as a technique to permit the escape of hilsa juveniles and other commercially important species (Fig. 1). Covers with very small mesh size are fixed over and on top of the square mesh windows to quantify the percentage of hilsa juveniles escaping through the window.
These experimental bag nets are under operation in Tribeni, Godakhali and Frasergunj in Hooghly river; at Bharbuth in Narmada river in Gujarat; and, at Odalerevu, Godavari, in Andhra Pradesh in a participatory mode as the BRD is under trial now. Once the design is optimised and the BRD is standardised, it will become mandatory. In Hooghly the mean escapement of all the species from the BRD was found to be 0.65 kg and juveniles of hilsa formed 11.6 per cent of the total catch excluded from the BRD. The length of the excluded hilsa ranged from 37 mm to 55 mm. The experiment is in the preliminary stage and seasonal and temporal variations in the catch and size classes need to be studied, for optimising the mesh sizes and position of the BRD in the net for enhancing the juvenile’s escapement, especially hilsa, from the bag nets.