Common names: English, Hindi and Punjabi - rohu; Bengali-rui; Burmese- nga-myit-chin

History of use: Distributed across the Indo-Gangetic floodplains of Bangladesh, India and Pakistan and also in the Irrawady and its tributaries in Myanmar. Labeo rohita is the most highly valued of all carp species farmed using traditional or newly-developed aquaculture systems in the Indian subcontinent. It has been introduced to other areas of India beyond its natural range for aquaculture in ponds, and reservoirs (Jagannatham 1946; Thyagarajan and Chacko 1950) and also to the Godavari and Krishna rivers (David 1983). Because of its fast growth and high quality flesh, it has also been introduced to other countries, including the former USSR, Japan and the Philippines (Jhingran 1982).

Production statistics: There are no reliable statistics on production of rohu from aquaculture.

Where farmed: Region - South and Southeast Asia, FAO Area - 04, Asia, Inland.

Countries: Bangladesh, Cambodia, China, India, Indonesia, Laos, Malaysia, Mauritius, Myamar, Nepal, Sri Lanka, Thailand and Vietnam.

Climate and environmental tolerance: Tropics; hardy, thrives well below an altitude of 550 m at temperatures ranging from 16.8 - 37.0°C; minimum tolerable temperature 13.9°C; reproduces between 22-31.0°C; tolerates salinity levels of 5 ppt.

Current farming methods: Originally, rohu seed for aquaculture were collected from floodplain rivers and or bred by simulating riverine conditions in special ponds called dry bundhs. Since 1957, induced breeding technology and large hatcheries have facilitated mass production of seed: for a review of methods, see Jhingran and Pullin (1988).

The postlarvae are called renu, dhani or jeera in Bengali and are also commonly called 'spawn'. After absorption of the yolk-sac, 3-day old postlarvae are ready for long-distance transport, rearing in nurseries or direct stocking in ponds prepared for aquaculture. For example, beel nurseries, each measuring over 50 ha, are stocked with carp seed (including rohu) in Bangladesh.

Rohu attain maturity when two to three years old (900 - 1,500 g) in India and spawn (only once a year in nature) during the south-west monsoon (July - August). However, the same females can be spawned two to three times in hatcheries, both before and after the monsoon, in addition to the normal spawning. The mature ovaries constitute about 10-20% of the body weight of the female. A 1-kg female usually releases about 100,000 - 200,000 eggs.

For induced spawning, two injections of crude carp pituitary gland extract (2-3 mg and 5 - 8 mg/kg body weight) are given to females at an interval of 4-6 hrs, as against only one (2-3 mg/kg) to males at the time of the second injection to the female. A female and two males in a running water spawning tank should then spawn within six hours. If spawning has not taken place, the female is then stripped and the eggs fertilized with semen stripped from males. The fertilized eggs hatch after 18-24 hrs, depending upon the water temperature. Optimum temperature for spawning and hatching is 28oC (Chaudhuri 1963).

The postlarvae feed exclusively on zooplankton - rotifers and small cladocerans. Nursery ponds (about 400 m2, 1m deep) are prepared by liming and fertilization with cow manure or poultry litter. Prior to stocking, large copepods and various insects (especially notonectids) and their larvae are predators on the postlarvae and are eradicated from the nursery ponds with appropriate chemicals (Jhingran and Pullin 1988). The postlarvae are given supplementary feed (a fine powder comprising a 50:50 mixture of groundnut or mustard oil cake and rice or wheat bran) daily at two, three and four times the weight of postlarvae stocked, increasing the amount at five-day intervals. Stocked at 5 million/ha, the postlarvae can grow to fry of about 25 mm in 15 days, with a survival of over 60%. The fry are then transferred to rearing ponds (800 - 1200m2, 1.5m deep) for raising to early (50-70 mm) or advanced (100-125 mm) fingerlings or to early juveniles (150 - 200 mm): the stocking material for small or seasonal ponds, perennial ponds and reservoirs respectively.

The feeding habits of rohu fry change when they have reached 25 mm in length. Consumption of rotifers and cladocerans drops to about 10% and they start to feed on unicellular algae (15%) and decaying vegetation (55%), the latter increasing in proportion as they grow further. Fry of about 200 mg are stocked at 100,000 - 125,000/ha, usually as a combination of catla, rohu and mrigal. The proportion of rohu is about 30-35%.

Supplementary feed (the 1:1 cake-bran mixture) is given to fry stocked in rearing ponds, at a daily rate equal to their body weight for the first month, followed by twice their body weight/day in the second and third months.

Grow-out ponds (0.2 - 5.0 ha) are prepared by liming and fertilization. Predators and weeds are eliminated before stocking. Monoculture of rohu is not economical and a cheap, balanced diet for rohu has yet to be produced commercially. Rohu is usually grown in South Asia as 20 - 25% of polycultures that comprise three to six species of Indian major and Chinese carps, and in other countries with various local species. Rohu is a midwater and marginal feeder, browsing on periphyton, decaying vegetation and debris. Experimental intensive aquaculture systems based on stocking 85% indigenous Indian carps with 10% silver carp and 5% grass carp have given annual production of 15 t/ha (Tripathi et al. 1994).

In Andhra Pradesh, India, over 100,000 ha of ricefields have been converted into fish ponds since the 1980's. For large ponds (1-4 ha, 1.5 - 3.0 m deep) the stocking density is around 10,000 fingerlings (>150 mm)/ha comprising rohu (70%), catla (20%) and mrigal (10%). Even at this high density, rohu attains a weight of 1 kg in about an year. The fish are fed initially on rice bran alone and later on rice bran and oilcake (40:60).

At least six intergeneric and five interspecific hybrids employing either male or female rohu have been produced experimentally (Khan and Jhingran 1975, Tripathi 1992). Hybrids of rohu with Chinese carps are short-lived, and the male common carp female rohu hybrid is sterile. The male catla x female rohu hybrid is fertile and combines the deep body of catla and the small head of rohu: characteristics preferred by some farmers. Catla-rohu hybrids are also found in the seed produced from dry bundhs.

Processing and Marketing: Rohu is usually marketed at 1 kg or above; fresh or packed in ice and transported to distant markets. The fish from Andhra Pradesh are transported to Calcutta - a distance of about one thousand kilometers - from where they are airlifted to markets in Assam and the north-eastern states of India. The fish are also sold in Dhaka (Bangladesh). Rohu has also been exported recently to Canada, the middle East and the U.K., though on a small scale, for Bengali citizens.

Likely future trends: Rohu is likely to become an even more important aquaculture species in near future, once research on selective breeding of rohu in India lead to the availability of the seed of faster growing strains. With further studies on feed development, monoculture of rohu in cages, pens, running waters and closed recirculatory systems might be possible. Both fresh and processed rohu might then become significant commodities, with much wider markets.

References:

Chaudhuri, H. 1963. Induced spawning of Indian carps. Proc. Nat. Inst. Sci. India (B), 29:478-87.

David, A. 1963. Studies on the fish and fisheries of the Godavari and Krishna river system. Part I. Proc. Nat. Acad. Sci. India (B) 33 : 263-286.

Jagannatham, N. 1946. A note on the introduction of rohu and mrigal into Madras waters. Indian Farming, 7: 292-296.

Jhingran, V.G. 1982. Fish and Fisheries of India. Hindustan Publishing Corporation (India), Delhi. 666p.

Jhingran, V.G. and R.S.V. Pullin, 1988. A hatchery manual for the common, Chinese and Indian major carps. ICLARM Studies and Reviews 11p. Asian Development Bank and ICLARM Manila, Philippines. 191 p.

Khan, H.A. and V.G. Jhingran. 1975. Synopsis of biological data on rohu Labeo rohita (Hamilton, 1822). FAO Fish Synop 111. 100p.

Thyagarajan, S.and P.I. Chacko, 1950. Introduction of rohu and mrigal from Orissa State to Madras State. Indian Com. J.: 1-4.

Tripathi, S.D. 1992. Three decades of research on carp hybridisation in India, p. 00 - 00. In Symposium on Zoological Research in Relation to Man and Environment, 1 - 4 March, 1992. Zoolical Society, Calcutta, India.

Tripathi, S.D., P.K. Arvindakshan, S. Ayyappan, J.K. Jena, H.K. Muduli, S. Chandra and K.C. Pani, 1994. A new high in carp polyculture : 15 tonnes per ha per year. In National Symposium on Aquacrops, 16-18 November 1994. Indian Fisheries Association and Central Institute of Fisheries Education (Abstracts): 1.

S. D. Tripathi