Freshwater aquaculture

Unit 4 - Cultivable fish and their culture methods

4.1.4. Grow-out culture
Carp culture is the main stay of Indian aquaculture. Catla rohu and mrigal are cultured traditionally in ponds and tanks due to their higher growth and consumer preference. Due to their compatible nature all the three species are grown under polyculture. Catla is a surface feeder feeding predominantly as zooplankton while rohu is a column feeder. It is an omnivorous planktophage. Mrigal is a bottom feeder feeding on benthic detritus. The exotic silver carp, grass carps and common carp are also stocked along with the three Indian major carps.
India possesses 2.3 million ha of potential freshwater resources in the form of tanks and ponds, but utilize only about 40% on the average for carp farming. The average national carp culture productivity is only 2 tons/ha/yr while the potential production is be 3-4 tones under semi-intensive culture. Hence there is an enormous scope for increasing carp production in India. This is possible through bringing more average under culture and also by following scientific technology of carp production.
The scientific carp culture practices involve the following steps.

Pre-stocking preparation
Control of aquatic weeds and eradication of predators and pest fishes are one of the important steps for achieving higher production. The ponds should be dried until the bottom soil cracks. Application of lime @ 200-500 kg/ha and ploughing the soil removes obnoxious gases, oxidizes the organic matter in ponds, sterilizes the soils and kills unwanted animals.

After filling the ponds to a depth of 1.5m the ponds should be manured with cow dung @ 3-4 tones/ha as a basal dose 15-20 days before stocking. Alternatively, poultry dropping @ 1.5-2.00 ha/ha can be used. If mahua oil cake is used for eradication of predatory and weed fish the basal dose of organic manure is avoided. 15-20 days after application of manures the ponds are ready for stocking.

The carp culture ponds are stocked with surface, column and bottom feeders at proper ratios to utilize all the pond niches efficiently. 30-40% of surface feeders, 30-35% of column feeders and 30-40% of bottom feeders should be stocked. Grass carp may be stocked if terrestrial grasses or aquatic weeds can be supplied from outside.

The grow–out ponds are stocked with fingerlings of 8-10g after proper acclimatization. A stocking density of 5000 fingerlings/ha is recommended to get a production of 3-5 tonnes/ha/yr. The density could be raised to 8,000 – 10,000 figerlings/ha for achieving production of 5-8 tons/ha/yr. The stocking density depends on the level of management and input use.

Post stocking management
The management of carp, ponds after stocking involves intermittent liming and fertilization, water management and health care.

In average production ponds cow dung is applied @ 500 kg/ha/fortnight, along with 15 kg/ha single superphosphate and 10 kg/ha area. However the dose and frequency of fertilization depends on plankton production and water quality. Other manures such as poultry manure or duck droppings can be used at half the dose of cow dung.

Supplementary feeding is provided to the stocked fish. At high stocking densities natural food produced through fertilization will not be sufficient. Supplementary feeds could be simple mixtures of rice bran and oil cake in a ratio of 1:1. The quality of supplementary feed can be improved by mixing fishmeal and soya bean meal with vitamin and mineral mixture to the simple mixture of rice bran and oil cake. Commercial fish pellets are also available which may give better yields. The carps are fed @ 5% of the body weight for first month which is gradually decreased to 3-2%. The biomass of the fish should be assessed monthly and feed ratio should be adjusted accordingly. The daily ratio is split into two doses and fed in the morning and evening. The mixture is made into dough by adding correct quantity of water and making in to balls.

The balls are kept in feeding trays at different places in the pond. In Andhra Pradesh simple mixtures of rice bran and powdered ground nut oil cake are filled into gunny bags with small holes, which are hung in the ponds on poles at different places. Fish will nibble at the holes and get feed. When grass carp is stocked in the ponds aquatic weeds such as Hydrilla, Ceratophyllum, Najas, duck weeds or tender terrestrial grasses should be provided.

Harvesting
The carps take about 10-12 months to grow to marketable size when fingerlings of 8-10 g are stocked. In this period catla grows to a size of about 1 kg, while, rohu and mrigal attain a size of about 600-750g. Silver carp grows to more than 1 kg while grass carp can grow to a size of 3-5 kg if fed at 100% of its body weight. The fish are usually harvested by large seine nets by repeated dragging. If the ponds are drainable a finally drain harvested in carried out. A production of 3-5 tones/ha/yr can be obtained through scientifically managed semi-intensive culture of carps.