Selection of pond

The main criteria to be kept in mind while selecting the pond is that the soil should be water retentive, adequate supply of water is assured and that the pond is not in a flood prone area. Derelict, semi derelict or swampy ponds can be renovated for fish culture by dewatering, desilting, repair of the embankments and provision of inlet and outlet.

Pond Management

Carp culture in ponds is basically a three-tier culture system where the first step begins with the rearing of spawn up to fry (2–3 cm) stage for 2–3 weeks in nursery ponds followed by rearing of 2–3 weeks old fry for about 3 months up to fingerling stage (8–12 cm) in rearing ponds before they are finally released in stocking ponds for growing up to table size fish. To ensure high rate of survival and growth during all the three stages of rearing, a package of management practices should be strictly followed, and slackness at any stage of the management procedure may affect farm productivity and profitability adversely.

Techniques of management involve (i) manipulation of pond ecology to ensure optimum production of natural fish food while maintaining the water quality parameters within tolerance limits of the stocked fish species; and (ii) the husbandry of fish through stock manipulation, supplementary feeding and health care. Broadly, the various steps involved in the management of ponds at all the three stages of culture may be classified as (i) pre-stocking, (ii) stocking and (iii) post-stocking management operations.

Prestocking

Pre-stocking management aims at proper preparation of ponds to remove the causes of poor survival, unsatisfactory growth, etc., and also to ensure ready availability of natural food in sufficient quantity and quality for the spawn/ fry/fingerlings to be stocked. Pre-stocking part of the management involves the following sequential measures.

Eradication and control of aquatic weeds and algae

Floating weeds such as water hyacinth, Pistia, etc., very often cover the entire water surface cutting off light drastically, thus resulting in critical reduction in primary productivity of the pond. Weeds are classified according to their nature of occurrence, into four major groups namely, floating, emergent, submerged and marginal. Control measures for all the above-mentioned classes of weeds and blooms fall into four major categories, viz. preventive, manual and mechanical, chemical and biological.

Preventive control

The preventive measures have to be taken well in advance. The measures include trimming of pond margins, dewatering and desilting of old ponds, uprooting or burning of dried marginal weeds during the summer and providing barriers to prevent the entry of floating weeds.

Manual and Mechanical control

The free-floating groups of weeds are either hand picked or dragged by wire or strong coir rope nets. In bigger ponds they should be removed part by part from the marginal areas and finally the centrally located weed mass is dragged towards the banks and lifted out. The manual removal of submerged weeds from a heavily infested water body is relatively much more difficult. They are either pulled by hand or hand-drawn bottom rakes or uprooted with bamboo poles having a cross piece tied strongly at the terminal end.

Chemical control

Certain commercially available chemicals (herbicides) can provide an efficient means of eradication of undesirable aquatic plants. Most herbicides are selective in nature and hence application of appropriate herbicide should be taken up. Floating weeds such as water hyacinth (Eichhornia crassipes) can be controlled using the herbicide 2–4-D at a dosage of 12-36 kg/m². Water lettuce (Pistia stratiotes) could also be controlled by foliar spray of aqueous ammonia (1%) at the rate of 50–75 kg/ha along with 0.2 % of any commercially available detergent as a wetting agent.

Emergent weeds such as water lily, lotus, and floating heart can be cleared by spraying the herbicide 2–4-D at the rate of 8–10 kg/ha with detergent (0.25%). The chemical is diluted at the rate of 300 l/ha and sprayed through a foot pump sprayer.

Submerged weeds like Vallisneria, Hydrilla, Najas, Potamogeton and Ceratophyllum can be controlled by paraquat at the rate of 3–4 ppm within two weeks. It can also be controlled by application of anhydrous ammonia at the rate of 15–20 ppm.

Marginal weeds like Ipomea, Jussiaea, etc., could be controlled by spraying the herbicide 2–4-D at the rate of 8 kg/ha.

Copper sulphate is perhaps the oldest and a very widely used algicide to control excess algal bloom. The recommended doses are 0.2 to 1.0 ppm, but it is not very effective in ponds having high pH (pH above 8.6), Microcystis bloom is cleared with 0.3 to 0.5 ppm of Diuron. Simazine also clears the bloom in 16–20 days and the rate of application is 0.3–0.5 ppm. Both the chemicals do not have harmful effect on fish.

Chemical control of aquatic weeds

Biological control of aquatic weeds

Another important controlling method is by introduction of weed-eating fishes. Grass carp is the most effective biological control agent against most of the submerged and floating weeds except the water ferns. About 300–400 fish, each of about 0.5 kg weight, are enough to clear 1 ha of Hydrilla infested water body in about a month.

Common weed eating fish and the weeds of their preference

Eradication of unwanted fish

Predatory fish prey upon the spawn, fry and fingerlings of carps and the weed fish compete with carp for food, space and oxygen. Therefore predatory and weed fish should be completely eradicated from nursery, rearing and stocking ponds before these ponds are stocked. The commonly encountered predatory and weed fish in undrainable ponds are listed below.

Common predatory and weed fish of undrainable ponds

Absolute removal of these unwanted fish by thorough and repeated netting is not possible and hence dewatering and poisoning the pond are the only alternative methods.

Fish toxicants

Although a number of chemicals and plant derivatives are available in the market which are poisonous for fish, only a limited number of such toxicants are safe and suitable for fish culture purposes. Based upon the following criteria a suitable fish poison is selected.

• Poisoned fish should be safe for human consumption
• Least adverse effect on the pond biota
• Toxicity period should be of short duration
• Should not have residual effect
• Easy commercial availability
• Simplicity of application
• Cost considerations.

Mohua oil cake, bleaching powder and ammonia are considered suitable. Mohua oil cake kills all the fish species within a few hours when applied at the rate of 250 ppm. The toxicity of doses up to 250 ppm lasts for about 96 hours and subsequently it serves as organic manure in the pond. It should be applied at least two weeks before stocking the ponds.

Bleaching powder or Calcium hypochlorite (CaOCl₂) is another practical and safe fish toxicant. It kills all the predatory and weed fish of the pond when applied at the rate of 25–30 ppm. The method of application is also relatively simple. The powder is mixed with water and uniformly spread over the entire water surface. Distressed and dead fish are removed by netting. Chlorine killed fish are safe for human consumption.

Anhydrous ammonia when applied at the rate of 20–25 ppm kills the predatory and weed fishes. Besides, it also controls the aquatic weeds and later acts as nitrogenous fertilizer. Toxicity of ammonia lasts for 4–6 weeks.

Recommended doses of fish poison

Liming

The tanks, which are acidic in nature, are less productive than alkaline ponds. Lime is used to bring the pH to the desired level. In addition lime also has the following effects -

• Increases the pH.
• Acts as buffer and avoids fluctuations of pH.
• It increases the resistance of soil to parasites.
• Its toxic effect kills the parasites; and
• It hastens organic decomposition.

The normal doses of the lime desired ranges from 200 to 250 Kg/ha. However, the actual dose has to be calculated based on pH of the soil and water as follows,

Fertilization

Fertilisation of the pond is an important means of intensifying fish culture by increasing the natural productivity of the pond. The fertilisation schedule has to be prepared after studying the quality of the pond soil. A combination of both Organic and Inorganic fertilisers may be used for best results. The fertiliser programme has to be suitably modified depending on the growth of the fish, available food reserve in the pond, physico chemical conditions of the pond and climatic conditions.