Freshwater Aquaculture

Freshwater Aquaculture resources of the country have been estimated of the order of 13.67 million ha, of which 2.25 million ha are in the form of ponds/tanks, 8.27 million ha beels/jheels/derelict water bodies and 3.15 million ha reservoirs. There has been a very impressive growth in fish production during 1990's in the country.

Fertilizer management in freshwater aquaculture

The potential areas for inland aquaculture have already been explored in India, additional production can only be achieved through successful manipulation of available resources that influence the productivity of various aquaculture systems. One of the main ways to achieve this is through maintenance of adequate levels of nutrients in the pond environment. Pond-management in fish culture is mainly concerned with fertilization requirements and strategies; and with good management of pond soil and water quality.

Fertilization requirements of flsh ponds

Phytoplankton, the floating microscopic plants, that give water its green colour, are the fIrst step in the food chain of fIsh ponds. Other organisms also feed on them and multiply, increasing the availability of natural food for fIsh stocked in the pond. Besides carbon dioxide (CO2), water and sunlight for carbohydrate synthesis, phytoplankton need mineral elements including nitrogen, phosphorus, potassium, calcium, sulphur, iron, manganese, copper and zinc for their growth and nutrition. To promote phytoplankton growth and to maintain the optimum natural productivity of ponds, the water must contain adequate amounts of these nutrients. Managing the pond soil and water effectively can, therefore, help to provide an adequate amount of natural food for stocked fIshes, promoting the healthy growth of fry and fingerlings. Pond soil plays an important role in regulating the concentration of nutrients in the pond water. Knowledge of the nature and properties of pond soil can help a farmer to develop efficient management practices that will boost production.

Properties of pond soils

The most important chemical properties of bottom soil influencing the nutrient management practices of ponds are as follows.

Soil reaction (PH): The pH of the soil is one of the most important factors for maintaining pond productivity since it controls most of the chemical reactions in the pond environment. Near neutral to slightly alkaline soil pH (7 and a little above) is considered to be ideal for fIsh production. If the pH is too low (strongly acidic), this can reduce the availability of key nutrients in the water and lower pond fertility.

Organic carbon content: Organic carbon acts as the source of energy for bacteria and other microbes that release nutrients through various biochemical processes. Pond soils with less than 0.5% organic carbon is considered unproductive while those in the range of 0.5-1.5% and 1.5-2.5% to have medium and high productivity, respectively. Organic carbon content of more than 2.5% may not be suitable for fIsh production, since it may lead to excessive bloom of microbes and oxygen depletion in the water.

Carbon to nitrogen ratio: The carbon to nitrogen (C: N) ratio of soil influences the activity of soil microbes to a great extent. This in turn affects the rate of release of nutrients from decomposing organic matter. The rate of breakdown (Inineralization) is very fast, moderately fast and slow at C: N ratios in the range of less than 10, 10-20 and more than 20, respectively. In general, soil C : N ratios between 10 and 15 are considered favourable for aquaculture and a ratio of 20 : 1 or narrower gives good results.

General nutrient status: Nitrogen, phosphorus and potassium are the major nutrients Single superphosphate (SSP) is most commonly used as a phosphate fertilizer in fish ponds. Depending on the available phosphate content of pond soil, application of 25-50 kg phosphate (pps)lha (i.e. 156-312 kg SSPlha) and 40--75 kg ppslha/year (i.e. 250--468kg SSPlha) in rearing and stocking ponds, respectively give good results. To get better utilization efficiency, phosphorus fertilizers should be applied in weekly intervals and the first installment should be given 7 days after initial organic manuring.

Muriate of potash (potassium chloride, KCl) and sulphate of potash (potassium sulphate, K2SO4)are commonly used as potassium fertilizers in fish ponds. Application of 10--20kg KPlha (i.e. 16-32 kg KCl/ha or 20--40kg ~S04lha) and 25-40 kg KP/ ha/year (i.e. 41-66 kg KCl/ha or 52-83 kg K2SO4lha/year)in rearing and stockingj>onds, respectively give good results. The fertilizer should be applied in equal monthly splits.

If thick green or blue-green blooms of algae develop in the pond, to avoid depletion of oxygen, application of manure and fertilizer should be suspended.

Careful use of organic manures and chemical fertilizers in combination is a sound strategy. Occasional development of unhygienic conditions in the pond may be avoided by using pre-decomposed organic manure. Use of excessive amounts of raw organic manure can result in excessive blooms of microbes during aerobic breakdown of large amount of raw organic manure, and may also cause oxygen depletion.

(Source:Hand book of Fisheries and Aquaculture. 2006. Indian Council of Agricultural Research. New Delhi).