Freshwater aquaculture (2+1)

Unit 3 - Nursery, Rearing and grow-out ponds

3.2.2. Pond fertilization

• Fry and fingerlings of most fish such as carps feed on zooplankton
• Sustained zooplankton production in ponds depends on good phytoplankton and bacterial base
• This is maintained through adequate availability of nutrients such as Nitrogen, Phosphorous carbon and micronutrients in ponds.
• Natural availability of these nutrients in ponds will be inadequate.
• Hence they need to be added through external sources for sustaining good plankton growth
• Nutrients are added to water through organic manures and inorganic fertilizers.

Organic manures

• Organic manures are are rich in carbon and contain nutrients such as N and P in small amounts.
• They decompose slowly and release the nutrients slowly
• They promote the growth of zooplankton through saprophytic food chain
• They promote sustained growth of phytoplankton and zooplankton for longer periods of time
• Several types of manures such as cow dung, poultry litter, pig dung, horse dung etc., can be used to fertilize fish ponds.
• Most common manures used in fish ponds are cow dung and poultry manure
• Raw cow dung is generally applied at a rate of 5-10 tons/ha 15 days before stocking.
• It can be also applied in phases; 2/3 of the amount as basal dose and a second dose after a week of stocking
• Poultry manure is 2-3 times richer than cow dung in the content of nitrogen and phophorus.
• Hence half the dose of cow dung is used, when poultry manure is applied to the ponds.

Inorganic fertilizers

• These are concentrated forms of nutrients such as N and P
• Urea or ammonium sulphate is used as a source of N while single or triple super phosphate is used as a source of P
• Inorganic fertilizers promote the production of phytoplankton on which zooplankton production depends.
• Their action is very fast and when used in excess quantities promote blooms of undesirable Blue Green Algae (BGA)
• Hence they should be used cautiously in fish ponds
• A combination of organic manure and inorganic fertilizers will promote the growth of phytoplankton quickly which will sustain for a longer period of time because of fertilizing with organic manures.