Twelve Thousand Fingerlings and Twelve Hundred Hens on 0.8 Hectares
A farmer near Barasat has a 0.8 ha pond with an average depth of 1.4 m. Heavy beds of Hydrilla and water hyacinth grow along the margins and in the shallows. He wants to move from single-species rohu culture to composite fish culture and has ₹60,000 available for seed. His plan is to stock 9,600 fingerlings, that is 12,000 per hectare, in the ratio silver carp 45%, catla 25%, rohu 20% and mrigal 10%, with no grass carp. He also wants to build a poultry shed for 1,200 layers directly over the pond and to stop all supplementary feeding to save money, relying only on the bird droppings. The pond is already green, there is no aerator, and the afternoon pH is 8.2 with total ammonia nitrogen 0.4 mg/L. He asks you to approve the plan so he can place the seed order this week.
1. Assess the proposed species ratio and the omission of grass carp in the light of the pond's condition.
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The ratio is wrong for this pond on three counts.
Silver carp at 45% is far too high. Silver carp is a plankton filter feeder; at that share it competes directly with catla (also a surface plankton feeder) and with rohu juveniles, and it commands a lower market price in West Bengal. In a six-species model, silver carp is typically 15–20% and catla 25–30%. As proposed, nearly three-quarters of the stock is surface/plankton dependent, and the column and bottom niches are barely used: mrigal at just 10% and no common carp leaves the bottom layer almost vacant.
No grass carp is a serious omission. The pond has dense Hydrilla and water hyacinth. Grass carp is the cheapest weed-control tool available: stocked at roughly 500–1,000 per hectare, it grazes submerged weeds, converts them to fish flesh, and reduces the decaying vegetation that consumes oxygen. Water hyacinth is tougher and should be removed manually from the margins rather than left to the fish.
A more workable six-species ratio for this pond would be catla 30%, rohu 25%, mrigal 15%, silver carp 15%, grass carp 10% and common carp 5%. That uses the surface, column and bottom, delivers weed control, and spreads market risk across species that sell at different sizes and times.
2. Calculate what the 1,200 birds would mean for the pond. Is that stocking appropriate, and what would you recommend instead?
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Manure load. A layer produces roughly 45–55 g of wet droppings per day, so 1,200 birds deposit about 55–65 kg of wet manure daily. The pond volume is 8,000 m² × 1.4 m ≈ 11,200 m³, so the daily load is about 5–6 g/m³ of wet manure - a heavy continuous organic input into an unaerated pond with no sludge removal history.
Nitrogen. Fresh poultry manure is roughly 1.5–2% nitrogen, so 60 kg of wet manure delivers about 0.9–1.2 kg N per day. Spread over 11,200 m³ that is about 0.08–0.1 mg/L of N per day, or 20–30 mg/L over a 250-day cycle if nothing were assimilated. Even allowing for fish harvest removing some and much being lost to denitrification, this is far beyond what the pond can process. The result would be a permanent dense bloom, night-time oxygen crashes, rising un-ionised ammonia at pH 8.2, and a bottom turning black and anaerobic - the classic pathway to a disease outbreak.
Recommendation. Limit the flock to about 400–500 birds per hectare, that is roughly 320–400 birds for this 0.8 ha pond, and place the shed over the pond only on the bund side with the droppings entering at one point, not over the whole surface. Even then, do not stop feeding: the droppings supplement the pond's natural food but do not substitute for a balanced supplementary ration. Reduce pellet feeding by about 30–50% relative to a pond without birds, monitor dawn DO, and keep a night aerator for August–September.
3. The farmer has ₹60,000 for seed at 12,000/ha. What practical stocking plan, seed budget and yield expectation would you give him, and why is his 'no feeding' assumption unsafe?
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Stocking. Keep the total near 9,000–10,000 per hectare rather than 12,000, since there is no aeration and the pond is shallow at 1.4 m. For 0.8 ha, that means about 7,500–8,000 fingerlings:
- catla 30% ≈ 2,300
- rohu 25% ≈ 1,900
- mrigal 15% ≈ 1,150
- silver carp 15% ≈ 1,150
- grass carp 10% ≈ 770
- common carp 5% ≈ 380 Use fingerlings of 8–10 cm, all from a single certified hatchery so that the seed arrives together and there is no size hierarchy.
Budget. At ₹6–8 per fingerling, 8,000 fingerlings cost ₹48,000–64,000, so the ₹60,000 is only just enough for seed and leaves nothing for the poultry shed, aeration or feed. He should either reduce numbers to about 6,500–7,000 and use the balance for lime and feed, or phase the poultry shed to the second year.
Why 'no feeding' fails. The bird droppings supply nitrogen and phosphorus that stimulate plankton, but they are not a balanced feed: grass carp needs plant material, rohu needs a protein supply, and a plankton bloom alone cannot carry 10,000 fish/ha to market size. Expected yield from manure alone in an unaerated 0.8 ha pond would be roughly 1.5–2 t; with a supplementary pellet ration at 2–3% body weight and weed for grass carp, 3–4 t is realistic. The extra feed costs perhaps ₹40,000–60,000 and returns far more than that in fish weight, so cutting feed is the most expensive saving he could make. Calculate the whole cycle before ordering seed.