Aquaculture systems and fish culture
How fish and shrimp are farmed: ponds, cages, raceways and RAS, polyculture, water quality, feeding and FCR, health, sustainability, and a pond sum.
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For most of history, fish came from the wild. Today, farmed fish, shrimp, molluscs and seaweeds supply around half of the aquatic animals people eat, and FAO reported that in 2022 aquaculture overtook capture fisheries in producing aquatic animals for the first time. Most of this comes from Asia, led by China, India, Indonesia, Viet Nam and Bangladesh, and much of it from small and medium farms raising carps, tilapia, catfish and shrimp. Aquaculture converts feed into protein efficiently and can be integrated with crops and livestock, but it also raises questions about feed sources, water pollution, disease and habitat loss.
Aquaculture systems, management and water quality are standard topics in fisheries science, animal science and integrated farming courses.
Systems by intensity
- Extensive: low stocking, no feeding; fish rely on natural food in ponds, reservoirs or rice fields. Low inputs, low yields.
- Semi-intensive: ponds are fertilised with manure or chemical fertiliser to grow plankton, and fish receive supplementary feed such as rice bran and oil cakes; the most common system for carps and tilapia in Asia and Africa.
- Intensive: high stocking densities, complete pelleted feeds and aeration; used for shrimp, catfish, tilapia and salmon.
Systems by structure
- Earthen ponds: the backbone of freshwater aquaculture.
- Cages and pens in lakes, reservoirs and the sea, as for salmon in Norway and Chile and tilapia in many lakes.
- Raceways: flowing water channels for trout.
- Recirculating aquaculture systems (RAS): indoor tanks with filtration and water reuse; high control and low water use but high cost and energy.
- Integrated systems: rice–fish culture, livestock–fish (manure fertilising ponds) and aquaponics (fish waste feeding plants).
- Mariculture of shrimp in coastal ponds, and molluscs and seaweeds on ropes and racks.
Polyculture
Stocking several species with different feeding habits makes full use of a pond's natural food.
The composite fish culture of South Asia combines the Indian major carps catla (surface), rohu (column) and mrigal (bottom) with Chinese silver carp, grass carp and common carp. Polyculture often raises yields well above single-species culture in the same pond.
Water quality
Fish live in their own wastes, so water quality limits production:
- Dissolved oxygen: ideally above about 5 mg/L; it falls at night and before dawn, when plants and algae respire, and fish gasp at the surface when it is too low. Aerators and water exchange help.
- Temperature: warm-water species such as tilapia and carps grow best at about 25 to 32 °C; trout and salmon need cool water.
- pH: about 6.5 to 8.5; liming corrects acidic ponds.
- Ammonia and nitrite from fish wastes and uneaten feed are toxic; un-ionised ammonia rises with pH and temperature.
- Turbidity, alkalinity and hardness affect productivity and buffering.
Feeding and growth
- Natural food (plankton, benthos) is boosted by fertilisation.
- Supplementary and formulated feeds supply protein (fish meal, soybean meal and other plant proteins), energy, vitamins and minerals.
- Feed conversion ratio (FCR) for well-managed tilapia and carp is often about 1.2 to 1.8 kg of feed per kg of fish; farmed fish convert feed more efficiently than most land animals, partly because they are cold-blooded and supported by water.
Health and biosecurity
Major problems include bacterial diseases (Aeromonas, Streptococcus, Vibrio in shrimp), viral diseases (white spot syndrome virus in shrimp, tilapia lake virus), parasites (sea lice in salmon, Argulus), and fungal infections. Prevention through good water quality, quality seed from certified hatcheries, appropriate stocking, quarantine and vaccines (widely used in salmon) reduces the need for antimicrobials.
Sustainability
- Feed: reliance on fish meal and fish oil from wild fisheries is falling as plant, insect and algal ingredients replace them.
- Effluent rich in nutrients can pollute water; integrated and recirculating systems reduce it.
- Habitat: shrimp farming has destroyed mangroves in some regions.
- Escapes and genetics: farmed fish escaping into the wild.
- Certification schemes and better management practices address these issues.
Planning a carp pond
A farmer has a 1 ha pond and stocks 8,000 fingerlings of mixed carps. Expected survival is 80 per cent, and fish are harvested at an average of 0.8 kg after about 10 months.
- Fish harvested = 8,000 × 0.80 = 6,400.
- Production = 6,400 × 0.8 = 5,120 kg, about 5.1 t/ha.
If about half the growth comes from natural food in a fertilised pond and the rest from supplementary feed with an FCR of 2.0:
- Feed needed ≈ 0.5 × 5,120 × 2.0 ≈ 5,120 kg of supplementary feed.
At 0.40 dollars per kg of feed and 1.80 dollars per kg of fish, feed costs 2,048 dollars against fish sales of 9,216 dollars, before fingerlings, fertiliser, labour and harvesting costs.
Key points
- Aquaculture now supplies about half of the aquatic animals eaten; most is in Asia.
- Systems range from extensive to semi-intensive and intensive; structures include ponds, cages, raceways, RAS and integrated systems.
- Polyculture of carps uses different feeding niches (catla, rohu, mrigal, silver, grass and common carps).
- Water quality: dissolved oxygen above about 5 mg/L, suitable temperature, pH 6.5–8.5, low ammonia and nitrite.
- FCR of well-managed fish is often about 1.2–1.8; good water, seed and stocking prevent disease.
- Sustainability concerns: feed sources, effluent, habitats, escapes and antimicrobials.
What the exams ask
- School-leaving and entrance papers: types of fish culture, cultivable species and composite fish culture.
- Degree and research-fellowship entrances: pond management, fertilisation and feeding, water quality parameters, stocking, polyculture, and fish diseases.
- Professional examinations: aquaculture economics, biosecurity, RAS and integrated systems, and sustainability certification.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Why do pond fish die at dawn?
Overnight, algae and fish use oxygen without photosynthesis replacing it, so dissolved oxygen reaches its lowest point at dawn. Overcrowded or over-fertilised ponds are most at risk.
Is farmed fish as nutritious as wild fish?
Farmed fish provide similar protein. Omega-3 content depends on feed: as fish oil is replaced with plant oils, levels can fall unless algal or other omega-3 sources are added.
Can fish be raised with rice?
Yes. Rice–fish systems are practised in China, South-East Asia and elsewhere; fish eat weeds and pests and add nutrients, and farmers get two products from one field.
Sources
- FAO. The State of World Fisheries and Aquaculture 2024: Blue Transformation in Action. FAO, Rome, 2024.
- Boyd, C. E. and Tucker, C. S. Pond Aquaculture Water Quality Management. Kluwer Academic Publishers, 1998.
- Lucas, J. S., Southgate, P. C. and Tucker, C. S. (eds) Aquaculture: Farming Aquatic Animals and Plants, 3rd edition. Wiley-Blackwell, 2019.
- Naylor, R. L. et al. "A 20-year retrospective review of global aquaculture." Nature 591, 551–563 (2021).
- Jhingran, V. G. Fish and Fisheries of India, 3rd edition. Hindustan Publishing, 1991.
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