Aquaculture systems

In short

An aquaculture system is the physical and management arrangement in which aquatic organisms are grown — the water body (pond, tank, cage, raceway), the degree of control over water and feed, and the species mix. Systems are classified by intensity (extensive to super-intensive), by water use (flow-through, static, recirculating), by species combination (mono-, poly-, integrated culture) and by location (inland freshwater, brackishwater, marine). India is the second largest producer of farmed fish in the world, and roughly three-quarters of its ~18 million tonnes of total fish production comes from aquaculture systems covered here (the rest being marine and inland capture), driven by carp polyculture and brackishwater shrimp. The topic is central to B.F.Sc aquaculture papers, ICAR-ASRB NET/JRF/SRF, ARS, state fisheries officer exams, and is the first thing asked in interviews at hatcheries, feed companies (Avanti, Growel, CP India, Godrej Agrovet), NFDB/PMMSY-funded projects and FAO/World Bank aquaculture programmes.

Explained

Classification of aquaculture systems

• By intensity — governed by stocking density, feed and water inputs:
  • Extensive: natural productivity only, little or no feed; stocking 2,000–5,000 fingerlings/ha; yield 0.5–1.5 t/ha/yr. Example: unfertilised village ponds, traditional *bheries* of West Bengal, *pokkali* and *khazan* fields.
  • Semi-intensive: fertilisation plus supplementary feed, partial water exchange; 5,000–10,000 fingerlings/ha; yield 3–8 t/ha/yr. The dominant Indian carp system.
  • Intensive: complete formulated feed, aeration, high exchange; yield 10–50 t/ha/yr. Example: *Pangasius* and tilapia ponds in Andhra Pradesh, intensive *Litopenaeus vannamei* ponds.
  • Super-intensive: closed systems (RAS, biofloc) with 20–100+ kg/m³; yield measured per m³, not per hectare.
• By water exchange: static (no exchange, only top-up), flow-through/raceway (continuous one-pass water), recirculating (water treated and reused), zero-exchange (biofloc).
• By species composition: monoculture (one species), polyculture (species of different feeding niches), monosex culture (all-male tilapia, all-male *Macrobrachium*), integrated culture (fish plus crop/livestock).
• By salinity: freshwater (<0.5 ppt), brackishwater (0.5–30 ppt), mariculture (>30 ppt).
• By rearing stage: hatchery (spawn to fry), nursery (fry to fingerling), rearing, grow-out, broodstock holding.

Pond systems and composite fish culture

• Pond types: nursery (0.02–0.05 ha, 1.0–1.5 m deep), rearing (0.05–0.1 ha, 1.5–2.0 m), stocking/grow-out (0.4–1.0 ha, 2.0–3.0 m), broodstock, and bundh-type ponds (dry and wet bundhs) used for induced carp breeding in Madhya Pradesh/West Bengal.
• Pond preparation sequence: de-watering and desilting → removal of weeds (manual, mechanical or biological using grass carp) → eradication of predatory and weed fish (mahua oil cake 2,500 ppm, or repeated netting) → liming (200–500 kg/ha on normal soil, more on acidic) → basal manuring (raw cattle dung 10,000 kg/ha/yr, or poultry manure ~5,000 kg/ha/yr) → inorganic fertiliser (urea and single superphosphate in split doses) → plankton check → stocking of fingerlings (50–100 mm) after acclimatisation.
• Composite fish culture (ICAR-CIFA package): six species occupying different niches so that food and space in the whole water column are used — Catla (surface zooplankton feeder), rohu (column, omnivore), mrigal and common carp (bottom detritus/benthos), silver carp (surface phytoplankton), grass carp (macrovegetation). Typical ratio 3 surface : 3 column : 4 bottom, 5,000 fingerlings/ha, yield 4–6 t/ha/yr without aeration, 8–10 t/ha/yr with aeration and quality feed.
• Management: supplementary feed of rice bran and oil cake 1:1 at 3–5% of body weight tapering to 1–2%; monthly sampling for growth; liming at 2-monthly intervals; multiple stocking and multiple harvesting to keep biomass below carrying capacity.
• Advantages: uses natural productivity, low capital cost, suits small holdings, spreads market risk across species. Limitations: land-and-water intensive, yields capped by natural food and dissolved oxygen, vulnerable to weather and seepage.

Cage, pen and running-water systems

• Cage culture: fish held in net enclosures suspended in an open water body (reservoir, lake, estuary, sea). Components — frame (GI/HDPE), floats, inner grow-out net, outer predator net, bird net, moorings. Types: fixed, floating, submersible, submerged.
  • Freshwater reservoir cages (6 m × 6 m × 4 m, or 4 m diameter GI) widely used in Jharkhand, Chhattisgarh, Maharashtra and Telangana reservoirs for *pangas*, tilapia and minor carps; typical yield 20–40 kg/m³/crop.
  • Marine cages developed by ICAR-CMFRI (6–15 m diameter HDPE) for Asian seabass (*Lates calcarifer*), cobia, silver pompano and lobster fattening.
  • Advantages: no land needed, easy harvest and observation, high stocking. Limitations: dependence on complete feed, risk of net biofouling and damage, escapes, disease spread, local nutrient loading, conflicts over water use; reservoir cage area is usually capped (commonly about 1% of water spread area as per state guidelines — check the current state order).
• Pen culture: an area of a shallow water body or wetland enclosed with net on all sides but using the natural bottom; used in Chilika, Kolleru and reservoir margins for raising advanced fingerlings before reservoir ranching.
• Raceway / running-water culture: long narrow concrete or earthen channels with continuous gravity flow; exchange 10–30 times per hour. Standard for rainbow trout in Jammu & Kashmir, Himachal Pradesh, Uttarakhand, Sikkim and Arunachal Pradesh (ICAR-DCFR package), at 20–30 kg/m³.
• Ranching/culture-based fisheries: stocking advanced fingerlings into reservoirs and open waters with no feeding; yields in Indian reservoirs rise from under 50 kg/ha/yr to 100–500 kg/ha/yr with regular stocking.

Closed and modern systems: RAS, biofloc, aquaponics, IMTA

• Recirculating aquaculture system (RAS): water is reused after treatment. Loop order — culture tank → mechanical filter (drum/screen filter, settling cone) → biofilter (moving bed or trickling filter: *Nitrosomonas* oxidises NH₄⁺ to NO₂⁻, *Nitrobacter* NO₂⁻ to NO₃⁻) → degasser/CO₂ stripper → oxygenation (oxygen cone or diffuser) → UV or ozone disinfection → back to tank. Daily make-up water only 5–10% of system volume.
  • Advantages: year-round control of temperature and quality, biosecurity, 90–99% water saving, siting near markets, effluent containment. Limitations: high capital (roughly ₹30–60 lakh for a small 10–20 t/yr unit, varies widely), high electricity dependence, total failure within hours if power or oxygen stops, and need for trained operators. PMMSY provides unit-cost-based subsidy for RAS and biofloc — current unit costs and subsidy rates are revised periodically.
• Biofloc technology (BFT): a near-zero-exchange tank system where added carbon (molasses, jaggery, wheat flour) at a C:N ratio of about 12–20:1 makes heterotrophic bacteria convert ammonia into microbial protein flocs which the fish re-eat. Needs continuous aeration, TSS/floc volume control (settleable solids 5–15 mL/L in an Imhoff cone), alkalinity maintenance. Suits air-breathing and tolerant species — *Clarias magur*, *Pangasianodon*, tilapia, *Anabas*, *Litopenaeus vannamei*; not suited to carps at very high density.
• Aquaponics: fish tanks coupled to hydroponic plant beds (media bed, NFT or deep-water culture); fish nitrate feeds the plants, plants strip nutrients. Fish-to-plant and feed-to-plant-area ratios decide performance.
• Integrated multi-trophic aquaculture (IMTA): finfish co-cultured with filter feeders (mussel, oyster) and extractive seaweed (*Kappaphycus*, *Gracilaria*) so that wastes of one become inputs of another; practised experimentally along Tamil Nadu and Gujarat coasts.
• Other: cold-water flow-through for trout and mahseer; sewage-fed culture in East Kolkata Wetlands; periphyton-based culture using bamboo/substrate; Aquamimicry for shrimp.

Integrated farming and other traditional Indian systems

• Fish-cum-paddy: rice field with trenches/refuges, about 8–10% of field area, stocked with carps or *Anabas*; practised in Kerala (*pokkali*/*kaipad*), Odisha, north-eastern states (Apatani system of Arunachal Pradesh).
• Fish-cum-duck: 200–300 ducks/ha; droppings manure the pond, ducks eat tadpoles and predatory insects. Fish-cum-poultry: 500–600 birds/ha. Fish-cum-pig: 30–40 pigs/ha. In all cases on-farm manure replaces most fertiliser.
• Fish-cum-horticulture: dykes used for vegetables, banana, coconut, fodder.
• Brackishwater traditional systems: *bheri* (West Bengal), *gheri* (Odisha), *khazan* (Goa), *pokkali* (Kerala) — tide-fed trapping-and-holding systems, extensive, mixed shrimp-and-finfish.
• Advantages of integration: nutrient recycling, two or more incomes per unit area, lower feed and fertiliser cost. Limitations: management complexity, hygiene and antibiotic-residue concerns with livestock manure, and export-market restrictions on manure-fed produce.

Site selection, design and carrying capacity

• Site criteria: soil with 15–30% clay and adequate water retention; soil pH 6.5–7.5; assured water of 25–32 °C (warm water) or 10–18 °C (trout); no flooding; road and market access; no pollution source upstream; land not classified as agricultural prime or ecologically sensitive.
• Pond design: inlet and outlet at opposite ends, screened; dyke side slope 2:1 (soil-dependent), freeboard 0.5 m, top width 1.5–3 m; central or peripheral drainage; nursery and quarantine ponds separate; effluent treatment/settling pond for intensive units.
• Carrying capacity is the biomass a system can hold with dissolved oxygen, ammonia and feed supply kept safe; exceeded, growth slows and mortality rises regardless of feed. Managed through aeration, exchange, partial harvest and biofloc/biofilter capacity.
• Aeration: paddle-wheel (2 hp aerates about 1,000 kg standing crop as a rule of thumb; verify against SOTR), aspirator, diffused air, Venturi; placed to create a circular current gathering sludge at the centre.
• Regulation and standards in India:
  • Coastal Aquaculture Authority Act, 2005, as amended by the Coastal Aquaculture Authority (Amendment) Act, 2023, with the 2025 rules/regulations framed under it — mandatory registration of coastal aquaculture farms, hatcheries, broodstock multiplication centres, nucleus breeding centres and feed mills; no farm within the no-development zone of CRZ; ban on antibiotics and pharmacologically active substances listed in the Schedule.
  • CRZ Notification, 2019 governs coastal siting; EIA Notification, 2006 applies to larger projects.
  • Wildlife (Protection) Act, 1972 and Environment (Protection) Act, 1986 restrict siting in protected areas and discharge quality; state pollution control board consent for effluent.
  • MPEDA handles export registration, pre-harvest testing and traceability; NaCSA organises farmer societies; NFDB implements PMMSY (launched 2020) which funds RAS, biofloc, cages, hatcheries, ponds and the Aquaculture Extension Service.
  • Exotic species: introduction and farming of exotics (e.g. *L. vannamei*, tilapia, *Pangasianodon*) need clearance; state rules on tilapia and *Pangasianodon* culture differ, so check the current state notification.
  • FAO Code of Conduct for Responsible Fisheries (Article 9), Better Management Practices (BMPs), and certification schemes (BAP, ASC, GlobalG.A.P., India Organic Aquaculture) guide practice.

Key points

• Extensive system — no feed, relies on natural food, 0.5–1.5 t/ha/yr.
• Semi-intensive — manure plus supplementary feed, 3–8 t/ha/yr; the mainstay of Indian carp culture.
• Intensive / super-intensive — complete feed plus aeration or water treatment; output per m³ matters more than per hectare.
• Composite fish culture — six-species carp polyculture in 3:3:4 surface:column:bottom ratio; raises yield by using all niches.
• Monoculture — single species, simpler feeding and harvest, but leaves niches and natural food unused.
• Cage culture — net enclosures in open water; no land, high density, but depends entirely on formulated feed.
• Pen culture — net enclosure using the natural bottom, used mainly for raising fingerlings in wetlands and reservoirs.
• Raceway — flow-through channel with 10–30 exchanges/hour; standard for trout in Indian hills.
• RAS — water treated and reused; mechanical filter, biofilter, degasser, oxygenation, UV; 5–10% daily make-up.
• Nitrification — *Nitrosomonas* then *Nitrobacter* convert ammonia → nitrite → nitrate in the biofilter; nitrate is the least toxic.
• Biofloc — carbon added to reach C:N ≈ 12–20:1 so heterotrophic bacteria turn ammonia into edible microbial protein.
• Aquaponics and IMTA — waste nutrients recovered by plants, or by filter feeders and seaweed, respectively.
• Integrated farming — fish with paddy, duck, poultry or pig; manure replaces fertiliser and gives a second income.
• Carrying capacity — maximum standing crop a system supports; oxygen and ammonia, not feed, set the ceiling.
• Stocking density governs growth rate and FCR; higher density needs more aeration and exchange per unit output.
• Multiple stocking and multiple harvesting keeps biomass under carrying capacity and gives repeated cash flow.
• Coastal Aquaculture Authority Act, 2005 (amended 2023) — compulsory registration of coastal farms and hatcheries; antibiotic ban.
• PMMSY through NFDB — the main funding route for RAS, biofloc, cages and hatcheries since 2020.

Formulas and examples

Example 1 — Composite carp culture yield and FCR. A 1.5 ha pond is stocked at 5,000 fingerlings/ha of 25 g each. After 10 months, 80% survive at a mean weight of 1.1 kg. Feed used: 9,500 kg.

• Number stocked = 5,000 × 1.5 = 7,500; survivors = 0.80 × 7,500 = 6,000.
• Harvest = 6,000 × 1.1 = 6,600 kg → 4,400 kg/ha.
• Stocked weight = 7,500 × 0.025 = 187.5 kg; weight of survivors at stocking = 6,000 × 0.025 = 150 kg.
• Gain of survivors = 6,600 − 150 = 6,450 kg → FCR = 9,500/6,450 = 1.47.
• Net yield = (6,600 − 187.5)/1.5 = 4,275 kg/ha over 10 months.

Example 2 — Cage culture economics and density. A floating cage of 6 m × 6 m × 4 m water depth is stocked with 9,000 pangas fingerlings; 85% survive at 900 g in 8 months. Feed 8,200 kg at ₹48/kg; fish sell at ₹110/kg.

• Cage volume = 6 × 6 × 4 = 144 m³; stocking density = 9,000/144 = 62.5 fish/m³.
• Harvest = 0.85 × 9,000 × 0.900 = 6,885 kg → 47.8 kg/m³.
• Feed cost = 8,200 × 48 = ₹3,93,600; revenue = 6,885 × 110 = ₹7,57,350.
• Gross margin over feed = ₹7,57,350 − ₹3,93,600 = ₹3,63,750 per cage per crop (before cage amortisation, seed, labour and mortality insurance).
• FCR on gain: stocked weight at ~10 g = 90 kg; gain of survivors = 6,885 − 76.5 = 6,808.5 kg → FCR = 8,200/6,808.5 = 1.20.

Example 3 — SGR and daily feed. Tilapia grow from 40 g to 320 g in 120 days; the tank holds 2,400 fish and is fed at 3% body weight.

• SGR = [(ln 320 − ln 40)/120] × 100 = [(5.768 − 3.689)/120] × 100 = (2.079/120) × 100 = 1.73% per day.
• Biomass at 320 g (assume all alive) = 2,400 × 0.320 = 768 kg; daily feed = 768 × 3/100 = 23.0 kg/day.
• At an average 180 g over the cycle, mean biomass = 432 kg → mean daily feed ≈ 13 kg; over 120 days ≈ 1,555 kg, giving an expected FCR of 1,555/(768 − 96) = 2.31 at that feeding rate, i.e. 3% is too high at the later stage — tapering to 1.5–2% is needed.

Example 4 — RAS water saving and biofilter ammonia load. A RAS of 120 m³ holds 4,800 kg of fish (40 kg/m³), fed 1.8% of body weight with 32% protein feed; 5% make-up water per day.

• Daily feed = 4,800 × 0.018 = 86.4 kg.
• N fed = 86.4 × 0.32 × 0.16 = 4.42 kg N; assume 30% excreted as TAN → 1.33 kg TAN/day, so the biofilter must nitrify about 1.33 kg/day (≈55 g/h).
• Make-up water = 0.05 × 120 = 6 m³/day, i.e. 180 m³/month, against roughly 6,000 m³/month for a pond system of similar output with 5% daily exchange — about 97% saving.

Example 5 — Biofloc carbon dose. 60 kg of 30% protein feed is fed per day in a biofloc tank; 50% of feed N is excreted; molasses has 40% carbon; target C:N = 15:1.

• N in feed = 60 × 0.30 × 0.16 = 2.88 kg; N excreted = 1.44 kg.
• Carbon needed = 1.44 × 15 = 21.6 kg → molasses = 21.6/0.40 = 54 kg/day (about 0.9 kg molasses per kg feed — consistent with the usual field thumb-rule of 0.5–1.0 kg/kg feed).

Where it is used

• Carp and pangas farming belts: Kolleru and Krishna/West Godavari (Andhra Pradesh), Punjab, Haryana, Bihar, West Bengal and Odisha — semi-intensive pond systems supplying most of India's inland fish; roles: farm supervisor, aqua-technician, pond manager.
• Reservoir cage and pen projects: state fisheries departments, NFDB/PMMSY-funded cage clusters in Jharkhand, Chhattisgarh, Maharashtra, Telangana, Madhya Pradesh — roles: cage operations officer, extension officer, culture-based-fisheries manager.
• Mariculture: ICAR-CMFRI open-sea cages for seabass, cobia and pompano; mussel and oyster farming in Kerala and Karnataka estuaries; seaweed farming in Tamil Nadu (Kappaphycus) — roles: mariculture technician, research assistant.
• Hatcheries and seed production: carp hatcheries using Chinese circular hatcheries, *magur*/*singhi* hatcheries, scampi and shrimp hatcheries, trout farms in J&K and Himachal — roles: hatchery manager, broodstock technician.
• RAS, biofloc and aquaponics ventures: urban/peri-urban start-ups, ornamental and trout RAS units, PMMSY-subsidised biofloc tanks — roles: system designer, water-quality officer, service engineer.
• Feed, input and certification companies and institutions: Avanti, Growel, Godrej Agrovet, CP India, Waterbase (technical sales, farm advisory); ICAR-CIFA, CIFRI, CIBA, CMFRI, DCFR, CIFE; MPEDA/NaCSA and NFDB for registration, BMPs and project appraisal.

Interview questions

• Q: Differentiate extensive, semi-intensive and intensive aquaculture. A: They differ in stocking density, feed and water inputs — extensive uses only natural food (0.5–1.5 t/ha/yr), semi-intensive adds manure and supplementary feed (3–8 t/ha/yr), and intensive uses complete feed with aeration and exchange (10 t/ha/yr and above).
• Q: Why is composite fish culture more productive than monoculture of rohu? A: The six species feed at different levels and on different food — surface plankton, column, bottom detritus and macrophytes — so the whole water column and all available natural food are used instead of one niche.
• Q: What is the basic species ratio in composite carp culture? A: Roughly 3 surface feeders : 3 column feeders : 4 bottom feeders, adjusted to local seed availability and the pond's plankton and benthos.
• Q: Name the treatment units of a RAS in order. A: Culture tank → mechanical/solids filter → biofilter for nitrification → CO₂ stripper/degasser → oxygenation → UV or ozone disinfection → back to the tank.
• Q: Why is a C:N ratio of about 12–20:1 maintained in biofloc? A: At that ratio heterotrophic bacteria out-compete nitrifiers and immobilise ammonia into microbial protein flocs, which the fish eat — controlling ammonia without water exchange and recycling feed nitrogen.
• Q: What limits production in a well-fed pond? A: Carrying capacity — dissolved oxygen and the build-up of ammonia and metabolites, not the quantity of feed; beyond it growth slows and mortality rises.
• Q: Cage culture gives high production per cubic metre, so why is it not used everywhere? A: It needs a suitable open water body with good depth and flow, total dependence on costly complete feed, and it loads the surrounding water with nutrients, so area is usually restricted and leases and clearances are needed.
• Q: Which system would you recommend for *magur* (*Clarias magur*) in a small plot with limited water? A: A biofloc or low-exchange tank system, because *magur* is air-breathing, tolerant of high ammonia and crowding, and fetches a high price — a plain earthen pond would waste its tolerance advantage.
• Q: What is the difference between cage culture and pen culture? A: A cage is enclosed on all sides including the bottom and hangs in the water body; a pen is enclosed only on the sides and uses the natural bottom of the water body as its floor.
• Q: Which Act regulates brackishwater shrimp farms in India and what does it require? A: The Coastal Aquaculture Authority Act, 2005, as amended in 2023 — farms, hatcheries and related units must be registered with the Coastal Aquaculture Authority, cannot occupy the no-development zone, and must not use banned antibiotics.
• Q: Why is mahua oil cake applied to a nursery pond? A: At about 2,500 ppm it acts as a piscicide to kill predatory and weed fish; it then breaks down and acts as a manure, so the pond can be stocked a few days later after a bioassay check.
• Q: What is the single biggest operational risk in RAS and how is it managed? A: Loss of power or oxygen supply, which can kill the entire stock within an hour — managed by standby generators, backup oxygen cylinders with automatic switchover, low-oxygen alarms and remote monitoring.

Common mistakes

• Quoting yields without the unit basis — t/ha/yr for ponds, kg/m³/crop for cages and tanks are not comparable.
• Calculating FCR against total harvested weight instead of weight gain, which understates FCR.
• Treating biofloc as a system needing no aeration or no feed — it needs continuous aeration and a quality feed; stopping aeration for even a short time collapses the floc and kills the stock.
• Confusing a biofilter (autotrophic nitrification, nitrate end-product) with biofloc (heterotrophic assimilation into microbial protein).
• Forgetting the clay content, water assurance and legal siting checks (CRZ, CAA registration) in a site-selection answer.
• Assuming higher stocking density automatically means higher profit — it raises aeration, feed and disease cost, and breaches carrying capacity.
• Mixing up cage, pen and raceway definitions, or calling reservoir ranching a "culture system" when it involves no feeding or enclosure.

Related topics

Aquatic environment and water quality · Fish nutrition and feed · Shrimp farming · Fish diseases and health management · Fish biology · Fisheries extension, economics and project formulation