1.Abstract

As one of the world’s most economically valuable cold-water salmonid species, rainbow trout have extremely stringent physiological requirements regarding water temperature (optimum 12-16 ℃), dissolved oxygen levels and water purity. Traditional raceway aquaculture relies heavily on high-quality alpine cold springs or groundwater; not only is it constrained by increasingly stringent environmental regulations on the direct discharge of effluent, but it also frequently faces the risk of large-scale yield reductions and feeding suspensions due to water source depletion and sudden rises in water temperature during the summer.

The new generation of industrial-grade, land-based, closed-loop recirculating aquaculture systems (Trout RAS) has been specifically developed for cold-water fish with high oxygen demands. With ‘year-round cold-water temperature control, natural-like circulating flow patterns, high-pressure pure oxygen closed-loop systems and highly efficient biochemical denitrification’ at the core of its technology, this system has completely overcome the geographical and water source constraints of rainbow trout farming. The system provides a highly reliable, industrial-standard process for high-quality triploid rainbow trout, significantly improving survival rates and feed conversion efficiency.

Table 1: Performance of land-based closed-loop aquaculture systems

2.Why choose RAS? The strategic value of land-based rainbow trout farming

Traditional cage farming and flow-through aquaculture have long dominated rainbow trout production, but mounting environmental pressures — including water scarcity, the spread of disease to wild populations and increasingly stringent regulations on effluent discharge — are driving the industry towards a transition to closed-loop recirculating aquaculture systems (RAS). RAS systems achieve a water recycling rate of over 90 per cent through mechanical and biological filtration, significantly reducing freshwater consumption and completely eliminating the risk of farmed fish escaping into natural water bodies.

Rainbow trout (Oncorhynchus mykiss) are typical cold-water fish with extremely stringent water quality requirements: water temperature must be maintained at 12-18 °C, dissolved oxygen levels must not fall below 6 mg/L, and ammonia-nitrogen concentrations must be strictly controlled. The key advantage of RAS systems lies in their ability to provide stable and precise environmental control throughout the year, regardless of geographical location or climatic conditions. This means that producers in tropical and subtropical regions can also farm high-quality rainbow trout locally, significantly reducing the distance over which cold-chain logistics are required and providing consumers with fresher produce.

3.Analysis of the Industry Context and Key Challenges

(1). Depletion of natural cold water resources and environmental red lines: Traditional raceway systems consume tens of thousands of tonnes of water for every tonne of rainbow trout produced. As ecological and environmental red lines become increasingly stringent, the direct discharge of effluent with high nitrogen and phosphorus levels is subject to strict controls, and the extensive farming model reliant on cold springs and reservoir outflow is facing complete phase-out.

(2). Frequent ‘life-or-death’ crises during summer heatwaves: Rainbow trout are extremely sensitive to water temperature; a drop in water temperature to below 20 °C results in severe feeding decline, whilst temperatures exceeding 24 °C lead to mass mortality. In recent years, extreme heatwaves have become increasingly frequent, and traditional open-pond farming often faces the risk of total loss.

(3). Health risks from external water sources: Open natural water sources are highly susceptible to introducing Infectious Pancreatic Necrosis Virus (IPNV) and brain flukes (which cause rotavirus disease). Once an outbreak occurs, not only does the survival rate plummet, but it also severely impacts product exports and the supply chain for high-standard fish feed.

4. Description of the closed-loop architecture for core processes

The rainbow trout’s extremely high oxygen demand and high excretion rates require the system to have a robust supply of dissolved oxygen and strong physical waste removal capacity:

Full-process process architecture for rainbow trout closed-loop recirculating aquaculture systems (RAS):

STEP 01-Natural-style high-flow-velocity rearing tanks

Taking into account rainbow trout’s preference for flowing water, a circular or circular-with-chamfered-corners tank design is employed. Through precise hydrodynamic regulation of the inflow, a cruising flow velocity of 0.5-1.0 times the body length per second is established, prompting the fish to swim continuously against the current. This not only ensures firm flesh but also rapidly expels faeces into the central discharge outlet via centrifugal force.

STEP 02-Dual Physical Interception of Solids and Liquids

Concentrated effluent from the pond bottom flows by gravity into a micro-vortex separator, where large particulates settle, before being channelled into a precision drum microfilter with a pore size of 40-60 μm. Organic faecal matter is rapidly intercepted and flushed out of the system before it can break down, dissolve and release ammonia nitrogen.

STEP 03-Precision-Loaded Biological Reaction Tower

The specific surface area of the suspended packing is precisely matched to the high protein metabolism rate of rainbow trout. Driven by ample aeration, the nitrifying bacterial community maintains total ammonia nitrogen (TAN), a highly toxic substance, at levels below 0.2 mg/L over the long term.

STEP 04-Counter-current Degassing and Ozone Decolourisation and Deodorisation

A high-performance degassing tower rapidly strips away free CO₂ exhaled by dense fish populations, preventing water acidification; the simultaneous injection of trace amounts of ozone oxidises dissolved organic matter, ensuring crystal-clear water and eliminating the earthy odour from the fish flesh.

STEP 05-High-Efficiency Closed-Loop Pressurised Pure Oxygen Dissolution

To meet rainbow trout’s extremely high oxygen demand, the system utilises a micro-pressurised pure oxygen cone to vigorously shear-mix pure oxygen with the water, achieving an oxygen dissolution utilisation rate exceeding 92 per cent. Even at a high stocking density of 80 kg/m³, the dissolved oxygen level in the effluent remains stable at 8.5 mg/L or above.

STEP 06-Precision Cooling, Constant Temperature and Gravity Recirculation

Combined with high-efficiency industrial chillers or ground-source heat pumps, the system precisely maintains the optimal growth water temperature of 14-16 °C. The main water treatment process utilises a stepped, gravity-fed layout, reducing pump head requirements and enabling low-energy-consumption, pure, flowing water recirculation.

5. Hydrodynamic optimisation, energy-saving design and actual water quality parameters

(1). Creation of a Circulating Flow Field and Meat Texture Shaping

This approach does away with aquaculture methods relying on still water or weak currents. The ‘treadmill’-style, bottom-hugging vortex flow, created through hydrodynamic optimisation, not only resolves the issue of faecal sedimentation but also compels rainbow trout to maintain healthy swimming activity. This significantly reduces the excessive accumulation of intramuscular fat, resulting in a firm meat texture rivalling that of wild salmon.

(2). Scientific Piping Network Layout and Extreme Energy Efficiency

The layout of the fluid piping network within the facility is highly compact. Through rigorous calculations of frictional resistance along the flow path and a stepped, gravity-fed layout, the head required by the high-flow recirculation pumps has been minimised, resulting in a significant reduction in the facility’s overall operational electricity consumption.

Table 2: Comparison of key performance indicators between traditional flow-through aquaculture and the new land-based recirculating aquaculture system (RAS)

Table 3: Measured water quality data for the system’s core water treatment units

Economic Benefit Analysis: Taking a standard 1,000 m³ commercial rainbow trout production facility as an example, thanks to a significant reduction in the feed conversion ratio and off-season, peak-season-avoiding marketing (commanding a premium of 20% or more), gross profit for a single year is robust, and the project’s overall static return on investment (ROI) is approximately 2.5-3.5 years.

6. Target Scenarios for Key Industries

(1). Green transformation of idle industrial and mining facilities in inland areas: Deployment can be achieved using idle factory buildings on the outskirts of cities or in industrial estates, without the need to rely on high-altitude cold springs. Combined with the dechlorination and purification of tap water or deep well water, this enables an efficient business model for the ‘direct supply of fresh sashimi to urban suburbs’.

(2). Utilisation of saline-alkali wasteland and slightly saline water: Rainbow trout possess excellent salinity tolerance; combined with desalination of slightly saline groundwater and the addition of trace elements, large-scale, high-quality aquaculture can be achieved on vast areas of saline-alkali wasteland inland.

(3). Relay Rearing of Large-Size Triploid Rainbow Trout/Salmon: In conjunction with the deep-sea cage farming industry, comprehensive immunisation and the centralised pre-growing of large-size fry (500g-1,000 g) are carried out in land-based facilities, significantly shortening the offshore farming cycle and reducing the risk of disease.

7. Frequently Asked Questions (FAQs)

Q1: High-density rainbow trout are extremely oxygen-demanding. How can safety be ensured in the event of a sudden power cut?

A: Oxygen deprivation can easily be fatal to rainbow trout; the system comes as standard with quadruple redundancy: dual-circuit mains power + diesel generator set with grid connection within seconds; a high-pressure, self-contained pure oxygen emergency bypass, which relies on the internal pressure of the gas cylinders to maintain a continuous oxygen supply for over 12 hours in the event of a power or water supply failure; active-standby hot-swap capability for the core water pumps; and millisecond-level audible and visual alerts for dissolved oxygen levels via the cloud.

Q2: Will the operation of the refrigeration units result in excessively high electricity bills?

A: Rainbow trout thrive at relatively low water temperatures (14–16°C). Within the fully enclosed, double-layered insulated facility, 95 per cent of the water is reused in a closed-loop system, with the vast majority of cooling capacity retained within the system’s circulation. Thanks to the significantly energy-saving design featuring ‘ultra-low head loss piping networks + high-efficiency pure oxygen cones’, the electricity cost per kilogram of fish is approximately US$0.59-0.74, which is fully offset by improved feed conversion rates, exceptionally high survival rates and the premium derived from zero wastage.

Q3: What is the payback period for land-based RAS rainbow trout farming?

A: Depending on project scale and management standards, the payback period for a typical RAS rainbow trout farm is 4-6 years, with an annual profit margin of 15-25 per cent once large-scale operations are established.

Q4: Can the RAS system be used to farm rainbow trout in high-temperature regions of southern China?

A: Yes. One of the core advantages of the RAS system is its precise temperature control capability; even in southern regions, cooling equipment can maintain water temperatures within the optimal range for rainbow trout (12-18 °C), enabling farming across different geographical areas.

Q5: How does the quality of rainbow trout from land-based RAS farming compare to that of wild rainbow trout?

A: In an RAS farming environment, the growth conditions for rainbow trout are highly controllable. The meat quality, colour and nutritional content can all meet or even exceed wild standards, whilst there is no risk of parasite or heavy metal contamination, resulting in higher food safety.

8. Conclusion

The breakthrough in land-based recirculating aquaculture systems (RAS) for rainbow trout represents not only a technological innovation in the field of aquaculture, but also a significant step towards implementing the strategy for the green transformation of the fisheries sector. With the implementation of the ‘General Specifications for Land-based Recirculating Aquaculture Facilities’ and the roll-out of further industrial-scale projects, RAS technology will play an increasingly important role in ensuring a supply of high-quality protein, reducing pressure on marine ecosystems, and driving the modernisation of the fisheries sector.

Should you be interested in equipment selection, scheme design or partnership models for land-based RAS recirculating aquaculture systems for rainbow trout, please do not hesitate to contact us for expert advice and bespoke solutions.

Figure 1: Rainbow trout recirculating aquaculture facility