Growth Performance and Water Quality Control Technology of Freshwater Fish in Recirculating Aquaculture System
With the continuous improvement of intensification in the aquaculture industry and increasingly stringent environmental protection requirements, traditional aquaculture models are facing numerous problems such as environmental pollution, water resource waste, and declining product quality. The Recirculating Aquaculture System (RAS), as a new type of aquaculture method, has advantages including water conservation, land saving, high stocking density, environmental controllability, and reduced tailwater discharge. It aligns with the current national strategic demands for circular economy and energy conservation and emission reduction, representing an important direction for the transformation and development of the aquaculture industry and has become a crucial model for the sustainable development of modern fisheries. In RAS, the aquaculture water is recirculated after undergoing physical filtration, biological purification, aeration, disinfection, and other treatments, requiring the system to continuously maintain water quality conditions suitable for fish growth. As the direct environment for fish survival, fluctuations in various water quality parameters directly affect the physiological functions, metabolic efficiency, and disease resistance of fish, ultimately manifesting as differences in growth performance. Therefore, in-depth exploration of the intrinsic relationship between water quality control and growth performance of freshwater fish in RAS holds significant theoretical and practical importance for improving aquaculture efficiency and promoting healthy industry development.
1 Overview of Recirculating Aquaculture System
The recirculating aquaculture model is a farming method in which culture water is recirculated after treatment through physical, chemical, and biological filter processes. Research on recirculating aquaculture technology began earlier abroad. In the 1960s, countries such as the United States, the Netherlands, and Denmark initiated relevant studies. The United States primarily used it for farming rainbow trout, striped bass, and black sea bass; the Netherlands mainly used it for European eel and African catfish; Denmark's recirculating aquaculture process system was an outdoor semi-enclosed system mainly used for rainbow trout production.
China introduced foreign recirculating aquaculture technology and facilities in the 1980s. Due to high investment and operating costs, most of the introduced facilities were quickly abandoned. In 1988, the Fishery Machinery and Instrument Research Institute of the Chinese Academy of Fishery Sciences, drawing on West German technology, designed and built China's first recirculating aquaculture production workshop. In recent years, Chinese scholars such as Qu Keming proposed high, medium, and low-level recirculating aquaculture technology models based on the different needs of various types of aquaculture enterprises and promoted them in coastal areas; Liu Bo from the Heilongjiang Provincial Fisheries Technology Extension Station proposed "container" recirculating aquaculture technology and models; Professor He Xugang from Huazhong Agricultural University proposed a pond "zero-discharge" green and efficient "captive" aquaculture model.
Recirculating aquaculture models are mainly divided into types such as "raceway," "container," and "captive." Taking the "raceway" aquaculture model as an example, it consists of a flow-through tank, waste collection area, aeration facilities, diversion facilities, purification area, wetland, and other components. The small-water-body water-pushing aquaculture area consists of rectangular tanks, occupying 2%–5% of the pond area. In recent years, domestic flow-through tank specifications are generally 20 m long, 4 m wide, and 2.5 m high, with 1–2 tanks set per 6670 m² of water body. The core component is the water-pushing aeration equipment. Early versions used impeller devices for water pushing and aeration devices for oxygenation, but now most use air-lift equipment composed of blowers, microporous aeration tubes, and baffles. Generally, two interconnected submerged waste collection tanks with a volume of 10 m³ are built for every three tanks, placed at the rear end of the flow-through tanks for collecting waste from the culture area. The large-water-body ecological purification area occupies 95%–98% of the pond area, with diversion dikes and water depth above 2 m. This area primarily cultures filter-feeding fish, with aquatic plant coverage controlled at 20%–30% of the purification area. It is equipped with paddlewheel aerators, impeller aerators, wave-making machines, etc., and microbial preparations are added as appropriate.
2 Effects of Recirculating Aquaculture Model on Growth Performance of Freshwater Fish
2.1 Growth Rate
The recirculating aquaculture model can provide a relatively stable growth environment for freshwater fish, which helps improve growth rates. In traditional pond aquaculture, water quality is greatly affected by external environmental factors such as temperature and rainfall, which can easily cause water quality fluctuations and affect fish growth. In the recirculating aquaculture model, the water quality control system can maintain relatively stable water quality parameters such as water temperature, dissolved oxygen, and pH value, creating suitable growth conditions for fish. For example, in the "raceway" aquaculture model, the water flow velocity in the flow-through tank can be adjusted through water-pushing aeration equipment. Appropriate flow velocity can promote fish movement, enhance physical fitness, increase feed intake, and accelerate growth.
2.2 Feed Utilization Rate
The recirculating aquaculture model can improve the feed utilization rate of freshwater fish. In traditional aquaculture, after feed is dispensed, some feed sinks to the bottom without being consumed, causing waste. Meanwhile, the feed that sinks to the bottom decomposes to produce harmful substances, affecting water quality. In the recirculating aquaculture model, due to the effect of water flow, feed can be better dispersed in the water, making it easier for fish to consume, thus reducing feed waste. Additionally, treatment units such as biofilters in the recirculating aquaculture system can remove organic matter like residual feed and feces from the culture water, reducing the content of harmful substances such as ammonia nitrogen and nitrite nitrogen in the water. This reduces the impact of these harmful substances on the digestive and absorptive functions of fish, thereby improving feed utilization rate.
2.3 Product Quality
The recirculating aquaculture model helps improve the product quality of freshwater fish. In traditional aquaculture, fish are susceptible to infection by pathogens such as parasites and bacteria, leading to disease occurrence and affecting product quality. In the recirculating aquaculture model, measures such as water quality control and disinfection can effectively reduce the number of pathogens in the water, lowering the risk of fish diseases. At the same time, the relatively clean growth environment of fish in the recirculating aquaculture model reduces the production of undesirable odors such as muddy smell, improving the taste and quality of the product.
3 Key Parameters and Methods of Water Quality Control in Recirculating Aquaculture Model
3.1 Key Parameters
3.1.1 Dissolved Oxygen
Dissolved oxygen is one of the important water quality parameters affecting fish growth. Fish require sufficient oxygen for respiration during growth. Insufficient dissolved oxygen can lead to slow growth, decreased immunity, and even death. Generally, dissolved oxygen in recirculating aquaculture systems should be maintained above 5 mg/L.
3.1.2 Ammonia Nitrogen
Ammonia nitrogen is one of the main pollutants in aquaculture water, mainly originating from fish excrement and decomposition of residual feed. Ammonia nitrogen is highly toxic to fish, damaging gill tissue, nervous system, and immune system, affecting growth and survival. Ammonia nitrogen concentration in recirculating aquaculture systems should be controlled below 0.5 mg/L.
3.1.3 Nitrite Nitrogen
Nitrite nitrogen is an intermediate product produced during the nitrification of ammonia nitrogen and has certain toxicity. Nitrite nitrogen combines with hemoglobin in fish blood, reducing its oxygen-carrying capacity and causing hypoxia and suffocation in fish. Nitrite nitrogen concentration in recirculating aquaculture systems should be controlled below 0.1 mg/L.
3.1.4 pH Value
pH value is an important indicator reflecting the acidity or alkalinity of water and has significant effects on fish growth and physiological functions. The pH value in recirculating aquaculture systems should be controlled between 7.0 and 8.5.
3.2 Water Quality Control Methods
3.2.1 Physical Control
Physical control mainly includes measures such as filtration, sedimentation, and aeration. Filtration is an effective method for removing suspended solids and particulate matter from water. Commonly used filtration equipment includes microscreen filters and sand filters. Sedimentation uses gravity to settle solid particles in the water to the bottom, thereby purifying water quality. Aeration is an important means of increasing dissolved oxygen in water. Commonly used aeration equipment includes blowers, paddlewheel aerators, and impeller aerators.
3.2.2 Chemical Control
Chemical control mainly involves adding chemical agents to the water to regulate water quality. For example, when ammonia nitrogen and nitrite nitrogen concentrations in the water are too high, nitrifying bacteria preparations can be added to promote nitrification reactions and reduce the content of ammonia nitrogen and nitrite nitrogen; when the water pH value is too low, quicklime can be applied to raise the pH value.
3.2.3 Biological Control
Biological control uses microorganisms, aquatic plants, and other organisms to purify water quality. Microorganisms can decompose organic matter in the water, converting harmful substances such as ammonia nitrogen and nitrite nitrogen into harmless substances. Commonly used microbial preparations include photosynthetic bacteria, Bacillus, and nitrifying bacteria. Aquatic plants can absorb nutrients such as nitrogen and phosphorus from the water, reducing the occurrence of eutrophication, while also providing habitats and shading for fish. Common aquatic plants include water hyacinth, alligator weed, and elodea.
4 Correlation Between Growth Performance of Freshwater Fish and Water Quality Control in Recirculating Aquaculture Model
4.1 Dissolved Oxygen and Growth Performance
When dissolved oxygen in the water is sufficient, fish respiration functions normally, metabolism is vigorous, feed intake increases, and growth rate accelerates. Conversely, metabolism slows down, and growth rate decreases. In the recirculating aquaculture model, reasonable aeration measures maintain stable dissolved oxygen levels in the water, providing a good respiratory environment for fish and promoting their growth and development.
4.2 Ammonia Nitrogen, Nitrite Nitrogen and Growth Performance
Ammonia nitrogen and nitrite nitrogen are toxic substances in aquaculture water that seriously harm fish growth and survival. High concentrations of ammonia nitrogen damage fish gill tissue, affecting respiratory function; they also damage the nervous system and immune system of fish, reducing their disease resistance. In the recirculating aquaculture model, treatment units such as biofilters can promptly remove ammonia nitrogen and nitrite nitrogen from the water, reducing their toxic effects on fish and ensuring healthy fish growth.
4.3 pH Value and Growth Performance
pH value has an important impact on fish growth and physiological functions. Different fish species have different adaptive ranges for pH value. In the recirculating aquaculture model, the pH value of the water is regularly tested, and corresponding adjustment measures are taken based on the test results.
5 Development Trends and Challenges of Recirculating Aquaculture Model
5.1 Intelligent and Precision Development Direction
With the development of Internet of Things, big data, and artificial intelligence technologies, the recirculating aquaculture model is evolving toward intelligence and precision. By integrating systems such as online water quality monitoring, automatic feeding, and equipment control, real-time regulation of the culture environment and automated management of the production process can be achieved.
5.2 Low-Carbon Environmental Protection and Sustainable Development Path
The recirculating aquaculture model meets the requirements of low-carbon environmental protection and sustainable development through water conservation, energy saving, and pollution reduction. Future efforts need to further optimize water treatment processes, reduce energy consumption and costs, and improve system stability and operability. For example, renewable energy sources such as solar and wind power can be used to supply electricity, reducing carbon emissions; microbial fuel cell technology can be used to achieve energy utilization of organic matter in wastewater, building an integrated "aquaculture-energy-environmental protection" system.
5.3 Challenges and Countermeasures
The current recirculating aquaculture model still faces challenges such as high investment, technical complexity, and high management requirements. It is necessary to strengthen technological research and development and integrated innovation to reduce system construction and operating costs; improve the standard system and operating specifications to enhance the technical level of farmers; and strengthen policy support and financial investment to promote the application of recirculating aquaculture models in rural areas.
6 Conclusion and Outlook
The recirculating aquaculture model, through reasonable water quality control, maintains stable levels of key water quality parameters such as dissolved oxygen, ammonia nitrogen, nitrite nitrogen, and pH value. This provides a good growth environment for freshwater fish, improving their growth rate, feed utilization rate, and product quality. Currently, in practical applications of the recirculating aquaculture model, there are still problems such as poor waste collection efficiency due to the impact of culture tank structure on hydrodynamic characteristics, and unstable treatment efficiency of biofilters. Future research should further optimize culture tank structure to improve waste collection efficiency; strengthen research on biofilm growth regulation and water circulation optimization to improve the treatment efficiency of biofilters; simultaneously, combine intelligent technologies to achieve real-time monitoring and automatic control of water quality parameters, further enhancing the scientific and precise nature of the recirculating aquaculture model, and promoting the sustainable development of the freshwater fish aquaculture industry.