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What are the factors that influence the good operation of RAS?

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By Milthon Lujan

Factors that influence a recirculating aquaculture system (RAS). Source: Li et al., (2023)
Factors that influence a recirculating aquaculture system (RAS). Source: Li et al., (2023)

The essence of modern intensive aquaculture is a high-density cultivation method based on water recycling, known as recirculating aquaculture systems (RAS).

RAS generally include aquaculture units for the breeding of aquatic organisms and for treating water containing filtration devices for the removal of particulate matter (feces, food waste), biological reactors for removing nitrogen, phosphorus, and other pollutants from the water, oxygenation equipment, and sterilization devices.

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Researchers from the Yellow Sea Fisheries Research Institute at the Chinese Academy of Fishery Sciences published a scientific review of the factors affecting the operation of recirculating aquaculture systems and revealed the mechanisms of their influence on the organisms being bred.

Advantages of RAS

The advantages of RAS include:

  • Aquatic organisms can be grown in a controlled environment;
  • The growth rate and harvest cycle can be controlled;
  • Water can be recycled through biofiltering, resulting in reduced water use;
  • The scale of cultivation is not limited by the environment;
  • Resistant to external risks (natural disasters, pollution, and diseases); and, Contaminants (chemicals and heavy metals).

Effects of environmental factors on RAS

Temperature

Most fish are poikilothermic, meaning they do not undergo adaptive genetic mutations when the temperature changes, but they reduce environmental stress through heat regulation behaviors such as tolerance, resistance, or preference.

The efficiency of many physiological processes in fish will change by between 6% and 19% for each 1oC change in body temperature, making temperature one of the most important environmental factors affecting fish growth and development.

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Additionally, the appetite, digestive functions, and feeding capacity of fish are also affected by temperature. As the temperature increases, antioxidant performance and enzymatic activities of the fish increase, and food intake increases until the optimal growth temperature is reached.

However, with an increase in temperature beyond the optimal value, the activities of functional enzymes decrease, and feeding capacities are inhibited, reducing the growth rate.

In recirculating systems for aquaculture, the composition of the microbial community and the efficiency of removing contaminants such as ammonia nitrogen and nitrite in a biofilter are also affected by temperature.

Dissolved oxygen

Dissolved oxygen is often the main limiting factor in RAS affecting system capacity and performance.

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In aquaculture, cultured animals obtain oxygen from the water through their gills. Asphyxiation will occur under conditions involving insufficient oxygen, affecting feeding, reproduction, and organism coverage behaviors.

When the dissolved oxygen concentration is within a reasonable range, it is suitable for the growth and development of aquatic animals and can improve aquaculture efficiency. However, decreasing the dissolved oxygen content severely limits the growth of aquatic organisms and may even cause massive mortality in severe cases.

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In recirculating aquaculture systems, oxygenation methods such as the cone oxygen contactor, jet aeration, and Venturi aeration can increase the dissolved oxygen content in the water, and oxygen utilization efficiency can reach 90%.

pH

pH is an important water quality parameter in RAS, reflecting the activity of hydrogen ions in the water. Generally, water suitable for aquaculture has a pH between 6.0 and 9.0.

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In suitable pH environments, cultured organisms have a higher oxygen consumption rate, adequate oxygen supply to tissues and organs, and strong enzymatic activities, facilitating normal respiration, growth, and development.

In aquaculture water, biological respiration and decomposition of organic matter consume oxygen and release CO2, causing a decrease in pH.

Acidic environments will stimulate an increase in mucus in fish gills, and excessive mucus and precipitated protein will cover the gills causing fish to suffocate.

Acidic conditions will also damage fish vision, hearing, and smell, affecting perception, motor function, metabolism disorders, and ultimately endangering the growth and development of cultured organisms.

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Likewise, low pH will affect fish gonad development and maturation, reducing sperm motility and damaging embryo development, ultimately affecting reproduction and population continuation.

Salinity

For brackish or seawater RAS, salinity is an important indicator of water quality.

In general, organisms have a degree of adaptation to salinity. For example, when salinity increases, ions in fish become imbalanced, gills and intestines usually compensate for ion loss passively, and excess water is drained from the body through kidneys and bladders.

When salinity changes beyond the tolerance range of aquatic organisms, the mechanism of osmotic pressure regulation will be affected and growth, development, reproduction, and other physiological activities will be negatively affected and, under some conditions, may cause death.

Stocking density

Population density of cultured organisms is one of the main factors determining the productivity of an aquaculture system. In intensive aquaculture, population density in water is often increased to obtain higher production and greater economic benefits.

An appropriate density is essential for the survival, growth, well-being, and health of cultured organisms.

As the population density of aquaculture increases, competition for animal food and living space will intensify, consuming a lot of energy, and the average food intake per individual will decrease, reducing fish growth rate and survival rate.

High population density can also have adverse effects on fish muscle development, reducing protein and crude fat content in the fish body, further influencing the quality of aquatic products.

Water flow rate and exchange rate

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Appropriate water flow rate is beneficial to improve fish growth rate, survival rate, feeding capacity, and environmental adaptability.

Differences in life habits and ecological niches determine the difference in organism requirements for optimal flow rate.

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A higher water flow rate may cause more suspended particles in the water, leading to filtration and removal of suspended particles. Therefore, increasing the water flow rate at the ammonia discharge point can contribute to ammonia degradation and water purification.

On the other hand, the water exchange rate plays an important role in the removal of feces, food waste, and the maintenance of water quality. In addition to affecting fish feeding and growth, a low water exchange rate can lead to the accumulation of nitrates, nitrites, suspended solids, organic matter, metal elements, and steroids, causing deformities, skin ulcers, oral cavity, and fin injuries in fish.

Light

Light is an important environmental factor in aquaculture, directly or indirectly affecting feeding, growth, reproduction, and other behaviors of fish, shrimp, crabs, sea cucumbers, and abalone.

The influence of light on the growth of aquaculture organisms mainly focuses on three aspects: light intensity, photoperiod, and spectrum.

Various studies have shown that inadequate light intensity can cause poor growth, reduce disease resistance, and increase mortality among fish.

However, high light intensities impose stress on fish, causing slow growth, liver oxidation, weak immune function, and even death.

The appropriate light intensity can not only improve the feeding efficiency, growth rate, survival rate, and feed conversion rate of farmed fish, but also improve water quality by enhancing the activities of nitrifying bacteria in RAS.

The relationship between light intensity, light quality, and photoperiod is inseparable and complementary. Understanding the integral influence of light on the growth of cultured organisms is helpful in optimizing the light conditions for aquaculture.

In recirculating aquaculture systems, lighting devices such as LED lamps can be installed, allowing specific lighting strategies to be designed according to the lighting needs of species to improve the quality of cultured organisms.

Interaction of environmental factors

In general, environmental factors should be kept within an appropriate range to ensure the growth of cultured organisms. Although the study of a single factor is easy to perform, the environment is a complex system, and there are mutual and promotional constraints among environmental factors.

For example, an increase in temperature leads to an increase in metabolic rate and thus an increased demand for oxygen.

In recirculating aquaculture systems, many environmental factors can be regulated to an appropriate range, creating a suitable environment for animal growth in culture. This is one of the main advantages of RAS.

Effects of disinfection methods in RAS

High-density aquaculture systems, such as RAS, can lead to potential proliferation of pathogenic microorganisms, which affect the growth of cultured organisms and the normal operation of the system.

In this sense, to prevent disease outbreaks, it is necessary to remove pathogenic microorganisms in the water.

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Ozone (O3) addition and ultraviolet (UV) eradication are the most commonly used disinfection methods in RAS. In recent years, other disinfection methods such as peracetic acid and hydrogen peroxide (H2O2) have also been applied in recirculating aquaculture systems.

Chlorine (Cl2) and sodium hypochlorite (NaOCI) can hydrolyze to produce hypochlorite (HCIO), which causes denaturation of proteins in bacteria and viruses through its strong oxidizing action. It is economical and widely used in RAS; however, because Cl2 and NaOCI are corrosive and toxic to organisms, they can remain in the water and affect the normal functioning of RAS.

Conclusion

The scientific review describes the factors that affect the stable operation of an RAS. Each factor plays an important role in the operation of RAS.

Among them, the authors believe that temperature, dissolved oxygen, and disinfection exert the greatest influence on the aquaculture unit and the water treatment unit in RAS.

In modern RAS, environmental parameters such as temperature, dissolved oxygen, pH, and salinity can be monitored online, and feeding strategies and disinfection measures can be optimized for implementation.

The study was funded by the National Key R&D Program of China, Ministry of Science and Technology of People’s Republic of China, Postdoctoral Application Fund of Qingdao, Qingdao Municipal Bureau of Human Resource and Social Security, and Basic Scientific Research Project of Chinese Academy of Fishery Sciences.

Contact
Zhengguo Cui
Yellow Sea Fisheries Research Institute
Chinese Academy of Fishery Sciences
No.106 Nanjing Road, Qingdao 266071, China.
Email: cuizg@ysfri.ac.cn

Reference (open access)
Li, H., Cui, Z., Cui, H., Bai, Y., Yin, Z., & Qu, K. (2023). A review of influencing factors on a recirculating aquaculture system: Environmental conditions, feeding strategies, and disinfection methods. Journal of the World Aquaculture Society, 1– 37. https://doi.org/10.1111/jwas.12976

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