Recirculating Aquaculture Systems (RAS) represent one of the most promising pathways for the sustainable development of the aquaculture sector. Their ability to conserve water, control the environment, and minimize environmental impact makes them a powerful tool. However, within these high-tech systems, there is a silent yet decisive factor: hydrodynamics.
The movement of water within a culture tank is much more than simple circulation; it is the engine that ensures cleanliness, water quality, and, fundamentally, the well-being and growth of the fish. This article, based on a recent scientific review published by researchers from the Beijing Academy of Agriculture and Forestry, Zhejiang Ocean University, and the Zhoushan Fisheries Research Institute, breaks down why understanding and optimizing hydrodynamics is crucial for any modern aquaculture operation.
Key takeaways
- Hydrodynamics, or the behavior of water, is a determining factor for the success of a Recirculating Aquaculture System (RAS), as it directly impacts the removal of solid waste.
- The tank design (circular, rectangular, or octagonal) defines the efficiency of the water flow in concentrating and evacuating waste such as feces and uneaten feed.
- Adequate water flow not only cleans the tank but also stimulates the fish, improves their behavior, reduces stress, and can optimize their growth and feed conversion.
- Current research utilizes physical models and computational fluid dynamics (CFD) simulations to predict and optimize flow patterns, improving tank design before construction.
Why is water movement so important?
In a RAS, fish are entirely dependent on the conditions within the tank. The accumulation of uneaten feed and feces can rapidly deteriorate water quality as they decompose, producing ammonia and nitrites. This is where hydrodynamics plays its critical role.
An effective tank design uses the energy of the water flow itself to create a self-cleaning mechanism. A uniform and well-directed flow pattern can sweep solid waste toward the central drain, facilitating its removal and reducing the load on the filtration systems. This not only keeps the water cleaner but also ensures a stable and healthy environment for the cultured organisms.
Tank design: A fundamental decision for flow
Not all tanks behave the same way. The structure of the tank is the primary factor defining its hydrodynamic characteristics. The most common types in the industry are:
- Circular tanks: These are the most studied and widely used. Their main advantage is that they facilitate the creation of a uniform circular flow, which, thanks to the “teacup effect,” generates a secondary flow that efficiently pushes solids toward the central drain. However, their space utilization is less optimal.
- Raceway-style tanks: With an elongated rectangular structure and semi-circular ends, these are easy to manage and widely used in shrimp farming. They often require a significant investment and high energy consumption to maintain the circulating flow.
- Rectangular tanks: These offer excellent space utilization (over 95%) and are simple to construct. Their major disadvantage is poor water flow uniformity, which creates multiple “dead zones” where waste accumulates and is not properly evacuated.
- Corner-Cut (Octagonal) tanks: These aim to combine the best of both worlds: the high space utilization of rectangular tanks with the efficient hydraulics of circular ones. By cutting corners, a pseudo-circular flow pattern is generated that directs solids to the center. Even so, the angles can create low-velocity zones that hinder complete cleaning.
Factors that refine system hydrodynamics
In addition to tank shape, other elements are key to optimizing water flow:
- Operational parameters: The configuration of the water inlet (the number of pipes, their angle, and position) and the hydraulic retention time are decisive. For example, studies have shown that a jet angle of 40° can optimize sediment consolidation, and that using two water inlets improves flow uniformity compared to just one.
- Water flow generation equipment: Devices such as jet pipes, paddlewheel aerators, or submersible propellers are essential for generating and maintaining water movement, especially in large tanks. Their correct selection and positioning not only improve self-cleaning but also increase dissolved oxygen levels.
The direct impact of water flow on fish
Hydrodynamics does not just clean the tank; it creates the habitat where fish live, directly affecting their behavior, health, and growth.
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- Behavior and well-being: Water flow stimulates fish, which naturally tend to swim against the current. A moderate flow (generally between 0.5 and 2.0 body lengths per second) promotes schooling cohesion, reduces aggression by diverting energy to constant swimming, and enhances the immune response.
- Growth and development: Constant exercise in an appropriate flow can improve growth and feed conversion efficiency. For instance, it has been observed that Atlantic salmon benefit from moderate flows throughout their development cycle, and common carp show significantly better weight gain under controlled flow conditions. However, excessive flow can be detrimental, increasing energy expenditure and reducing the growth rate.
- Culture environment: A good flow ensures the homogeneous distribution of key parameters like oxygen and temperature, preventing stratification and ensuring all fish have access to optimal conditions.
Conclusion: Towards smarter, more efficient systems
Research in hydrodynamics demonstrates that the design and operation of an RAS must go beyond mere water containment. The interaction between tank structure, water flow, and fish behavior is a fundamental field of study for the future of aquaculture.
Optimizing hydrodynamics translates into more efficient systems with lower operating costs, better water quality, and, above all, healthier and more productive fish. For the producer, paying attention to how the water moves is, ultimately, a direct investment in the profitability and sustainability of their operation.
Reference (open access)
Wu, Y., Chen, J., Jia, C., Gui, F., Xu, J., Yin, X., Feng, D., & Zhang, Q. (2025). Recent Advances in the Hydrodynamic Characteristics of Industrial Recirculating Aquaculture Systems and Their Interactions with Fish. Sustainability, 17(17), 7946. https://doi.org/10.3390/su17177946
Editor at the digital magazine AquaHoy. He holds a degree in Aquaculture Biology from the National University of Santa (UNS) and a Master’s degree in Science and Innovation Management from the Polytechnic University of Valencia, with postgraduate diplomas in Business Innovation and Innovation Management. He possesses extensive experience in the aquaculture and fisheries sector, having led the Fisheries Innovation Unit of the National Program for Innovation in Fisheries and Aquaculture (PNIPA). He has served as a senior consultant in technology watch, an innovation project formulator and advisor, and a lecturer at UNS. He is a member of the Peruvian College of Biologists and was recognized by the World Aquaculture Society (WAS) in 2016 for his contribution to aquaculture.
