Recirculating Aquaculture Systems (RAS) have revolutionized the way we farm fish, providing a sustainable and efficient method to produce high-quality seafood. However, optimizing the performance of these systems requires a deep understanding of the complex interactions between the flow field and the fish being farmed.
A recent study published in the journal Ocean Engineering by researchers from Dalian Ocean University sheds light on the crucial role of fish swimming in shaping the hydrodynamics of aquaculture tanks, emphasizing the importance of incorporating fish behavior into computational fluid dynamics (CFD) models.
Hydrodynamic Studies of Tanks
Traditionally, studies on aquaculture tank hydrodynamics have overlooked the presence of fish, simplifying flow field dynamics as a purely physical phenomenon. However, this approach disregards the significant impact fish swimming can have on the flow regime.
The researchers behind this study used CFD to simulate the flow field dynamics of an aquaculture tank, accounting for the swimming movements of fish.
Computational Fluid Dynamics (CFD)
Computational Fluid Dynamics (CFD) has accelerated the rapid development of recirculating aquaculture systems, offering numerous advantages over traditional experiments. CFD simulations facilitate the implementation of multi-model optimizations, reduce experimental durations, and lower costs. Experiments increasingly validate the accuracy of numerical simulations, with CFD demonstrating superior precision in predicting circulation times, retention, and mixing in aquaculture tanks.
CFD techniques monitor hydrodynamic indicators, enabling structural parameter adjustments to establish optimal flow field environments in culture tanks. Geometric structures, including water inlet designs and tank configurations, are optimized to improve flow field conditions. CFD also provides a promising platform for the distribution and optimization of particulate matter and dissolved oxygen in aquaculture tanks.
Key Findings
The findings demonstrate that the presence and swimming of fish can profoundly affect flow conditions within the tank, with varying degrees of impact depending on factors such as fish size, swimming speed, and tank geometry.
The study’s authors validated their CFD model using physical modeling experiments, confirming the accuracy of their simulations. They also identified adjusting the inlet velocity as one of the most effective methods to optimize flow field conditions in the tank.
Implications for Tank Design
The study presents several important implications for fish farmers using recirculating aquaculture systems (RAS). Key points include:
- Impact of Fish Presence and Movement: The research highlights that the presence and movement of fish significantly influence tank hydrodynamics. Moving fish generate turbulence and energy dissipation, altering flow patterns and energy distribution within the tank. This means fish farmers must be aware that tank flow dynamics are affected not only by tank design and water inlet/outlet conditions but also by fish activity.
- Reduction in Flow Velocity: Fish swimming can reduce flow velocity within the tank. Specifically, fish swimming against the current decrease flow velocity by 12.5% to 16.7%, while circular swimming reduces it by 25% to 35.4%. This is critical for tank management, as reduced flow velocity can affect dissolved oxygen distribution, waste removal, and feeding efficiency. Fish farmers must adjust their systems to compensate for this velocity reduction and ensure adequate hydrodynamic conditions for fish welfare.
- Decrease in Flow Uniformity: The presence of fish also reduces flow uniformity in tanks. Research shows that tanks with fish exhibit flow homogeneity 3.5% to 7.1% lower than those without fish. Less uniform flow distribution can lead to uneven oxygen and waste distribution, creating less favorable conditions for fish. Farmers should consider this when designing and operating their tanks, seeking ways to promote more uniform flow distribution, such as adjusting water inlet velocity or tank design.
- Importance of Inlet Velocity: The study indicates that water inlet velocity is a key factor in determining average flow speed and uniformity. While increasing inlet velocity may enhance average speed, it can also result in higher energy consumption and reduced flow uniformity, especially in the presence of fish. Farmers should optimize inlet velocity to ensure proper flow and efficient distribution of oxygen and nutrients while minimizing energy consumption.
- Impact on Energy and Vortices: The presence of fish increases energy loss in the system due to collisions and turbulence. Fish also alter vortex formation in the tank, which can affect self-cleaning capabilities. Farmers should be aware of how fish influence these parameters and take measures to maintain good water quality and reduce energy consumption. This might involve adjusting tank shape to optimize flow and minimize dead zones.
- Need for Numerical Modeling: The research demonstrates that CFD models can accurately simulate the impact of fish movement on the flow field. Farmers can use these models to optimize tank design and water management strategies, improving fish welfare and reducing costs.
Study Limitations
The study presents several limitations that should be considered when interpreting the results. These include:
- Artificial Fish Distribution: In the numerical simulation, fish were uniformly distributed, which does not reflect real aquaculture conditions, where fish distribution is influenced by the environment and social hierarchies. This simplification may affect the accuracy of water flow simulations.
- Limited Swimming States: Fish in the model only swam in two modes: upstream and circular. These modes are idealizations and do not fully represent the diverse swimming behaviors observed in real aquaculture environments. Further research on autonomous fish swimming trajectories is needed.
Conclusion
This study underscores the critical importance of considering fish swimming in aquaculture tank hydrodynamics. By incorporating fish behavior into CFD models, researchers and practitioners can gain a deeper understanding of the complex interactions between the flow field and the fish being farmed.
Contact
Xiao-Zhong Ren
Key Laboratory of Environment Controlled Aquaculture (Dalian Ocean University) Ministry of Education
Dalian, 116023, China
Email: renxiaozhong@dlou.edu.cn
Reference
Wu, G., Liu, H., Ma, C., Xu, H., Ren, X., & Sun, W. (2025). Developments and application of fish school swimming model in recirculating aquaculture systems. Ocean Engineering, 319, 120196. https://doi.org/10.1016/j.oceaneng.2024.120196
