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Review

Recent Advances in the Hydrodynamic Characteristics of Industrial Recirculating Aquaculture Systems and Their Interactions with Fish

by
Yanfei Wu
1,†,
Jianeng Chen
1,2,†,
Chengxia Jia
1,
Fukun Gui
2,
Jianuo Xu
1,
Xiaolong Yin
3,
Dejun Feng
2,* and
Qingjing Zhang
1,2,*
1
Beijing Key Laboratory of Fishery Biotechnology, Institute of Aquatic Sciences, Beijing Academy of Agriculture and Forestry, Beijing 100068, China
2
National Engineering and Technology Research Centre for Marine Facility Aquaculture, Zhejiang Ocean University, Zhoushan 316022, China
3
Zhoushan Fisheries Research Institute, Zhoushan 316000, China
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Sustainability 2025, 17(17), 7946; https://doi.org/10.3390/su17177946
Submission received: 9 April 2025 / Revised: 11 July 2025 / Accepted: 23 July 2025 / Published: 3 September 2025
(This article belongs to the Section Sustainable Water Management)
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Abstract

Industrial recirculating aquaculture systems (RAS) constitute an energy-saving and environmentally friendly approach to modern aquaculture production. The hydrodynamic characteristics within these systems, coupled with the ecological environment of the fish, are essential for the efficient operation of the system and for promoting optimal fish growth and welfare. These systems provide several advantages, such as high intensification, efficient water resource utilization, enhanced environmental control, and minimal environmental pollution. Consequently, it has emerged as prominent avenue for advancing aquaculture development in China. This paper begins with an examination of the fundamental concepts and primary tank structures underpinning industrial RAS. It then proceeds to elucidate the hydrodynamic characteristics within RAS and their interrelationship with fish growth and welfare. Furthermore, it offers a thorough review of tank hydrodynamic characteristics and fish interactions from various perspectives, including operational parameters, hydrodynamic drive equipment, fish behavior, and the aquaculture environment. Finally, the limitations of current studies are assessed, and potential future research directions are proposed.

1. Introduction

The Food and Agriculture Organization of the United Nations [1] released the 2024 State of World Fisheries and Aquaculture report, highlighting a significant increase in global fishery and aquaculture production. Global production reached 223.2 million tons in 2022, reflecting a 4.4% increase from 2020. Aquatic animal production accounted for 185.4 million tons, while algae production contributing 37.8 million tons. This represents the first instance where aquaculture surpassed capture fisheries in animal production (see Figure 1), establishing its dominance in this sector. Reaching a record 130.9 million tons, global aquaculture production marks a pivotal industry shift. Internationally, recirculating aquaculture systems (RAS) have advanced significantly. it is estimated that up to 70% of smolts stocked in sea cages in Norway are from RASs [2]. Denmark currently has more than 50 factory farming systems with an annual output of 150–300 tons of aquatic products. Through advanced RAS technologies, these nations have improved production efficiency while reducing environmental impacts. As the global leader in aquaculture production, China is also the sole nation where aquaculture exceeds capture production. According to the 2023 Fisheries Statistics Yearbook reports that the total volume of aquatic products in China amounted to 68,659,100 tonnes, of which aquaculture comprised 55,654,600 tonnes (81.06%) [3]. China’s aquaculture sector has progressed substantially since the early 2000s. China’s aquaculture trade volume has grown steadily [4], fueled by policies promoting sustainable, high-quality development and supporting aquaculturists in boosting output and income. These initiatives enhance operational quality and efficiency while advancing green aquaculture [5,6]. Traditional methods have historically shaped aquaculture production processes. However, they present persistent challenges: outdated techniques, inadequate infrastructure, product contamination, and yield reduction [7]. Facility proliferation has degraded water quality in aquaculture zones, triggered disease outbreaks, and caused pollutant accumulation from unregulated wastewater discharge. Mitigation efforts focus on concentrating and optimizing aquaculture wastewater discharge [8]. Consequently, recirculating aquaculture systems (RAS) have emerged as a research and development priority for achieving intensive, efficient, and sustainable production.
The Recirculating Aquaculture System (RAS) is an environmentally sustainable model for aquaculture that utilizes physical filtration, biological purification, and other treatment cycle components (Figure 2). RAS offers higher yields per unit area compared to other aquaculture systems [9], with water reuse rates exceeding 90% [10]. The internal circulation within the recirculating aquaculture tank facilitates the rapid removal of feed, feces, and other solid pollutants generated during the aquaculture process. This process reduces the burden on both the physical filtration and biological purification systems, as well as on the subsequent recycled water treatment processes. As a result, the aquaculture water remains stable, ensuring the health of the aquaculture environment. The core advantage of this technology is its ability to improve water resource efficiency, reduce operational costs, and achieve energy savings, emission reductions, and the development of green, sustainable, and high-density aquaculture [11,12,13]. In compared to traditional aquaculture, RAS integrates advanced industrial technologies, including engineering applications, automation, modern biological systems, mechanical equipment, and control instrumentation. The objective is to facilitate comprehensive control and scientific management throughout the aquaculture life cycle, thus addressing resource limitations such as seasonal constraints and land availability. The benefits of this approach include a controllable aquaculture environment, high water resource utilization, high intensification, minimal pollution, and assurance of both quality and quantity [14]. Moreover, the area dedicated to industrialized recirculating aquaculture has consistently expanded over the past decade (Figure 3).
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Figure 2. Process flow of recirculating aquaculture system. Source: Ren et al. [15].
Figure 2. Process flow of recirculating aquaculture system. Source: Ren et al. [15].
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Figure 3. China’s industrialized recirculating aquaculture area from 2013 to 2023.
Figure 3. China’s industrialized recirculating aquaculture area from 2013 to 2023.
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Current research by domestic and international scholars predominantly focuses on areas including: biological seed selection, water treatment, fish diseases, and aquaculture equipment. However, a significant research gap persists regarding the self-cleaning characteristics of aquaculture tanks and their interaction with fish. The energy from water inlet systems constitutes the primary hydrodynamic driver in recirculating aquaculture systems (RAS). A uniform horizontal flow field effectively concentrates solid pollutants (e.g., residual feed and fecal matter), facilitating removal. Consequently, investigating tank self-cleaning mechanisms is critical. This paper synthesizes recent research on the interactions between RAS hydrodynamic characteristics and fish, examining multiple perspectives. These include: tank structure, operational parameters, hydrodynamic drive equipment, fish behavior, the aquaculture environment, and fish growth and welfare. The objective is to apply flow field characteristics to tank engineering design, thereby establishing a scientific foundation for advancing sustainable RAS development.

2. Main Structural Types of Recirculating Aquaculture Tanks

Currently, the sewage collection from aquaculture tanks is primarily achieved through water flow, which facilitates the discharge of waste or the treatment of aquaculture effluent for recycling. The structure and flow characteristics of aquaculture tanks are critical in enhancing the inherent drive and automatic cleaning capacity for waste movement at the bottom of the tank [16]. Currently, four main types of recirculating aquaculture system (RAS) tanks are commonly used: runway-type tanks [17], circular tanks [18,19], rectangular tanks [20], and square cut corner aquaculture tank. Each of these tank types exhibits distinct characteristics.

2.1. Runway Type Aquaculture Tank

Runway-type tanks [21,22] consisting of a closed rectangular structure basin, with a cement-mixed long side and two semi-circular sections on the short side. This configuration is referred to as a runway-type aquaculture tank (Figure 4). A gap is maintained between the tank wall and the water wall, with an artificial wave device creating a circulating water flow around the wall. The shape resembles a track and field runway, which is the basis for the tank’s name, ‘runway-type aquaculture tank.’ Runway-type tanks offer several advantages, including ease of management, operational convenience, and simplicity in fish capture. As a result, they are widely used in shrimp aquaculture. However, the construction of these tanks and their ancillary facilities often requires substantial financial investment. Additionally, the operational process involves significant investment in infrastructure and high power demands.

2.2. Circular Aquaculture Tank

Circular tanks (Figure 5) are widely utilized and extensively studied. They facilitate the efficient exchange of water and the development of a uniform circular flow pattern, ensuring that no dead zones accumulation dirt. This type of tank was the most widely used and studied in earlier years [23,24,25]. The formation of secondary flow in circular tanks is of significant importance. Secondary flow refers to the movement of the main stream in the aquaculture tank, such as the “teacup effect” [26]. This phenomenon has been demonstrated to enhance the efficiency of waste collection. The system’s effective waste collection directs solids to the vicinity of the outlet. However, circular tanks exhibit suboptimal space utilization and present challenges in construction. In Figure 5a, The Q-criterion is of paramount importance in fluid mechanics, particularly in the study of turbulence and vortex dynamics. The Q-criterion is a widely used method for identifying and visualizing vortex structures in fluid flows. It is derived from the second invariant of the velocity gradient tensor, which enables the identification of regions with pronounced rotational characteristics. This method is extensively used in the post-processing of computational fluid dynamics (CFD) simulations to provide deeper insights into complex flow phenomena. In Figure 5b, the inlet flow rate was 10 m3/min, and the HRT was 35 min, which is similar to the HRT reported by Summerfelt et al. [27]. Figure 5c has two inlets and was designed to analyze the effect of the difference in the number of inlets.
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Figure 5. (a) Circular aquaculture tank, (b) Circular tank secondary vortex and vortex structures using Q-criterion (Q = 0.01). Source: Choi et al. [28]. (c) Velocity magnitude patterns in the tank. Source: Sin et al. [29].
Figure 5. (a) Circular aquaculture tank, (b) Circular tank secondary vortex and vortex structures using Q-criterion (Q = 0.01). Source: Choi et al. [28]. (c) Velocity magnitude patterns in the tank. Source: Sin et al. [29].
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2.3. Rectangular Aquaculture Tank

The primary advantage of rectangular tanks (Figure 6) over circular tanks lies in their high space utilization rate, exceeding 95% [30]. Moreover, their construction process is straightforward. However, these tanks exhibit poor water mixing uniformity, characterized by multiple stagnant zones and a suboptimal overall flow rate, precluding the achievement of ideal piston or mixed flow regimes. Consequently, effective aggregation and discharge of pollutants within the tank are impeded. In Figure 6b, different colors represent the magnitude of the flow velocity. The inlet flow rates represented by A1, A2, A3, and A4 are shown in Table 1. Zhang et al. [31] results show that the tanks with corner structures have better flow field characteristics, which include a higher flow velocity, turbulence intensity, and discharge effect.
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Figure 6. (a) Rectangular aquaculture tank, (b) Situation groups, (c) Flow field characteristics. Source: Zhang et al. [31].
Figure 6. (a) Rectangular aquaculture tank, (b) Situation groups, (c) Flow field characteristics. Source: Zhang et al. [31].
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Table 1. Simulation working condition groups.
Table 1. Simulation working condition groups.
Situation GroupsCorner RatioCorner Distance (m)Water Inflow (m3/s)Water Outflow (m3/s)
A10:10.043.43694.81
A20.26:11.5642.00672.04
A30.33:12.041.06656.88
A40.40:12.440.02640.34

2.4. Square Cut Corner Aquaculture Tank

The square-cut aquaculture tank (commonly termed the octagonal aquaculture tank) constitutes an improvement over rectangular aquaculture tanks [32]. This modification introduces right angles at the corners, generating a horizontal circulation pattern analogous to circular tank, thereby directing suspended solids toward the drainage outlet. Essentially, this configuration represents a rectangular tank transformed into a pseudo-circular geometry. Consequently, octagonal tanks integrate the high space utilization efficiency of rectangular tanks with the efficient hydraulic exchange characteristics of circular tanks. However, these right angles adversely impact flow velocity and hydraulic mixing efficiency, the inflow velocity is shown in Table 2, creating low-velocity zones (Figure 7d). This phenomenon hinders solid pollutant aggregation at the tank bottom and impedes discharge of accumulated sediments. Consequently, the tangent-angle culture tank wall design (Figure 7a) may be optimized through transformation into a square-arc configuration (Figure 7b).
Table 2. Inlet setup.
Table 2. Inlet setup.
Inlet ConfigurationHydraulic Retention Time (min)Single Inlet Flow Rate (L/min)Total Inlet Flow Rate (L/min)
Single-pipe inlet458.68.6
Four-pipe inlet22.54.317.2
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Figure 7. (a) Square-cut corner aquaculture tanks, (b) Square rounded corner aquaculture tank, (c) Diagram of the aquaculture tank, (d) Flow field maps under various rolling conditions in the four-pipe inlet configuration. Source: Li et al. [33]. Note: T represents the period and θ represents the amplitude, The arrow indicates the direction of the jet.
Figure 7. (a) Square-cut corner aquaculture tanks, (b) Square rounded corner aquaculture tank, (c) Diagram of the aquaculture tank, (d) Flow field maps under various rolling conditions in the four-pipe inlet configuration. Source: Li et al. [33]. Note: T represents the period and θ represents the amplitude, The arrow indicates the direction of the jet.
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Within recirculating aquaculture systems (RAS), cultured organisms are entirely dependent on tank conditions for viability. Water quality parameters directly regulate the organisms’ developmental environment. Consequently, hydrodynamic characteristics and flow characteristics critically govern organism performance, Optimal tank configuration selection is fundamental to aquaculture productivity. This is attainable through structural parameter optimization, jet pipe geometry modification, and adjustment of operational variables to enhance autonomous purification capability and establish ideal hydrodynamic characteristics. Resolution of these hydrodynamic constraints constitutes an essential for improving practical aquaculture operations.

3. Methodology for Hydrodynamic Studies of Recirculating Aquaculture Systems

During the early 1970s, China’s recirculating aquaculture system (RAS) model underwent preliminary development [34]. Concurrently, research focused on optimizing solid waste removal from aquaculture tanks expanded progressively. Experts consensus identifies “before keeping fish, condition the water first.” as the fundamental aquaculture principle. Flow pattern distribution and pollutant aggregation within tanks directly determine organism growth conditions. Strategic deployment of hydrodynamic drive equipment leverages tank self-cleaning functionality to establish circular flow regimes, enabling direct detritus collection at drainage outlets [35,36]. This investigation yields following findings: First, literature analysis identifies physical scale modeling and numerical simulation as dominant research methodologies (Table 3). Second, key determinants include: (1) tank geometry [37]; (2) operational parameters (inlet configuration, orifice count, hydraulic retention time); and (3) hydrodynamic equipment deployment (positioning, quantity, and distance).
Table 3. Methods of hydrodynamic research in aquaculture tanks.
Table 3. Methods of hydrodynamic research in aquaculture tanks.
Research MethodResearch ObjectResearch ToolAnalysis ParameterReferences
Physical model testRunway tankADVCyclone velocity, TSS removal rateCheng et al. [22]
Rectangular tankADVWater velocity, dirt accumulationLiu et al. [38]
Runway type aquaculture tankADVTurbulence intensityZhang et al. [39]
Circular tankADVBottom displacement, Rotation speedOca et al. [40]
Circular tankPIVAverage flow velocity, flow field uniformity coefficientZhu et al. [41]
Octagonal tankImage processingSewage collectionZhang et al. [42]
Square arc-angle aquaculture tankImage Acquisition Processing SystemCharacteristics of pollutant aggregation and distributionZhao et al. [43]
Numerical
simulation		Engineered runway recirculating aquaculture system for tanksCFDFlow rate distribution, solid phase particle deposition rateWang et al. [44]
Octagonal aquaculture tankADVVelocity distribution, particle removal rateLiu et al. [32]
Octagonal aquaculture tankCFDWater velocity, dirt accumulationGorle et al. [45]
Circular aquaculture tankSTAR-CCM+Flow field distribution, solid particle motion characteristicsHu et al. [46]
Square arc-angle maricultural tankCFDArc-width ratio, effluent efficiencyRen et al. [47]
Rectangular single-side arc angle tankCFDFlow velocity distribution, solid particulate matter depositionXue et al. [48]
Note: Acoustic Doppler Velocimetry (ADV), Particle Image Velocimetry (PIV), Computational Fluid Dynamics (CFD).

3.1. Physical Modelling Test Methods

The physical modeling approach employs geometrically scaled or isometric prototypes for experimental analysis. Quantitative flow velocity data are collected using instrumentation including Acoustic Doppler Velocimetry (ADV) [49] and Particle Image Velocimetry (PIV) [50].
In 1989, Du et al. [25] conducted an experimental study using a circular aquaculture tank as the test subject. The study aimed to investigate the influence of the inlet pipe on the flow rate and discharge performance when water was introduced from the tank wall. The results demonstrated that the flow rate at all points on the bottom of the tank was critical in initiating fouling and discharge. Davidson et al. [23] investigated the effect of the inlet pipe structure on the self-cleaning capacity in a circular Cornell-type aquaculture tank. Their findings indicated that the rotational velocity of the water in the circular tank could be influenced by adjusting the orientation of the inlet pipe structure. Oca et al. [40] measured the flow field distribution in tanks with various inlet configurations using ADV. The results demonstrated that altering the inlet configuration from vertical to horizontal improved the flow field performance [51]. They employed an ADV flow velocity meter to collect data on the flow field at various depths within the tank, indicating that the rotational velocity of the water was approximately 5–6% of the velocity of the water exiting the injection port. Conventional flow measurement techniques, such as ADV, are constrained by the limitations of single-point measurements. In contrast, PIV enables full-field, instantaneous measurements without interference. Zhu et al. [41] conducted a model test to investigate the effects of the distance and angle of the inlet pipe on the hydrodynamic characteristics of a circular aquaculture tank with dual inlets. The test employed PIV to measure the flow field distribution within the tank. The results indicated the optimal sewage collection and hydrodynamic characteristics when the inlet pipe was positioned at a distance of d = 1/4 r and an angle of α = 30°–40° (the inlet pipe is set at a distance d and the radius of the aquaculture tank is r).
Previous studies have employed indirect methods to assess self-cleaning capabilities in aquaculture tanks, primarily analyzing system flow patterns. Alternatively, direct approaches like sewage collection dynamic test have been utilized to evaluate tank self-cleaning efficacy. Standard image acquisition systems typically include high-definition cameras with computer interfaces, control software, and structural components. This technology’s objective outputs have led to its broad adoption for imaging solid particle distributions in RAS tanks. Typical image processing workflows use R2019b to determine debris maximum distribution distance (Lmax) from captured images. This method indirectly reflects tank hydrodynamic characteristics. Venegas et al. [52] experimentally investigated jet mixing effects on circular tank hydrodynamics. Optimal mixing time, tangential velocity, uniformity, and solids removal occurred at 45° jet angles. Zhao et al. [9] used control variables to examine jet angle/velocity effects on flow fields and fouling aggregation in square tanks. Dual jet operation at 40° optimized debris aggregation and improved self-cleaning efficiency. Zhang et al. [42] applied image processing to study fouling collection in square tangent tanks. Optimal fouling collection occurred at 24 cm/s flow rates and ≈40° jet angles. Liu et al. [32] systematically examined hydraulic drivers (jet velocity/position), tank conditions (bottom smoothness/slope), and debris density in rectangular shrimp tanks. Higher jet flow rates produced smoother tank bottoms, while proximal jet placement near outlets enhanced dirt aggregation. Gui et al. [53] tested paddle wheel aerator effects (deployment distance, speed, angle) on hydrodynamics in square tangent tanks.

3.2. Numerical Simulation Methods

Numerical simulation denotes the application of numerical methods and mathematical modeling to solve applied problems in engineering and physical sciences. Diverse tank designs in recirculating aquaculture systems require numerical modeling to optimize structural configurations. CFD techniques enable this implementation. These models further facilitate optimization of structural and operational parameters.
Wei et al. [54] analyzed structural/operational parameter effects on rotational velocity distributions in circular tanks using numerical simulations and fluid dynamics principles. Liu et al. [55] established a CFD model for RAS that quantified flow velocity distributions and solid particle removal efficiencies. Chen et al. [56] simulated submersible pusher arrangement effects on experimental tank flow fields. A three-pusher configuration was proposed, optimizing flow fields through deflector integration and pusher angle adjustment. Wang et al. [44] modeled total suspended solids (TSS) using a dense discrete phase model (DDPM). Results demonstrated significant influence of collection zones on deposition of 0.60–2.00 mm solid particles. Hu et al. [46] investigated flow field distributions and solid particle motion in circular RAS tanks using STAR-CCM+. Zhang et al. [57] applied a solid-liquid two-phase model to evaluate particle-mediated water purification efficacy in square tangential tanks. Analysis of two-phase flow across slopes revealed optimal conditions at 0.25 rad/s rotational velocity and 12° slope. This configuration showed superior purification efficacy relative to alternatives. Gorle et al. [45] analyzed jet angle effects on debris aggregation in double-drain octagonal tanks via CFD. Two novel inlet configurations were proposed to enhance tank self-cleaning performance. Optimal hydrodynamic drive equipment enables effective circulation patterns and debris aggregation. Ren et al. [47] conducted numerical simulations to assess the impacts of daily circulation frequency, jet angle, diameter-to-depth ratio, and arc width ratio on collection/discharge characteristics in single-pass seawater systems. Computational Fluid Dynamics (CFD) simulation provide several advantages [58], including rapid computational speed, cost-effectiveness, and reduced time cycles when investigating hydrodynamic characteristics, especially in comparison to physical model tests. However, certain limitations must also be acknowledged. On one hand, CFD simulation depend on numerical analysis software for simulation and prediction, which may yield results inconsistent with field tests. On the other hand, CFD numerical simulations are subject to inherent uncertainties, such as model import, parameter settings, numerical discretization, and other software-related factors. Therefore, it is essential to establish a suitable and accurate numerical model in relevant research and validate the computational method through experimental data.

4. Factors Affecting the Hydrodynamic Characteristics of Recirculating Aquaculture Systems

4.1. Operational Parameters

Key operating parameters that govern flow field characteristics in RAS tanks include inlet pipe configuration—encompassing structure, number, aspect ratio, angle, and orifice count—as well as hydraulic retention time. Jet pipes are a common hydraulic driving mechanism in aquaculture systems and typically feature single- or multi-tube jet inlets driven by pump systems. Researchers worldwide have extensively studied hydrodynamic properties across diverse tank configurations. Ren et al. [59] used physical model experiments to investigate how square-arc-angle inlet structures affect flow field characteristics. Their results indicated that single-pipe arc walls produced higher average velocities, while double-pipe arc inlets further outperformed single-pipe configurations by yielding more uniform bottom flow distribution and reducing low-velocity zones near arc angles and sidewalls. Shi et al. [60] developed a two-phase CFD model to investigate the effects of tank aspect ratio (length-to-width ratio, L/W) on discharge characteristics. In rectangular arc-angle tanks with dual inlets, aspect ratios between 1.0 and 1.5 optimized sewage discharge efficiency, achieving over 95% particle exclusion. Jet angle also significantly influences hydrodynamic behavior; Zhao et al. [9] used a controlled-variable approach to assess how jet angle and velocity influence flow field properties and sediment aggregation, demonstrating that under dual-jet operation, a 40° jet angle resulted in optimal sediment consolidation across the tank. Yu et al. [61] investigated how inlet configuration and the number of orifices affect hydrodynamic properties in single-channel rectangular arc-angle tanks, finding that arc-angle inlet positioning with 100–120 daily recirculation cycles (18–21 orifices) yielded optimal bottom flow conditions. Finally, Summerfelt et al. [27] demonstrated that large tanks (500–1300 m3) and land-based circular systems with 35–50-minute hydraulic retention enabled higher feed loading and improved metabolic waste removal.

4.2. Hydrodynamic Drive Equipment

Hydraulic drive equipment plays a critical role in RAS by promoting efficient solid-waste aggregation via induced flow, thereby enhancing water quality and supporting higher stocking densities [62]. Primary RAS hydraulic drive devices include jet pipes [63], jet-mixing injectors [52], waterwheel aerators [64], and submersible actuators [65]. Jet pipes are more commonly employed than jet-mixing injectors and, when mechanically powered, outperform them in terms of tangential velocity, flow uniformity, mixing efficiency, and waste-discharge performance. Waterwheel aerators are extensively used in tank-based aquaculture—particularly in large-scale operations—to boost dissolved oxygen concentrations while facilitating efficient waste collection and conveyance to discharge points. Research shows that positioning these aerators at a 45° angle with a layout distance ratio of 1:4 optimizes sediment aggregation and flow field characteristics in square arc-angle tanks [53]. Guo et al. [66] employed numerical simulations to analyze flow field distributions across various tank designs and determined that at least two waterwheel aerators are necessary to achieve optimal circulation. Submersible actuators perform essential mixing and propulsion—similar to waterwheel aerators—in both oxidation ditches and RAS; however, improper layout of any flow-inducing equipment can result in solid deposits near tank sidewalls, where low velocities promote deposition and low dissolved oxygen. Li [67] demonstrated that submersible actuators enhance oxidation ditch oxygenation, foulant mixing, and horizontal flow velocities in refinery wastewater treatment. Wu et al. [68] used physical model experiments to evaluate how actuator deployment angle (θ), actuator count (n), and placement patterns (perpendicular bisector vs. diagonal) affect collection performance in octagonal tanks, finding that doubling actuator quantity markedly improved collection efficiency.
Traditional jet pipes are limited in power and can circulate flow only within a restricted zone. Consequently, novel hydrodynamic drive devices should be explored to achieve more efficient solid-waste aggregation. This challenge represents a key direction for future investigation. The positioning and orientation of these devices similarly affect tank hydrodynamics and flow-field behavior. Therefore, choosing optimal hydrodynamic drive devices and configuring them appropriately for each tank type not only enhances self-cleaning efficiency but also elevates dissolved oxygen levels.

5. Influences of Hydrodynamic Properties on Fish

The hydrodynamic characteristics of aquaculture tanks pertain to the movement of and behavior of water within the system. These characteristics directly govern the collection and discharge of solid waste, including residual feed and feces, within the system. Key parameters include flow uniformity, velocity distribution, vortex formation, the water resistance coefficient, and energy gradient. Hydrodynamic conditions significantly influence the behavior, survival, growth, and development of aquaculture organisms, thereby shaping their environmental conditions (Table 4). Moreover, hydrodynamic conditions stimulate the sensory organs of fish, prompting them to congregate in the middle and upper layers of the tank.
Table 4. Influence of flow field in aquaculture tank on fish.
Table 4. Influence of flow field in aquaculture tank on fish.
Aquaculture ObjectEntrained VelocityAnalysis ParameterReferences
Hybrid sturgeon0, 0.1, 0.3 and 0.5 m/sSwimming states, rheotaxis frequency, tail beating frequency and Oxygen consumption rate,Li et al. [69]
Anabarilius grahami0,0.1, 0.2 and 0.25 m/sSwimming performance, swimming speed, rheotaxis frequency, and tail beat frequencyZhong et al. [70]
Zebrafish13 Bl/s Length, growth and developmentPalstra et al. [71]
Rainbow trout 0, 0.9 Bl/sSpecific growth rate, energyMcKenzie et al. [72]
California halibut0.5, 1.0 and 1.5 Bl/sFeed conversion ratio, growth rateMerino et al. [73]
Atlantic salmon0.10–0.27, 0.36–0.63 Bl/sFillet quality, growth rateArve et al. [74]
Carps0, 1.5 and 2.5 Bl/s Body weight, specific growth rate, feed conversion efficiency, and hepatosomatic indexShrivastava et al. [75]

5.1. Influence of Hydrodynamic Characteristics on Fish Behavioural Traits

Fish display distinct flowtal groups maintes and sense relative motion between their bodies and the surrounding water through their lateral line system. Within optimal flow ranges, fish exhibit significant increases in schooling cohesion, tail-beat frequency, and oxygen consumption as flow velocity rises. Optimal velocity ranges, however, vary markedly across species. Water flow also reduces intersense relate aggression by diverting energy from antagonistic behaviors toward sustained swimming.
Qian et al. [76] evaluated the effects of flow velocity on fish physiology and metabolism, finding that velocities below 0.5 BL/s had no discernible behavioral impact, whereas velocities above 2 BL/s significantly impaired behavior, survival, and metabolic function. Moderate velocities (0.5–2 BL/s) enhanced growth, physiological performance, and metabolic efficiency, thereby improving immune response and antioxidant capacity. Polverino et al. [77] demonstrated that tank dimensions influence mobility and behavior in Gambusia affinis. Similarly, Li et al. [69] reported a significant increase in schooling cohesion (p < 0.05) in juvenile hybrid sturgeon (Huso duricus Georgi♂ × Acipenser schrencki Brandt♀) at higher velocities, with cohesion rising proportionally to velocity and reaching approximately 100% at 1.5 BL/s. Zhong [70] quantified the effects of flow velocities of 0, 0.1, 0.2, and 0.25 m/s on the swimming mode, speed, schooling cohesion, and tail-beat frequency of Anabarilius grahami. Significant linear correlations emerged between swimming speed, tail-beat frequency, and velocity within specific ranges. Experimental groups maintained higher velocities than static controls, highlighting the role of flow in promoting sustained swimming performance.
In recirculating aquaculture systems, fluctuations in flow velocity and direction influence tail-beat frequency, swimming patterns, and foraging behavior. Concurrently, fish swimming alters tank turbulence, affecting dissolved oxygen distribution and overall system hydrodynamics [4,78]. Therefore, integrating fish behavior with hydrodynamic analysis is essential for effective system management and welfare enhancement.

5.2. Influence of Hydrodynamic Properties on the Aquaculture Environment

Optimal hydraulic flow and water quality maintenance are critical for fish survival and infrastructure integrity in recirculating aquaculture systems (RAS). These parameters directly govern organism health and overall water quality [79]. High stocking densities and feeding regimes lead to the accumulation of uneaten feed and fecal matter on tank surfaces if not promptly removed. The decomposition of this organic matter produces nitrite and ammonia, thereby accelerating water quality deterioration. Thus, optimizing hydrodynamics for efficient solids removal is therefore essential to maintaining high water quality.
Most solid waste in RAS originates from feeding activities, with uneaten feed and feces accumulating on tank substrates. These particles constitute the primary suspended matter source, consisting mainly of proteins, lipids, dietary fiber, and trace inorganic nutrients and vitamins. Timely removal is critical, as decomposition produces nitrite and ammonia nitrogen, impairing water quality and organism health [80]. Suspended particulate matter (SPM) includes colloidal, dissolved, and macroscopic fractions; particles smaller than 35 μm account for 90% of SPM, with size distributions spanning 0.4–900 μm [81]. Particle size is influenced by tank hydrodynamics, organism activity, and pump operation. McMillan et al. [82] examined the effects of pump cycling on particle size distributions, while prolonged hydraulic retention times and equipment degradation lead to the formation of recalcitrant particles. Foss et al. [83] demonstrated that elevated ammonia nitrogen concentrations inhibit turbot (Scophthalmus maximus) growth, and noncentrations inhibit turbot (Scophthalmus maximus) demonstrated that elevated itrogen is critical for water quality management. Hydraulic flow patterns and mixing regimes directly govern RAS water quality by altering physical parameters such as temperature and dissolved oxygen—and thereby affecting organism physiology. Therefore, optimized hydrodynamic regimes are essential for maintaining stable water quality and facilitating efficient solids removal.

5.3. Influence of Hydrodynamic Characteristics on Fish Growth and Development

The effect of hydrodynamic characteristics on fish growth and development is primarily reflected in their effects on feed intake and satiation. Appropriate flow conditions can enhance fish growth and development; however, excessive flow rates may lead to increased feed loss and energy expenditure, thereby negatively affecting growth performance. Additionally, spatial variations in flow modify feed distribution and availability, thereby affecting feeding behavior and growth [84].
Hydrodynamic characteristics strongly influence fish growth and development. Palstra et al. [71] investigated the impact of water flow on zebrafish (Danio rerio) by subjecting them to flow training (six hours per day, five days per week) for four weeks. The study revealed that zebrafish could swim continuously at 13 BL/s. Compared to untrained fish, the trained group exhibited a 5.6% increase in body length and a 41.1% increase in body weight [85]. McKenzie et al. [72] examined the effects of two stocking densities and continuous aerobic exercise on the growth, energy metabolism, and welfare of rainbow trout (Oncorhynchus mykiss). Their findings indicated that higher flow rates exacerbated the negative impacts of high-density rearing, thereby reducing the specific growth rate. The optimal flow rate for California halibut (Paralichthys californicus) fry was determined to be 1.0 BL/s, yielding the highest feed conversion efficiency and fastest growth rate [73]. Similarly, moderate flow rates of 0.36–0.63 BL/s benefited the growth of Atlantic salmon (Salmo salar) throughout their development cycle [74]. Shrivastava et al. [75] studied carp (Cyprinus carpio) in a recirculating swim channel, exposing them to 0, 1.5, and 2.5 BL/s for four weeks. The group at 2.5 BL/s showed significantly improved weight gain, specific growth rate, and feed conversion efficiency, indicating that such flow conditions promote carp growth. Overall, moderate flow conditions enhance energy utilization and support optimal fish development. In aquaculture systems, flow velocities of 0.5–2.0 BL/s are considered optimal, as they maintain normal respiration, enhance muscle strength, and support healthy growth.
Hydrodynamic regimes are vital for aquaculture, as they facilitate waste removal, oxygen circulation, and water-quality maintenance [86,87]. Furthermore, these flow conditions significantly influence fish metabolism, growth, and physiological function. Therefore, the strategic regulation of hydrodynamic regimes is essential for optimizing fish health and maintaining environmental stability.

6. Conclusions

6.1. Problems

The hydrodynamic characteristics of recirculating aquaculture systems (RAS) are critical determinants of internal flow field distribution, solid waste collection and discharge, and the dispersion of soluble and suspended pollutants; moreover, they are critically linked to the health and welfare of farmed species. Despite this importance, hydrodynamic research specifically targeting RAS applications is constrained by several persistent challenges:
(1)
Current research on recirculating aquaculture systems (RAS) in China primarily focuses on species selection, water treatment technologies, fish disease management, and equipment optimization. However, a significant research gap persists regarding the self-cleaning capabilities of aquaculture tanks and their interactions with fish.
(2)
Operational instability and insufficient standardization of hydrodynamic drive equipment characterize current recirculating aquaculture systems. Inconsistent water treatment efficiency leads to substantial water quality fluctuations and increases the risk of equipment-induced stress on fish. Therefore, optimizing both the selection and operational parameters of hydrodynamic drive equipment is essential.
(3)
Studies employing physical model tests or numerical simulations often neglect the influence of aquaculture organisms on both sewage collection and hydrodynamic characteristics within recirculating tanks. Hydrodynamic conditions significantly affect fish behavior, which is high variability. Consequently, accurately predicting fish behavioral responses remains a major challenge.

6.2. Outlook

To address the challenges currently encountered in the hydrodynamic characterization of recirculating aquaculture systems, it is recommended that future research focus on the following areas:
(1)
This study aims to elucidate the interactions between fish and hydrodynamic forces within recirculating aquaculture systems (RAS). Specifically, it will examine interrelationships among fish behavior, locomotion, and flow patterns in recirculating systems. The findings will help establish a theoretical framework for understanding tank hydrodynamics and optimizing solid waste discharge.
(2)
Future research should prioritize advancing the understanding of component interactions within recirculating aquaculture systems. This requires developing a comprehensive hydrodynamic model for RAS, supported by specialized analytical tools. Broadening research perspectives will further strengthen the foundation for successful RAS implementation.
(3)
Promoting interdisciplinary collaboration across aquaculture, biology, environmental science, engineering, and computer science is essential. Such integration will enable rigorous investigation of tank hydrodynamics-fish behavior interactions, providing the scientific basis for efficient and sustainable RAS operation.
(4)
Furthermore, optimizing hydrodynamic drive parameters and advancing research on large-scale tank hydrodynamics and fish-system behavioral interactions are recommended.

Author Contributions

Conceptualization, Q.Z., F.G. and D.F.; Data curation, J.X. and X.Y.; Software, J.C. and Y.W.; formal analysis, Q.Z.; Writing—original draft, J.C. and Y.W.; writing—review and editing, Y.W. and J.C.; visualization, J.C. and Y.W.; Validation, C.J. and Q.Z.; supervision, C.J. and Q.Z.; Resources, Q.Z., F.G. and Q.Z. funding acquisition, Q.Z., D.F. and Y.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research was provided by the Special Project for Building Scientific and Technological Innovation Capacity of Beijing Academy of Agricultural and Forestry Sciences (KJCX20251205); National Natural Science Foundation of China (Grant No. 32273189); Director Fund Project of Fisheries Science Institute, Beijing Academy of Agriculture and Forestry Sciences (JJPY-2025-05) Beijing Fishery Innovation Team of the Modern Agricultural Industrial Technology System (BAIC07-2025-07);Zhou Finance and Agriculture [2024] No. 15 Research and Demonstration on High-Effciency Facility-Based Cultivation of Shrimps; Zhejiang Provincial Regional Test Station Project (2024QYSC02).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding author due to privacy.

Acknowledgments

We greatly appreciate the careful works and constructive suggestions of the editor and all anonymous reviewers.

Conflicts of Interest

The authors declare no conflicts of interest.

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Sustainability 17 07946 g001
Figure 1. Global fisheries and aquaculture production of aquatic animals.
Figure 1. Global fisheries and aquaculture production of aquatic animals.
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Figure 4. Runway type aquaculture tank.
Figure 4. Runway type aquaculture tank.
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Wu, Y.; Chen, J.; Jia, C.; Gui, F.; Xu, J.; Yin, X.; Feng, D.; Zhang, Q. Recent Advances in the Hydrodynamic Characteristics of Industrial Recirculating Aquaculture Systems and Their Interactions with Fish. Sustainability 2025, 17, 7946. https://doi.org/10.3390/su17177946

AMA Style

Wu Y, Chen J, Jia C, Gui F, Xu J, Yin X, Feng D, Zhang Q. Recent Advances in the Hydrodynamic Characteristics of Industrial Recirculating Aquaculture Systems and Their Interactions with Fish. Sustainability. 2025; 17(17):7946. https://doi.org/10.3390/su17177946

Chicago/Turabian Style

Wu, Yanfei, Jianeng Chen, Chengxia Jia, Fukun Gui, Jianuo Xu, Xiaolong Yin, Dejun Feng, and Qingjing Zhang. 2025. "Recent Advances in the Hydrodynamic Characteristics of Industrial Recirculating Aquaculture Systems and Their Interactions with Fish" Sustainability 17, no. 17: 7946. https://doi.org/10.3390/su17177946

APA Style

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

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