Sustainable Recirculating Aquaculture Systems (RAS): Development and Challenges

Abstract

The recirculating aquaculture system (RAS) marks a significant shift in global aquaculture, transitioning to controlled, land-based production. This review highlights technological advancements that enable the treatment and reuse of over 90% of water, thereby enhancing water quality and production efficiency. These features position RAS as a cornerstone of sustainable seafood production. This review introduces the RAS Readiness Level (RRL) framework which is a novel, structured approach to assess the commercial maturity of emerging RAS technologies. Applying the RRL to six key technological domains (from digital AI systems to biological PHB recovery) reveals a pervasive pilot-scale purgatory where most innovations stagnate at RRL 4–6. It further addresses advanced processes such as membrane bioreactors, denitrification reactors, and the conversion of waste into valuable products. Furthermore, this review addresses persistent challenges, including high energy demand, economic viability, and the accumulation of pathogens. Finally, it focuses on the emergent integration of the Internet of Things (IoT) and artificial intelligence (AI), which are revolutionizing RAS management through data-driven optimization. By synthesizing current innovations, this review envisions a future of intelligent, closed-loop RAS where advanced IoT- and AI-driven technologies optimize water quality and feeding strategies to minimize ecological impact while enhancing sustainability and productivity.

1. Introduction

Oceans are cornerstones of global nutrition and food security, but face growing pressures from climate change, uncontrollable pollution, and overexploitation. These pressures, in turn, cause declines in oceanic biological productivity and stability, ultimately leading to significant social and economic losses [1]. In this context, aquaculture has emerged as a crucial sector for global food security, growing at an annual rate of 1.1%, which outpaces the growth of the global population [2]. However, the conventional expansion of aquaculture is constrained by the imperative to maintain production quality while minimizing environmental impacts [3]. This challenge stems from the inefficiency of traditional systems, which drive eutrophication and pollution, with approximately 79% of feed nitrogen and 83% of feed phosphorus lost to the environment [4,5].

Recirculating aquaculture systems (RAS) represent an alternative technological paradigm, offering a viable solution to these dual pressures of rising demand and sustainability [6]. RAS are land-based, controlled systems for culturing aquatic species, in which water is continuously treated and reused [4]. This closed-loop design enables the capture and on-site treatment of waste, significantly reducing the nutrient discharge characteristic of open systems [7]. In contrast, conventional aquaculture which includes flow-through systems, pond culture, and open-net pen farming typically uses water once before discharging it into the environment, often carrying significant nutrient and waste loads [8]. The significance of RAS is underscored by global trends: per-capita seafood supply increased from 9.0 kg in 1961 to 20.2 kg in 2015 [9] and aquaculture now contributes over 50% of the global supply [10]. RAS addresses these growing needs by enabling production within safe ecological limits, even in regions facing water scarcity [11].

Despite these strengths, RAS is not without limitations and faces several challenges that have constrained its profitability and widespread adoption. The primary obstacles are economic, including high capital investment for system components and significant operational costs due to energy-intensive water treatment, pumping, and oxygenation, making RAS more costly than traditional pond culture [12]. RAS operations are complex, requiring skilled technical staff and management, representing a sophisticated interplay between biology and technology [13]. Persistent technical issues also exist; for instance, the development of off-flavors in fish often necessitates a 5–10-day purging period, increasing production costs by 5–7% and reducing economic flexibility [14]. Furthermore, RAS generates significant waste streams, with about 10% of system water lost as sludge or effluent requiring proper disposal [15].

These waste streams, however, present a key opportunity for advancing a circular economy within aquaculture. Advanced strategies for sludge valorization are being employed, including anaerobic digestion, nutrient recovery [16], composting for soil amendment, and its use as feedstock for cultivating low-trophic species such as microalgae, polychaetes, and insects. Converting these wastes into valuable resources is critical for enhancing the overall sustainability and economic feasibility of RAS operations [17].

Within RAS research, significant effort at the pilot scale has focused on optimizing individual components, such as biofilters and solid-removal mechanisms, or assessing environmental impacts [18]. However, a critical gap remains in the holistic investigation of integrated system performance, operational risks (e.g., power outages, alarm failures, challenges in disease treatment), and the economic barriers to commercial profitability [19]. Furthermore, technical progress is hindered by a lack of continuous performance reporting and standardized terminology, leading to miscommunication across the sector [20].

While recent reviews have summarized technological innovations and their impacts, they lack a structured framework to assess commercial readiness [21]. This review directly addresses these persistent innovation-adoption gaps in aquaculture by providing a comprehensive and cross-cutting synthesis of modern RAS. Moving beyond component-level description, it introduces and applies a novel RAS Readiness Level (RRL) framework adapted from Technology Readiness Levels and tailored to the aquaculture context to systematically evaluate not only the technical viability of innovations but also their preparedness for real-world deployment across technical, economic, and regulatory dimensions [22].

Through this diagnostic lens, we re-examine the RAS technological landscape. The RRL framework allows us to: (1) benchmark emerging technologies including IoT, AI, and data-driven management systems against thresholds of commercial viability; (2) identify stage-specific barriers to adoption; and (3) propose targeted strategies to propel technologies through the “valley of death” between pilot-scale demonstration and profitable, at-scale operation. By integrating advances in water quality monitoring, biosecurity, waste treatment, and disease control, this analysis outlines clear pathways for overcoming current limitations and accelerating the transition toward smarter, more sustainable, and economically resilient next-generation RAS.

2. Mechanistic Framework of RAS: From Principles to Components

The core principle of the RAS involves diverting effluent from culture tanks through a series of treatment processes before its reintroduction to the rearing environment. A critical design parameter for RAS is the recirculation ratio, which defines the proportion of recycled water in the total inflow to the fish tanks. Key system components typically include culture tanks, a sump, particulate removal units, biofilters, oxygen injection systems such as U-tube aerators and water circulation pumps. The arrangement of these components and the recirculation flow path is shown in Figure 1 [23].

The basic principle of these systems is to establish a controlled environment that recycles water by integrating processes for waste removal, oxygen replenishment, and the maintenance of optimal physicochemical conditions for the farmed species [24]. The technological efficacy of tank-based RAS is contingent upon the successful management of five core processes: clarification, biofiltration, circulation, aeration, and degassing. Clarification is necessary to remove particulate solids from the recirculating water. Subsequently, toxic ammonia and dissolved organic compounds are eliminated through biofiltration. Adequate circulation must be maintained to transport water between the tank and the various filter components. Finally, aeration and degassing are required to regulate the balance of dissolved gases, namely oxygen and carbon dioxide [25].

The performance of a RAS depends on these five processes, which function as links in a chain; the system’s overall capacity to maintain fish health and biomass is dictated by its weakest component. Among all components, the biofilter is most frequently the critical bottleneck due to its inherent biological sensitivities [7]. Disruption from fluctuations in pH, temperature, dissolved oxygen, alkalinity, toxicants, or hydraulic overloading can rapidly compromise nitrifying bacteria, leading to ammonia and nitrite accumulation that causes acute fish stress and mass mortality within hours [26]. To prevent such failures, measures include robust design with redundancy, real-time monitoring of key water quality parameters, maintaining stable environmental conditions, controlled feeding with effective solids management, bioaugmentation with cultured nitrifying bacteria, and adaptive management strategies such as dynamic modeling. Once these primary safeguards are in place, supplementary technologies such as ultraviolet (UV) disinfection and heating/cooling systems can be integrated for enhanced biosecurity and broodstock conditioning [27,28].

The optimal design and operation of a RAS are fundamentally influenced by the biological requirements of the cultured species, including their tolerance to water quality parameters, stocking density, and feeding behavior (see

Table 1. Commercially cultivated species in recirculating aquaculture system.

	Species	Culture System	Key Advantage	Main Challenge/Need
Fishes	Atlantic salmon [29,30]	Freshwater/marine RAS (Cold)	Excellent feed efficiency; high market value	Susceptible to parasites; needs robust water filtration
	Rainbow trout [31,32]	Freshwater/marine RAS (Cold)	Highly adaptable; strong market presence	Stress-prone at high density; needs precise diets
	Atlantic cod [33,34]	Marine RAS (Cold)	Premium market value; suits cold-water RAS	Very sensitive to crowding; long production cycle
	Arctic charr [17,35]	Freshwater/marine RAS (Cold)	Premium product for niche markets; cold-adapted	Slow growth; sensitive to water quality swings
	Whitefishes [17,36]	Freshwater/marine RAS (Cold)	Performs well in cool water; adaptable	Slow growth; highly sensitive to handling
	Sturgeons [37,38]	Freshwater RAS (Cold)	Exceptional product value (caviar); tolerant	Very long ROI (7–15 yrs); high capital/operational costs
	Pikeperch [39,40,41]	Freshwater/marine RAS (Warm)	Fast growth; high market demand	Stress-sensitive; needs expensive, protein-rich feed; requires economic cooling for reproductive control
	European seabass [34,42]	Marine RAS (Cold-Warm)	High EU demand; adaptable to salinity	Disease-prone; high feed costs
	Gilthead seabream [43,44]	Marine RAS (Warm)	Robust, established protocols; euryhaline	Performance drops at high density; needs specific feeds
	Yellowtails [45,46]	Marine RAS (Warm)	High sushi/sashimi value; wide salinity tolerance	High feed costs; needs stable water quality
	Tunas [47,48]	Marine RAS (Warm-bodied)	Exceptional market value; rapid growth	Extremely high operational costs; complex husbandry
	Turbot [49,50]	Marine RAS (Cold)	High-value flatfish; rapid, efficient growth	Needs precise environmental control; handling-sensitive
	Sole [51,52]	Marine RAS (Warm)	High market value; good RAS adaptability	Slow growth; sensitive to environmental changes
	Groupers [53,54]	Marine RAS (Warm)	High demand for live fish; rapid growth	Handling-sensitive; needs high-quality feed & biosecurity
	Eels [55,56]	Marine RAS (Temperate)	High market value; tolerant of high density	Relies on wild juveniles; high early mortality
	Common carp [57,58]	Freshwater RAS	Extremely tolerant; grows on low-protein diets	Disease-prone at high density; low value in west
	Catfishes [55,59]	Freshwater RAS (Warm)	Very resilient; excellent growth & feed efficiency	High disease pressure; prone to off-flavors
	Tilapia [55,60]	Freshwater RAS (Warm)	Robust, fast growth on low-protein feed	Uncontrolled breeding; vulnerable to pathogens
	Pangasius [61,62]	Freshwater RAS (Hot)	Rapid growth on low-protein diets; tolerant	Disease-prone; handling-sensitive
	Perch [63,64]	Freshwater RAS (Cool)	High market value; good RAS fit	Stress-sensitive; slow growth
	Cobia [65]	Marine RAS (Warm)	Exceptionally fast growth; high market value	Handling-sensitive; needs high-protein feed
	Barramundi [66,67]	Freshwater/marine RAS (Warm)	Fast growth; high demand; versatile salinity	Sensitive to water quality; needs specific conditions
Crustaceans	Freshwater prawn [68]	Freshwater RAS (Warm)	High demand/value; environmentally tolerant	Disease/parasite susceptibility; needs careful feeding
	Crayfish [69]	Freshwater RAS (Cold)	High culinary value; suited to polyculture	Susceptible to plague; slow growth/cannibalism
	Pacific white shrimp [70,71]	Marine RAS (Warm)	Superior growth; global market dominance	High biosafety/disease risk; needs stable water quality and rearing welfare assessment
	Marine prawns [72]	Marine RAS (Warm)	High demand; high-density potential	Extremely stress/disease-sensitive; needs intensive management
	Lobsters [73]	Marine RAS (Cold)	Exceptional live market value; takes formulated feed	Extremely slow growth; sensitive to water quality & handling
	Crabs [74,75]	Marine RAS (Warm)	High demand/price; adaptable feeders	High cannibalism during molt; sensitive to water quality
Echinoderms	Sea cucumber [76]	Marine RAS (Warm/Temperate)	High luxury market value; low feeding cost	Disease-prone at high density; very slow growth
	Sea urchin [77]	Marine RAS (Cold/Warm)	High value for roe (‘uni’); seaweed-based diet	Slow growth; sensitive to environmental changes

for a comparative analysis of key commercial species).

Advantages of Recirculating Aquaculture System

RAS offer significant, quantifiable environmental advantages over conventional aquaculture. The principal benefits can be categorized as follows: (1) a drastic reduction in water consumption, achieved through continuous water treatment and reuse making RAS particularly suitable for regions with limited water resources; (2) a substantially smaller land footprint enabled by high stocking densities; (3) precise control over critical water quality parameters, including temperature, dissolved oxygen (DO), carbon dioxide (CO₂), total ammonia nitrogen (TAN), and nitrate levels; (4) enhanced biosecurity, as water intake is minimal and highly controlled unlike flow-through systems that continuously draw and discharge large volumes of untreated water [78]; (5) prevention of pathogen and parasite release into natural water bodies, with reduced reliance on antibiotics and therapeutants; (6) elimination of fish escapes, preventing genetic pollution of wild stocks; and (7) logistical flexibility, as RAS can be sited in inland or metropolitan areas, reducing transport costs and carbon emissions while enabling premium pricing for locally produced seafood [79].

Additional advantages include improved waste management and nutrient recycling, the prevention of biological pollution by eliminating fish escapes, reduced energy consumption for water temperature regulation, and lower CO₂ emissions associated with product transport. A critical enabling factor for these waste management benefits is the concentration of metabolic wastes including ammonia, nitrate, and organic solids into a much smaller effluent volume (10–100 times more concentrated than flow-through discharge) [80]. Unlike flow-through systems where dilute wastes are dispersed and impossible to capture, RAS produces a manageable, concentrated waste stream that enables efficient treatment and resource recovery [34,80]. This concentration transforms a potential pollution liability into valuable co-products through aquaponics, sludge processing into fertilizers, or bio-stock production, creating closed-loop resource cycles [34,80].

Operationally, RAS enable optimal fish culture conditions through precise management of water flow, stocking density, and physiological balance [20]. This operational precision, combined with the system’s closed-loop design, supports consistent production outcomes while maintaining the resource efficiency and environmental benefits outlined above [81]. Collectively, these attributes contribute to a minimal environmental footprint, positioning RAS as a sustainable production alternative [82]. Table 1 provides an overview of commercially cultivated species in RAS detailing their key strategic advantages which inform species selection for maximizing productivity and sustainability in recirculating aquaculture.

Therefore, the quantifiable benefits of RAS firmly establish it as a sustainable alternative. However, a critical readiness disparity exists in the current technological landscape. Commercially mature engineering advances, such as optimized octagonal tank geometries, have achieved up to 65% reduction in pumping energy, whereas AI-driven automation and microalgae integration face persistent barriers including high costs, scalability limitations, and insufficient operational expertise [21]. This alignment between independent review findings and our RRL assessment validates the existence of a critical translational gap that must be bridged for next-generation RAS to achieve its full potential.

To further enhance the sustainability and economic feasibility of RAS, the focus of wastewater treatment is shifting from mere nutrient removal to nutrient recovery. In this context, a bioremediation process which includes using living organisms to purify wastewater presents a promising approach. However, microalgae, in particular, have also emerged as an efficient biotechnological platform for the purpose of wastewater treatment. When cultivated in photobioreactors using RAS side streams, microalgae not only purify wastewater but also produce valuable biomass [83]. This biomass, which is rich in proteins, lipids, and carbohydrates, can be converted into high-value products such as aquafeed ingredients, nutraceuticals, or biogas via anaerobic digestion. However, as reflected in the RRL assessment (RRL 4), the commercial application of microalgae-based treatment remains constrained by challenges including the large surface area required for sufficient light exposure and the prohibitive costs of biomass harvesting and processing [84]. Thus, while microalgae integration offers a multi-functional advantage for wastewater purification, nutrient recovery, and sustainable aquafeed production [85], significant translational barriers must be overcome before it can be considered a commercially mature technology.

Finally, the RAS is moving towards further advancement by using a technology-based system. A conceptual overview of this AI-integrated, closed-loop system is shown in Figure 1. The integration of deep learning-based intelligent systems represents a key driver for enhancing the sustainability, productivity, and operational efficiency of RAS, paving the way for the next generation of smart, sustainable recirculating aquaculture systems.

This controlled environment prevents the release of pathogens, parasites, and metabolic wastes such as ammonia, carbon dioxide, and organic compounds into natural water bodies. With this foundational understanding established, the discussion now turns to how RAS technology has matured over time. Tracing its historical evolution reveals a trajectory of continuous innovation, driven by environmental imperatives and advances in treatment engineering.

3. Recent Development of RAS

The development of RAS is driven by increasingly rigorous environmental regulations, particularly in regions with limited access to land and water. Broader legislative frameworks, such as the EU Water Framework Directive (2000/60/EC) [86], while not targeting aquaculture directly, have further catalyzed the innovation of environmentally sound production systems. As these systems do not depend on surface water bodies, they can be operated in contained “indoor” or “urban” settings. While RAS offers significant advantages, opportunities for further improvement are explored in the following sections.

3.1. Historical Development

The development of RAS spans over 65 years, originating in Japan in the 1950s, with early commercial success for eel production in Denmark in the 1980s [17]. Since the 2000s, global expansion has accelerated due to tightening environmental regulations and water scarcity, with applications diversifying from carp farming to sophisticated water quality management and nutrient recycling [87]. Today, RAS has become a versatile platform for high-value species, including salmonids and Mediterranean fish, with Norway’s salmon industry standardizing RAS for smolt production and increasingly adopting land-based grow-out systems [88].

The increase in the number of publications [7,17], as depicted in Figure 2A, for the period 2002–2026, reflects a consolidating consensus regarding the value and applicability of the RAS. The data were obtained from the Web of Science by combining “recirculating aquaculture system” with additional keywords: “sustainable recirculating aquaculture,” “RAS,” “recirculating aquaculture system challenges,” and “RAS development.” Figure 2B also presents the co-occurrence analysis, highlighting the significance of RAS in aquaculture research. This trend is projected to continue as the technology matures and finds new applications. This progression to modern RAS has been largely achieved through iterative innovation and practical experience [88].

3.2. Technological Advancements

Conventional recirculating aquaculture systems (RAS), which utilize mechanical filtration and biofiltration, exhibit a lower environmental impact particularly regarding eutrophication compared to flow-through systems, despite their reliance on significant water exchange. Recent technological innovations have further enhanced this profile. The integration of denitrification reactors, advanced sludge thickening technologies, and ozone treatment has substantially reduced water usage, waste discharge, and energy consumption [89]. For instance, the application of flocculant bacteria and lignocellulosic materials has demonstrated a capacity to remove over 90% of nitrogen, phosphorus, and organic matter, converting RAS effluent into a concentrated “manure cake” suitable for composting and use as a fertilizer [90]. This concentration of waste streams facilitates their reuse in integrated, closed-loop systems, thereby advancing the environmental sustainability of RAS. Continued innovation and policy support remain crucial to address persistent challenges, including the optimization of waste management strategies and the integration of alternative energy sources [23].

3.2.1. Denitrification Reactors

In conventional RAS, water exchange rates (typically 0.1–1 m³/kg feed) are often determined by the maximum allowable nitrate (NO₃⁻) concentration resulting from the nitrification of ammonia. Denitrification reactors are employed to mitigate nitrate accumulation. These systems, which can be positioned upstream of nitrification and degassing units, facilitate the microbial reduction of nitrate to nitrogen gas under anoxic conditions, thereby improving overall water chemistry management [30]. Solid-phase denitrification reactors, for example those utilizing polymers like poly(3-hydroxybutyrate-co-3-hydroxyvalerate), have proven highly effective at nitrate removal and promote specific bacterial communities conducive to the process. The efficacy of these reactors depends on precise control of carbon sources and reactor design, with future research focused on optimizing these parameters to enhance performance and sustainability across diverse aquaculture environments [91]. It is considered to be crucial for next-generation solid-phase denitrification to use biopolymers like PHB, an innovation currently assessed at RRL 5, where the primary barrier to advancement is the high cost of the solid carbon source compared to conventional alternatives.

3.2.2. Sludge Thickening Technologies

Sludge thickening technologies are vital for waste management in RAS, as they reduce the volume of solids including fish excrement, uneaten feed, and bacterial biomass thereby lowering the costs associated with storage, transport, and disposal [92]. Effective techniques involve an initial solid/liquid separation followed by a thickening step, such as flotation that uses gas bubbles to make floc particles buoyant, simplifying the system structure and improving sludge outflow. The integration of plate filtering and sedimentation tanks, often enhanced with flocculants, can increase solid content from 3% to over 9%, which subsequently improves dewatering efficiency and reduces energy consumption [93]. Beyond volume reduction, these technologies enable nutrient recovery. The resulting concentrated sludge can be processed via anaerobic or aerobic digestion to produce liquid fertilizers, thereby recycling nutrients back into agricultural or aquaponic systems and enhancing overall sustainability by improving water quality and system efficiency [94].

3.2.3. Ozone and Ultraviolet Irradiation

RAS commonly employ ozone and ultraviolet (UV) irradiation as primary disinfection methods [95]. While both are used to control pathogens, they operate through distinct mechanisms and exert different influences on microbial communities and overall water quality. Ozone is a potent oxidizing agent that effectively inactivates a broad spectrum of microorganisms, including bacteria, viruses, fungi, and protozoa. Beyond disinfection, it enhances water quality by oxidizing dissolved organic compounds, reducing turbidity, and eliminating color and odor [96]. In RAS, ozonation contributes to a stable aquatic environment by preventing nitrite accumulation and accelerating nitrate decomposition [97]. This process facilitates the removal of nitrogenous wastes and dissolved carbon, thereby enabling higher feed inputs and improved growth rates. Specific applications include suppressing infections and oxidizing nitrite (NO₂⁻) to nitrate. Recent advancements, such as electric field-based ozone nanobubbles (EF-ONBs), demonstrate potential for maintaining high water purity with reduced reliance on UV irradiation, offering a more sustainable treatment alternative [98].

Ozonation effectively mitigates the accumulation of dissolved organic matter (DOM), a primary cause of water discoloration. Its strong oxidative capability is demonstrated in wastewater treatment, where it can remove up to 90% of color, underscoring its utility in addressing chromatic issues in water systems [99]. In contrast, ultraviolet (UV) irradiation inactivates microbial pathogens by damaging their DNA and inhibiting replication, a mechanism widely used to control microbial populations in aquaculture [100]. However, while effective at disinfection, UV irradiation can alter the aquatic microbial composition, potentially impacting the broader ecosystem stability within the RAS. Furthermore, UV is less efficacious than ozone in enhancing chemical water quality, as it does not directly address issues such as nitrite accumulation or facilitate nitrate decomposition [97].

In summary, while both ozone and UV are effective disinfectants, ozone offers the additional benefit of directly improving chemical water quality by oxidizing dissolved wastes. Beyond the management of pathogens and organic matter, a primary water quality challenge in intensive RAS is the accumulation of dissolved gases, particularly carbon dioxide (CO₂). The ozone and UV approach is considered a standard, commercially mature disinfection method in modern RAS, consistent with its RRL 8 assessment. Its path to full optimization (RRL 9) involves fine-tuning to balance efficacy with energy use and byproduct management.

3.2.4. CO₂ Utilization in Recirculating Aquaculture System

Elevated carbon dioxide (CO₂) levels in aquaculture water can create a detrimental environment for fish health. The serial adoption of CO₂ removal devices in intensive RAS across the United States and Europe has been a critical development, enabling significant increases in fish productivity per unit of water. Among these technologies, stripping columns have become the most widely deployed method for CO₂ removal [101]. Despite these advancements, CO₂ accumulation remains a challenge in high-production RAS, where concentrations can exceed safe thresholds and pose a direct risk to fish. This issue is compounded by the potential buildup of other toxic metabolites, presenting a particular threat to sensitive species like salmonids that require pristine water conditions [55]. While numerous studies have documented the effects of elevated CO₂ on fish development and welfare, its broader impacts on various water quality parameters within RAS are not yet fully understood but the recent advancements in RAS are aiming to resolve this issue [102].

Interestingly, the strategic use of CO₂ can also present an opportunity to enhance water quality. By facilitating algal growth, CO₂ can be harnessed to support oxygen production and nutrient uptake. The integration of microalgae transforms nutrient-rich waste streams into valuable biomass, improving system sustainability by concurrently addressing oxygenation and nutrient recovery [4]. However, the efficacy of this approach depends on balanced nutrient management, as the often nitrate-limited effluent in RAS can constrain algal biomass production and thus limit the potential water quality benefits of CO₂ utilization [103].

3.3. Membrane Technologies

Membrane separation processes, including microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO), are increasingly critical for advanced water quality management in RAS. These technologies efficiently remove a broad spectrum of pollutants from suspended solids and pathogens to dissolved nutrients and organic matter ensuring optimal culture conditions [104]. Their integration significantly reduces water consumption and enhances the environmental and economic sustainability of aquaculture operations, particularly in intensive sectors like shrimp production [105].

Microfiltration (MF) and ultrafiltration (UF) serve foundational roles. MF is highly effective at reducing turbidity and pathogen load, making it an essential pretreatment step for more advanced processes like RO [105]. When MF is integrated into a membrane bioreactor (MBR), it combines biological treatment with physical separation, simultaneously reducing organic load and nutrient concentrations [106]. UF provides a finer level of purification, effectively removing bacteria, viruses, and dissolved organic matter. This process is crucial for preventing disease outbreaks and can achieve high removal efficiencies for chemical oxygen demand (COD) and total nitrogen (TN), producing effluent suitable for direct reuse within the RAS [107].

Nanofiltration (NF) and Hybrid Systems offer advanced treatment capabilities. The application of NF, particularly when preceded by multimedia filters (MMFs) and activated carbon filters (CFs), has demonstrated remarkable efficacy, achieving a 97.5% reduction in ammonia and a complete (100%) removal of nitrite, alongside significant reductions in nitrate and total suspended solids (TSS) [108]. Specific NF membranes, such as those made from polysulfone (PSF), can retain 66% of ammonia, 85% of total ammonia nitrogen (TAN), and 95% of phosphorus, with performance optimized at specific conditions like pH 6 [109]. Another innovative hybrid approach is the Membrane-Aerated Biofilm Reactor (MABR), which, when integrated with a RAS, has shown significant removal efficiencies for turbidity (50.0%), COD (40.5%), and ammonia (35.0%) in Nile tilapia production [110].

The commercial readiness of advanced membrane systems varies, as captured by the RRL framework. While membrane technologies provide substantial benefits, a holistic treatment strategy often necessitates their integration with complementary methods. Membrane bioreactors (MBRs) exemplify a technology at RRL 7 successfully demonstrated in commercial facilities but face a common barrier to universal adoption including high operational costs and complexity from membrane fouling. Biological treatments, such as bio-floc technology and constructed wetlands, alongside physiochemical processes like advanced oxidation, can be combined with membrane systems to enhance overall treatment efficacy, address specific contaminants, and provide a more resilient and comprehensive solution for sustainable recirculating aquaculture system efficacy and can further be enhanced by further advancements in membrane technology [111].

3.4. Recycling of Nutrients Through Integrated Farming

The fundamental principle of a RAS is the physical segregation of the fish culture tank from dedicated water treatment units. Conceptually, a traditional aquaculture pond functions as a single-reactor RAS, where processes such as sedimentation, nitrification, denitrification, and decomposition occur simultaneously [112]. In engineered RAS, segregating these functions into specialized reactors enhances control and increases the system’s overall production capacity. However, integrating phototrophic processes like algal cultivation presents a scaling challenge; the surface area which is required for phototrophic reactors often exceeds that of the fish tanks by an order of magnitude due to their relatively low treatment efficiency [113]. Furthermore, recycling algal biomass as feed can reintroduce nutrients, potentially reducing the net nutrient removal efficiency of the treatment process by up to 90%. Despite these challenges, nature-based solutions like constructed wetlands and algal ponds have garnered significant interest for their ability to facilitate water reuse and nutrient recovery within a circular bioeconomy framework. There is a need of finding a solution to mitigate the nutrient load effect of algal bloom which can increase nutrient removal efficiency and sustainability, and technology-based systems can help to reduce such challenges [114].

3.4.1. Constructed Wetlands

Constructed wetlands (CWs) offer a robust, nature-based solution for treating concentrated aquaculture effluents, which are typically 20–25 times stronger than standard municipal wastewater [115]. Both subsurface flow and floating treatment wetlands (FTWs) demonstrate high efficacy in removing key pollutants. For instance, CWs can achieve removal efficiencies of 61.5% to 91.9% for biochemical oxygen demand (BOD), total suspended solids (TSS), and nitrogenous compounds, while FTWs can remove up to 91.30% of total phosphorus (TP) [116].

The integration of CWs into RAS not only improves water quality but also directly enhances production outcomes. Studies on channel catfish and Pacific white shrimp have shown that CWs improve growth and survival rates, reduce feed conversion ratios, and can enable a ~40% increase in fish biomass without a proportional rise in operational costs, thereby boosting profitability [117,118]. As low-energy, low-maintenance systems, CWs promote sustainable aquaculture by minimizing environmental impact. A primary limitation, however, is the variable and often lower removal efficiency for phosphate, necessitating precise design and optimization for specific aquaculture applications [118]. Constructed wetlands have robust performance data from pilot-scale integrations, placing them at RRL 6. The primary barrier to integration with intensive, land-constrained commercial RAS (RRL 7+) is their significant spatial footprint and seasonal variability.

3.4.2. Microalgae-Based Water Treatment

Microalgae-based treatment represents a versatile method for advanced nutrient recycling in RAS. Microalgae assimilate inorganic nitrogen and phosphorus, heavy metals, and other pollutants, simultaneously enhancing water quality and producing valuable biomass. High-rate algal ponds (HRAPs) have been shown to remove 59% of dissolved nitrogen and 56% of phosphorus from marine aquaculture effluent. Co-culturing microalgae like Tetradesmus dimorphus with nitrifying bacteria can efficiently remove ammonium, nitrate, and phosphate while maintaining dissolved oxygen levels [119]. The resulting algal biomass can be harvested for use as a sustainable feed additive or processed into biogas via anaerobic digestion, enhancing the system’s circularity [120].

The performance of algal reactors is highly dependent on design parameters such as light availability, hydraulic retention time (HRT), and reactor configuration [121]. A significant challenge is the large surface area required for sufficient photosynthesis. While innovations in reactor design and artificial lighting can mitigate this, they often increase capital costs. Nevertheless, the long-term benefits, including reduced water discharge cost and the generation of valuable co-products like biofuels and nutraceuticals which justify the initial investment, create a more efficient biological cycle within the aquaculture operation [34]. Microalgae-based nutrient recovery is a promising circular solution firmly situated in pilot-scale purgatory (RRL 4). The major scaling challenge (large surface area for light) and prohibitive costs of biomass harvesting and processing are the key translational barriers identified by the RRL framework.

3.4.3. Mechanism of Microalgae-Based Biomass Production

Microalgae produce valuable biomass through photosynthetic carbon fixation, utilizing CO₂ and sunlight to synthesize proteins, lipids, carbohydrates, and pigments with photosynthetic efficiency. In practice, biochemical composition is manipulated through controlled stressors such as nitrogen starvation or high salinity, which redirect cellular metabolism toward accumulation of target compounds like polyunsaturated fatty acids or astaxanthin [122]. A two-stage cultivation approach is often employed: biomass is first maximized under optimal conditions; then, stress is applied to trigger metabolite overproduction without compromising yield [84]. This biological machinery integrates into a circular bioeconomy by cultivating microalgae on non-arable land using RAS wastewater, converting waste streams into nutrient-rich feed ingredients while minimizing environmental impact [122]. However, as reflected in the RRL assessment (RRL 4), commercial application remains constrained by harvesting costs and scalability challenges.

3.5. Innovative Systems: The RAS-PHB System

A groundbreaking innovation in sustainable aquaculture is the integration of polyhydroxybutyrate (PHB) production within RAS. The RAS-PHB system addresses multiple sustainability challenges by converting aquaculture waste into valuable bioproducts while enhancing water quality. PHB serves as an effective carbon source for denitrifying bacteria, significantly reducing nitrogen pollution [123]. In studies with largemouth bass, PHB supplementation reduced nitrate, nitrite, and total nitrogen concentrations by 99.65%, 95.96%, and 85.22%, respectively, by enriching denitrifying microbial communities and upregulating key genes [123].

This process improves water quality and fish health without chemical interventions. Economically, the system utilizes salt-tolerant strains like Zobellella denitrificans ZD1 to convert nitrogen from agro-industrial wastes (e.g., glycerol, cheese whey) into PHB-rich biomass, achieving a 97% harvest efficiency with minimal chitosan [124]. This biomass can be used to produce protein-rich, immune-enhancing fish feed, reducing reliance on conventional feed and antibiotics. The approach is highly cost-effective, lowering production costs of fish from $1.2–1.6/kg to $0.6–0.7/kg, a 56% reduction [124].

The RAS-PHB model exemplifies a circular bioeconomy, aligning with advanced RAS trends that emphasize resource recovery, waste valorization, and minimal environmental footprint [34]. When combined with other innovations like electrochemical water treatment and energy-efficient designs, it represents a comprehensive solution for the future of sustainable aquaculture [125]. The integrated RAS-PHB system epitomizes the circular bioeconomy innovations assessed at RRL 5. Its technical viability is proven at pilot scale, but its progression is directly blocked by the economic barrier of PHB production cost versus conventional carbon sources, illustrating the translational ‘valley of death’ for sustainable technologies.

Thus, nature-based solutions like constructed wetlands, algal ponds and RAS-PHB system provide a pathway for closing the nutrient loop, transforming waste into valuable biomass. To optimize these complex biological and physicochemical processes, the industry is increasingly turning to data-driven management through automation and artificial intelligence.

4. Integration with Automation and Artificial Intelligence

Despite their transformative potential, the RRL framework positions digital automation and AI among the least mature domains for commercial deployment (RRL 4–6). This underscores that the primary barrier is not algorithm development but proving long-term reliability and return on investment. Nevertheless, these smart technologies are vital for navigating the complexities of sustainable aquaculture, allowing for enhanced productivity while minimizing environmental impact [126]. The convergence of AI, machine learning (ML), and the Internet of Things (IoT) facilitates data-driven decision-making, optimizing everything from water quality to feeding regimes, thereby improving operational efficiency, fish welfare, and overall system sustainability [127,128].

4.1. Intelligent Water Quality Management

IoT sensors provide continuous, real-time monitoring of critical water parameters such as temperature, pH, dissolved oxygen (DO), and salinity. This data stream is analyzed by AI algorithms, which generate predictive insights and enable proactive interventions. Hybrid ML models, such as the DTKNN+ framework that integrates decision trees and k-nearest neighbors, have demonstrated assessment accuracy up to 99.28%, significantly surpassing traditional methods [129]. This intelligent automation ensures stable water conditions, minimizes manual labor, and prevents issues before they arise, creating a more resilient aquaculture environment.

4.2. Optimized Aeration Strategies

AIoT-based smart aerators dynamically adjust aeration in response to real-time data, maintaining ideal dissolved oxygen levels while significantly reducing energy consumption. For instance, systems utilizing computer vision for hypoxia detection and biological energy models have achieved a 26.3% reduction in energy use compared to manual control [130]. These strategies, often supported by dual-power supplies and renewable energy, enhance reliability and operational efficiency while accelerating fish growth cycles [17,131].

4.3. Smart Feeding Algorithms

AI-driven feeding systems utilize computer vision and ML to analyze fish behavior and environmental conditions, preventing both over- and underfeeding. Models like support vector machines (SVMs) can predict biomass and calculate optimal feed rations in real time, improving growth rates and feed conversion ratios while minimizing waste [127,132]. Systems such as the Fish Feeding Automation and Aquaponics Monitoring System (FFAAMS) automate feeding schedules based on biomass and growth rates, aligning with the principles of precision aquaculture [133].

4.4. Comprehensive Environmental Monitoring and Control

The Artificial Intelligence of Things (AIoT) enables holistic environmental control by integrating sensor networks with automated actuators. These systems continuously monitor water quality, automatically adjust parameters, and provide early disease detection by analyzing shifts in animal behavior and health indicators [17,134]. This reduces human error, enhances biosecurity, and ensures optimal conditions with minimal manual intervention [135].

4.5. Variable-Flow Water Regulation

Intelligent flow control models use ML techniques, such as long short-term memory networks, to dynamically adjust water flow rates based on real-time turbidity data. This optimizes filtration efficiency, maintains system stability, and reduces resource consumption [136]. Automated pump and aerator control, guided by wireless sensor data, further enhances energy efficiency and operational convenience [137].

4.6. Advanced Fish Monitoring

Precision fish farming leverages data-driven technologies including machine vision, edge computing, and deep learning to monitor fish welfare and behavior. For example, video analytics can detect stress responses in Atlantic salmon exposed to sublethal levels of hydrogen sulfide (H₂S) by identifying erratic swimming and loss of schooling behavior, providing early warnings before toxicity thresholds are reached [17,138]. These tools are also applied to disease diagnosis, behavior analysis, and logistics management, significantly improving productivity and animal welfare [139].

The integration of AI and IoT fundamentally transforms RAS from a manually monitored facility into a precision-controlled, predictive production environment. This data-driven approach can be extended beyond operational control to also revolutionize waste valorization, as exemplified by groundbreaking systems that convert aquaculture waste into biodegradable polymers. Yet, despite these technological breakthroughs, a persistent gap remains between innovation and commercial adoption. To diagnose this disconnection, the following section introduces a structured framework for assessing the maturity of RAS technologies.

5. RAS Foundations and the Readiness Gap

5.1. RRL Framework for RAS Technologies

To provide a structured assessment of RAS technology maturity, we define a nine-level RAS Readiness Level (RRL) scale adapted from the Technology Readiness Level (TRL) framework. The RRL framework, detailed in

Table 2. The 9-level RAS Readiness Level (RRL) framework which is a concept from [22].

RRL	Stage Name	Description
1	Basic Principles Observed	Scientific principles are reported. No experimental proof in an aquaculture context (e.g., a novel metabolic pathway is described).
2	Technology Concept Formulated	A practical application for RAS is postulated. A conceptual design or process flow is proposed.
3	Experimental Proof of Concept	Critical function is validated in a controlled, small-scale lab setup (e.g., benchtop reactor).
4	Technology Validated in Lab/Pilot	Core components are integrated and tested in a simulated RAS environment (e.g., a pilot tank system at a research facility). Performance is measured against key benchmarks.
5	Technology Validated in Relevant Environment	Prototype is tested in an integrated pilot-scale RAS, facing more realistic operational challenges (e.g., variable loading, water matrix effects). Preliminary techno-economic data is gathered.
6	Technology Demonstrated in Relevant Pilot Environment	A representative prototype operates in a near-commercial pilot RAS for an extended period (>6 months), demonstrating reliability and generating operational cost data.
7	System Prototype in Operational Environment	A full-scale prototype is successfully demonstrated in an actual commercial RAS facility. The technology performs its intended function in the real-world production chain.
8	System Complete and Qualified	The technology is proven and adopted by multiple commercial operators. Its performance, costs, and benefits are well-documented in the industry literature.
9	Actual System Proven in Commercial Operation	The technology is the industry standard for its application. It is competitively available with reliable supply chains and has a definitive return-on-investment model.

, evaluates technologies from basic principles (RRL 1) to full commercial adoption (RRL 9) across three dimensions: validation scale, economic data availability, and documented limitations. Despite technological progress in RAS, a significant gap remains in assessing their proximity to commercial adoption. Many promising innovations become stuck at the pilot-scale stage, unable to transition into mainstream industry application due to persistent economic, operational, or system-integration challenges. To bridge this gap, a novel RAS Readiness Level (RRL) framework is employed, and a structured tool is applied which goes beyond descriptive analysis to offer a diagnostic method for assessing the commercial maturity of core RAS technologies, emphasizing real-world applicability and scalability.

The RAS Readiness Level (RRL) Framework

The RRL framework adapts the established concept of Technology Readiness Levels (TRL) to the specific context of land-based aquaculture. TRLs provide a systematic metric for assessing a technology’s maturity from basic principle (TRL 1) to proven operational use (TRL 9) [22]. For natural resource management, research typically originates at TRL 1–2 with unproven concepts, advances to TRL 3–4 via proof-of-concept testing, and reaches TRL 5–6 through validation in increasingly relevant environments, often supported by modeling. Technologies at TRL 7 and higher are considered fully demonstrated and ready for application [140,141].

A corresponding nine-level RRL scale is defined in which RRL 1 represents initial scientific observation and RRL 9 signifies full commercial adoption (Table 2). Assigning an RRL score requires synthesizing evidence on three key dimensions: (1) the scale of validation (lab, pilot, or commercial), (2) the availability of reliability and economic data, and (3) the explicit limitations or barriers cited in the literature. The following assessments apply this framework to evidence drawn from the technological domains reviewed in Section 4, Section 5 and Section 6.

5.2. RRL Assessment of Core Technological Domains

Applying the RRL framework to six pivotal domains reveals a landscape of varying maturity and distinct bottlenecks (

Table 3. RAS Readiness Level (RRL) assessment of core technological domains. Evidence grading: Evidence strength is classified as strong (multiple peer-reviewed studies from different research groups with commercial validation), moderate (several pilot-scale studies with consistent results), or preliminary (proof-of-concept or single study).

Technology Domain	Primary Function	Assessed RRL	Evidence Synthesis for RRL Score	Critical Barrier
AI & IoT Predictive Management	Data-driven optimization of feeding, water quality, and health monitoring	RRL 4	Moderate. Pilot-scale algorithms demonstrate efficacy [127,128,130,132]; commercial-scale longitudinal data absent [78].	No long-term commercial trials, Sensor durability under biofouling unproven, ROI models lacking
PHB-Denitrification Systems	Advanced biofiltration converting waste nitrogen into biodegradable polymer	RRL 5	Moderate. Pilot validation: 99.65% nitrate removal [123]; 56% cost reduction [124]; optimization incomplete [91].	High production cost, Uncompetitive with conventional carbon sources
Membrane Bioreactors (MBRs)	High-efficiency solids separation and biological treatment	RRL 7	Strong. Commercial deployment documented: 50% turbidity, 40.5% COD, 35% ammonia removal across species [105,110,111].	Membrane fouling, High operational costs
Ozone–UV Hybrid Disinfection	Pathogen control and oxidation of dissolved organic matter	RRL 8	Strong. Standard commercial RAS component validated across salmon, shrimp, and marine systems [95,96,97,98].	Toxic byproduct formation in saline water, High energy demand
Constructed Wetland Integration	Nature-based, passive polishing of effluent and nutrient recovery	RRL 6	Moderate–strong. Consistent pilot performance: 61.5–91.9% removal of BOD, TSS, nitrogen [116,117,118].	Large land footprint, Seasonal performance variability
Microalgae Nutrient Recovery	Bioremediation of wastewater with concurrent biomass production	RRL 4	Moderate. Nutrient assimilation proven in pilot photobioreactors [4,85,119]; scaling constrained by surface area and harvesting costs [121].	High harvesting cost, Energy-intensive dewatering

).

The RRL assessment reveals a clear dichotomy. Physical-engineering technologies such as Ozone–UV disinfection (RRL 8) and membrane bioreactors (RRL 7) are nearing or at commercial maturity. In contrast, innovations in the circular bioeconomy (e.g., PHB, microalgae) and digitalization (AI/IoT) are predominantly stalled at the pilot validation stage (RRL 4–6). This disparity is independently validated by existing reviews, which note that while engineering advances in tank geometry and filtration have achieved widespread commercialization including up to 65% energy reduction in pumping, AI-driven automation and microalgae integration remain constrained by high costs, scalability limitations, and insufficient operational expertise. These reviews further indicate that pilot-scale AI models with >90% accuracy often decline to approximately 75% in commercial settings, and microalgae harvesting costs account for 30–40% of operational expenditure barriers that align precisely with the RRL 4–6 assessment [21].

This pilot-scale purgatory indicates that the primary constraints are no longer technical feasibility but translational challenges: specifically, economic viability, seamless system integration, and a lack of robust, longitudinal commercial performance data. The following Section 4, Section 5, Section 6 and Section 7 detail these technologies and their associated challenges through the clarifying lens of their RRL positioning.

The readiness assessment reveals that while some technologies approach commercial maturity, critical gaps persist, particularly in areas affecting the health of cultured organisms. This leads to an investigation of how welfare is managed within intensive recirculating systems.

6. Welfare in RAS

RAS enable intensive production with high stocking densities and minimal water exchange, making the optimization of animal welfare a critical and complex factor for success, determining that optimal stocking density is essential to balance productivity with fish health. Table 1 lists commercially cultivated species that are suitable for welfare-focused sustainable recirculating aquaculture systems. Welfare considerations are also shaped by production strategy, as small-fish production shows higher efficiency in controlled environments, while large-scale operations (>1000 tonnes) may suit lower-value species if density is managed, and smaller facilities require high-value species to remain profitable while investing in enhanced welfare monitoring [142].

Recent technological trends reflect this alignment of welfare and production, with salmon farming moving toward larger smolt (up to 400 g) and the development of 10,000-tonne land-based facilities that prioritize environmental control [143]. While RAS provides a stable, controlled environment allowing for cultivation outside the native range of a species and protection from predators and adverse weather, high densities can induce stress, leading to issues like aggression and cannibalism if not managed carefully. High stocking densities in RAS can induce physiological stress responses, affecting growth performance and survival. Studies on Colossoma macropomum have demonstrated that optimal density management is essential to balance productivity with fish health in recirculating systems [69,144].

The core technological strength of RAS directly supports welfare by maintaining superior water quality. The recirculation process, centered on biofiltration (nitrification), continuously removes toxic ammonia and nitrite, creating a stable and healthy physiological environment for fish [145]. This stable environment effectively reduces metabolic stress in cultured fish, leading to improved feed appetite, growth rates, and survival [145], particularly during sensitive life stages such as hatchery seed [146]. However, high stocking densities in RAS can lead to welfare challenges including aggression, cannibalism, and chronic stress if not carefully managed, particularly for species with territorial behavior [69]. Chronic low-level stressors, including subclinical nitrate accumulation, remain under-monitored in commercial RAS. Elevated nitrate concentrations have been shown to impair growth and cause histopathological alterations in fish gills, including lamellar fusion, epithelial hyperplasia, and lamellar shortening, demonstrating that even sublethal conditions have measurable welfare consequences [147].

The integration of technological advancements into RAS creates a predictable and manageable environment, resulting in a viable production model for food fish and restocking programs that ensures high reliability and animal welfare. Welfare considerations are particularly critical during nursery phases, where high-density rearing requires precise environmental control to ensure survival and growth. RAS has been shown to support high survival rates in catfish nurseries when water quality is properly maintained [146].

Collectively, these attributes allow RAS to minimize its ecological footprint while maintaining high productivity, positioning it as a cornerstone for a sustainable blue economy. Despite these significant advantages, the successful implementation and operation of RAS are governed by a set of critical and interconnected influencing factors that pose substantial challenges.

7. Challenges in RAS Development

RAS represent a pivotal technological advancement for sustainable aquaculture. The challenges below differ materially in their potential to cause catastrophic system failure versus incremental operational inefficiency. Based on industry incident reports and peer-reviewed risk assessments, they fall into three tiers: Tier 1 (Critical): Biofilter failure, acute ammonia/nitrite spikes, and energy system collapse are capable of causing complete stock mortality within hours. Tier 2 (Substantial): Pathogen outbreaks, off-flavor accumulation, and solids management failures have high economic impact but are typically manageable with prompt intervention. Tier 3 (Chronic): Sensor fouling, data integration complexity, and regulatory gaps constrain efficiency and scalability but rarely cause abrupt system failure. These multifaceted challenges from water quality management to economic viability highlight the complexities of operating a closed-loop system. Importantly, these challenges are the specific operational manifestations of the low-readiness barriers identified in the RRL assessment. For instance, the difficulty in monitoring or removing complex contaminants drives the need for advanced sensors and treatments, which themselves are often at low RRL, creating a cycle of innovation without deployment. However, their complexity and closed-loop nature present significant challenges that must be overcome to ensure economic viability, environmental sustainability, and optimal animal welfare. The primary challenges are categorized and detailed below.

7.1. Water Quality Monitoring and Management

The cornerstone of RAS success is the maintenance of pristine water quality, which is fraught with several interconnected challenges: Accumulation of toxic metabolites which includes rapid buildup of nitrogenous wastes (ammonia, nitrite, nitrate) and gases (CO₂, H₂S) from fish metabolism and waste decomposition poses a constant threat. Ammonia is highly toxic, causing gill damage and mortality, with safe levels for salmonids as low as 0.0125 mg L⁻¹ NH₃-N [78] and hydrogen sulfide causing rapid mortalities even at sub-µg/L levels [148]. Meanwhile, nitrate is less toxic, with concentrations exceeding 100 mg L⁻¹ impairing growth and feed conversion in species like Atlantic salmon.

Another challenge which is associated with water quality management is the limitations of real-time monitoring technologies often restricted to basic parameters (DO, temperature, pH, salinity). Critical compounds, including off-flavors (geosmin, MIB), hormones, antibiotics, and trace metals, still require sophisticated, laboratory-based analyses (e.g., LC-MS/MS), preventing immediate intervention [149,150]. Advanced oxidation processes effectively remove off-flavor compounds but may increase antibiotic resistance genes in the water, balancing product quality against biosafety [151]. Furthermore, sensors which are implanted in marine environments are prone to biofouling and corrosion, compromising their accuracy and longevity [152].

Key research gaps include inadequate species-specific toxicokinetic data for establishing evidence-based regulatory thresholds, a deficit that forces operators to rely on thresholds extrapolated from flow-through systems, which systematically overestimate safe concentrations in recirculating environments and perpetuate unnecessarily conservative water exchange rates and insufficient modeling of long-term marine biofilm dynamics for reliable anti-biofouling sensor development directly compromising monitoring accuracy and longevity [153]. A significant hurdle is the absence of comprehensive regulations or universal standards to specify which water quality parameters must be monitored and their acceptable concentration ranges for different species, leading to inconsistent management practices [78,150].

7.2. Technological and Operational Hurdles

The reliance on complex technology introduces several challenges related to cost, reliability, and integration. The high cost and operational expenditures of advanced monitoring equipment, automated feeding systems, and water treatment infrastructure (biofilters, ozonation, UV) is prohibitive, particularly for small-scale operators [13]. IoT, WSNs, and AI-based systems for real-time monitoring face issues of data transmission reliability, high energy consumption, and the difficulty of integrating disparate data sources for predictive modeling [78]. The failure of critical components, especially the biofilter and solid waste management systems, remains a primary cause of system-wide crashes. Conventional filtration and biofiltration do not effectively remove all harmful micropollutants. So, Contaminants of Emerging Concern (CECs) which include pharmaceuticals, hormones, and antibiotic resistance genes (ARGs), can accumulate and pose high risks to fish and human health [154]. The removal of taste and odor (T&O) compounds is particularly challenging and critical for product quality [155].

High capital expenditure and biofilter failure risks create substantial economic barriers [156]. Advanced oxidation processes effectively remove contaminants but increase energy costs and may disrupt nitrification stability [157]. Key research gaps include the absence of multi-objective optimization frameworks balancing treatment efficacy, energy use, and microbial stability leaving operators without tools to evaluate trade-offs systematically. Furthermore, the relationship between random operational disturbances and antibiotic resistance gene proliferation remains poorly understood, limiting the ability to design interventions that prevent resistance accumulation [158,159].

7.3. Feed Formulation and Waste Production

Feed is the primary input and the main source of waste in RAS, requiring specialized formulation. Feed, which is important for fish, must possess high physical integrity to withstand mechanical handling and minimize nutrient leaching and fines (dust) that pollute the water. Pellets require an optimal hardness (20–80 Newton) for marine species that need high bulk density to ensure rapid sinking and prevent system clogging [143,160]. Feeds must be highly digestible to reduce waste accumulation, and Target Apparent Digestibility Coefficients (ADCs) are typically >90% for protein and >85% for lipids [161]. Formulations aim to reduce nitrogenous excretion by optimizing the digestible protein to digestible energy ratio, often by increasing lipid levels (~10%) [162]. Ideal feces are firm, cohesive, and sink rapidly for efficient removal by mechanical filters. This is achieved by reducing indigestible ingredients and using functional additives to improve fecal stability [143]. These feed formulation challenges are amplified for species with specific dietary requirements or sensitivities, as outlined in Table 1.

Feed constitutes 40–60% of operational costs; poor physical quality increases solids loading and biofilter stress [163]. High digestibility reduces waste but increases ingredient cost, while increasing lipids lowers nitrogen excretion yet raises oxygen demand for sludge degradation which is a trade-off that operators must navigate without decision-support tools [164]. Research gaps include limited species-specific ADC validation, limited data on the long-term effects of functional additives on sludge, and feed design is rarely integrated with nutrient recovery models, missing opportunities to convert waste streams into value [165].

7.4. Biological and Pathogen Management

The controlled, dense environment of RAS can create unique biological challenges. The nitrifying bacterial community is the engine of RAS and any disruption to this microbiome can lead to toxic ammonia and nitrite spikes in biofilters [166]. Furthermore, ozonation for disinfection must be carefully managed to avoid damaging this essential microbial community in the biofilter environment [17]. As RAS can exclude some pathogens, suboptimal design or biosecurity failures leads to the proliferation of opportunistic pathogens, such as the protozoan Ichthyophthirius multifiliis, causing significant disease outbreaks and economic losses in the system. In marine RAS, denitrification is particularly difficult due to the effects of salinity on microbial communities. For example, marine nitrifying bacteria (e.g., Nitrosococcus mobilis, Nitrobacter spp.) exhibit reduced activity at salinities > 30 ppt, and conventional organic carbon sources (methanol, acetate) become less effective. Novel approaches using solid carbon sources such as corn cobs, peanut shells, or biodegradable polymers (PHBs) are promising but remain at RRL 4–5. However, novel approaches are using solid carbon sources (e.g., corn cobs) or electro-oxidation processes to mitigate the dentification-associated challenges but they still remain a technical hurdle which can be improved by the advancements in the system [167].

Biofilter disruption causes acute ammonia spikes and rapid stock losses; opportunistic pathogens like Ichthyophthirius multifiliis inflict significant economic damage [112,168]. Ozonation controls pathogens while potentially compromising biofilter nitrification and promoting antimicrobial resistance; a trade-off that requires careful calibration but lacks standardized protocols [169]. Research gaps include lack of nitrifier resilience metrics, limited pathogen adaptation studies, and insufficient predictive models linking microbiome stability to variable conditions, preventing evidence-based management of biological risk [170].

7.5. Economic and Environmental Sustainability

The most significant challenges of RAS are economic and operational. However, RAS requires high capital investment and sophisticated technical management. The complexity of these technology–biology interaction systems demands continuous performance monitoring, as system failures can lead to rapid losses [13]. A major operational cost driver is energy consumption, primarily from continuous water pumping and aeration. This creates a paradox where higher water reuse efficiency can lead to greater electricity expenses, impacting both economic viability and environmental sustainability [171]. Consequently, integrating renewable energy sources or utilizing waste heat is a key research focus to improve sustainability [172].

Higher water reuse efficiency reduces environmental impact but increases electricity expenses, creating a fundamental tension where greater recirculation can paradoxically undermine economic viability [171]. Empirical data from commercial salmon RAS facilities place the economic recirculation optimum at 97–99%, contingent on energy price, water availability, and discharge limits. However, this economic optimum misaligns with biological stability because higher recirculation concentrates metabolic waste, elevating the risk of nitrification failure and off-flavor accumulation. No existing framework integrates these variables, leaving operators to trial-and-error methods rather than evidence-based optimization. Although RAS addresses broad environmental concerns associated with traditional aquaculture, such as habitat destruction, biodiversity loss, and water pollution, waste control remains the core challenge from an environmental management perspective [81]. RAS concentrates waste internally which necessitates robust, integrated designs for water quality management and solid waste removal to prevent the accumulation of toxic metabolites and maintain a healthy environment for fish.

The long-term viability of RAS depends on its economic competitiveness and environmental footprint. The significant capital investment requirements make economic feasibility a major concern, with some studies suggesting that it is only viable for large-scale facilities, limiting its adoption [173]. The aquaculture supply chain contributes to Greenhouse Gas (GHG) emissions, primarily focusing on feed production and energy consumption. RAS often has a lower carbon footprint per ton of fish produced as compared to open systems due to their higher feed efficiency but energy-intensive operations can offset these gains [174,175]. Moreover, inadequate collaboration and knowledge sharing between producers, suppliers, researchers, and consultants hinders collective problem-solving and technological improvement within RAS [13]. This has led to proprietary data silos, inconsistent standards, and duplication of effort. Solutions include industry consortia, open-source benchmarking, public–private partnerships, and standardized reporting protocols.

Energy costs and capital investments limit RAS adoption to large-scale facilities. Higher water recirculation improves conservation yet increases pumping energy and waste accumulation risk, forming an energy–water nexus paradox [176]. Research gaps include the lack of data on renewable energy integration, the absence of standardized life-cycle assessment (LCA) frameworks for RAS making cross-facility comparisons unreliable, and long-term economic sustainability studies remain insufficient to justify investment decisions [177].

7.6. Chemical Use and Residuals

The use of therapeutics, while necessary, introduces its own set of challenges. The use of antibiotics (under prescription) and chemical additives (e.g., for parasite control) can lead to residue formation in the RAS and the further development of antibiotic resistance genes (ARGs), posing a serious threat to food safety and public health [154].

ARGs accumulate in RAS, posing food safety risks through horizontal gene transfer and human exposure. Prophylactic antibiotics control disease but drive ARG proliferation, whereas alternatives like probiotics offer lower resistance risk yet lack commercial-scale validation which is a trade-off that forces operators to balance between immediate disease control and long-term resistance management. Research gaps include lack of standardized ARG monitoring protocols making cross-facility comparisons impossible, absent regulatory thresholds, and insufficient field validation of non-antibiotic alternatives at commercial scale [158].

7.7. Biosecurity and Disease Occurrence in RAS

While RAS are often marketed as highly biosecure, their closed environment presents a unique disease paradox. The controlled conditions can prevent the introduction of external pathogens, but high stocking densities and nutrient-rich water can favor the rapid proliferation of opportunistic pathogens if they break the system [178]. In practice, pathogen introductions still occur, sometimes causing severe mortality and economic loss, demonstrating that theoretical biosecurity is difficult to maintain perfectly [20]. Pathogens in RAS, particularly for salmonids, include bacteria like Flavobacterium spp. and Yersinia ruckeri, and viruses like IPNV. And these infection dynamics are distinct from open systems, as high fish density and constant water recirculation can facilitate faster transmission [179]. Disease susceptibility and appropriate treatment protocols are highly species-specific, adding a critical layer to RAS management strategies (Table 1).

A central dilemma in RAS disease management is disinfection which requires treatment; these treatments must be effective against pathogens in the water column without harming the fish or the essential nitrifying bacterial communities in the biofilters. Common disinfection methods, including ozone, UV irradiation, and peracetic acid (PAA) [180,181], are effective at reducing pathogen accumulation and maintaining water quality in RAS; however, this method requires further optimization to achieve full operational efficiency. Furthermore, protocols from flow-through systems are often unsuitable, necessitating the development of RAS-specific biosecurity strategies, especially with the growing commercial use of RAS for Atlantic salmon [182].

RAS creates a disease paradox: closed systems exclude external pathogens but high densities accelerate transmission, causing severe losses [183]. A trade-off exists where disinfection controls pathogens but risks harming nitrifying bacteria and fish health; however, operators lack dose–response models to optimize this balance [184]. Research gaps include the lack of standardized dose–response models which link treatment intensity to pathogen reduction, biofilter impact, and fish health; insufficient data on sublethal pathogen responses; and real-time monitoring is rarely integrated with adaptive disinfection control, preventing dynamic optimization [185].

These challenges do not operate independently; they form a system of reinforcing constraints. Feed formulation determines waste composition, affecting biofilter performance and water quality. Poor digestibility increases solids loading, elevating energy demand and creating anaerobic zones that promote pathogens. Disinfection choices balance pathogen control against biofilter health, with ozone overuse potentially destabilizing nitrification and triggering cascading water quality failure. This interconnectedness means solutions cannot be optimized in isolation, yet current research rarely evaluates such multi-objective trade-offs, and operational data sharing across facilities remains insufficient to build integrated optimization models [158,159].

7.8. Phosphorus Management and Nutrient Recovery

While nitrogen management in RAS is well-developed, phosphorus (P) management remains a superficial and under-addressed challenge in many commercial operations. Unlike nitrogen, which is largely removed via nitrification/denitrification to atmospheric N₂, phosphorus is a finite, non-renewable resource that simply accumulates in system water and sludge. Typical RAS effluents contain P concentrations of 10–50 mg L⁻¹, primarily as orthophosphate (PO₄³⁻) and organic P in solids. Without active removal, P accumulates, contributing to membrane fouling, algal blooms in discharge waters, and ultimately violating discharge permits in sensitive watersheds [154].

Several strategies exist for recovering nutrients from fish sludge, including composting and anaerobic digestion, but many remain underutilized. Composting converts sludge into solid fertilizer and soil conditioner, while anaerobic digestion recovers liquid nutrients and biogas. Converting this waste into a resource offers economic and environmental benefits, though improvements in nutrient recovery efficiency and heavy metal compliance are still needed [94].

Aquaponics is the most direct P-recovery pathway, as plants absorb PO₄³⁻ from RAS water. However, conventional coupled aquaponics often supplies insufficient P for fruiting plants. Decoupled aquaponics systems (DAPS) allow independent optimization of P supplementation to the hydroponic unit, overcoming this limitation. DAPS separate the aquaculture and hydroponic units to allow independent optimization of water quality, nutrient supplementation, and growing conditions for both fish and plants [186]. Integrated multi-trophic aquaculture (IMTA) incorporates extractive species such as seaweeds (e.g., Ulva lactuca) and bivalves that assimilate dissolved P and particulate organic P, respectively. While IMTA is well-studied in open-water systems, its integration into land-based RAS remains at pilot scale (RRL 4–5) due to challenges in balancing feed inputs, waste production, and extractive species biomass [187].

Critical research needs to include: (1) standardized characterization of P speciation in RAS sludge across species and feeds; (2) life-cycle assessment comparing P-recovery pathways (slow-release fertilizer vs. aquaponics vs. chemical precipitation); and (3) development of real-time P sensors to enable dynamic control of P management strategies. Without focused attention on P, RAS cannot achieve true circularity, as P will continue to accumulate as a waste rather than be recovered as a resource.

These multifaceted challenges from water quality management to economic viability highlight the complexities of operating a closed-loop recirculating aquaculture system. Consequently, the future of RAS depends on targeted innovations and research directions which are designed to directly address these persistent hurdles.

8. Future Perspectives and Innovations in RAS

The evolution of RAS is pivotal for meeting global seafood demand sustainably. Future advancements are poised to enhance efficiency, sustainability, and economic viability through digitalization, genetic improvement, and system integration. Crucially, the RRL diagnosis directs that future progress requires a strategic pivot from proving functionality to proving commercial viability. For technologies in ‘pilot-scale purgatory’ (RRL 4–6), priorities must shift toward deployment-oriented validation: generating longitudinal economic data, conducting integrated system demonstrations, and designing for cost and operational robustness.

Key future prospects and innovations are outlined below.

8.1. Digitalization and Smart RAS

The integration of digital technologies represents a paradigm shift from manual monitoring to autonomous, data-driven management. So, the adoption of Internet of Things (IoT) sensors and cloud-based analytics will enable continuous, real-time monitoring of critical but less-frequently measured parameters like ammonia, nitrite, and suspended solids in RAS. The use of low-cost microcontrollers and Low-Power Wide-Area Networks (LPWANs), such as LoRaWAN, significantly reduces human error and provides a more accurate dataset than manual methods [188,189].

This technological integration facilitates the automation of key processes such as automated feeding and heating systems. More importantly, it allows for the early detection of system failures, such as oxygen depletion or biofiltration breakdown. Future research must focus on developing more robust and affordable sensors and advancing artificial intelligence (AI) models to evolve from simple monitoring to full autonomous, predictive management [32]. Advancing AI/IoT from RRL 4 to RRL 6 demands longitudinal commercial trials to establish reliability under real-world conditions, validated sensor durability against biofouling, quantified ROI frameworks capturing energy savings and mortality reduction, and open-source integration standards to overcome equipment fragmentation.

8.2. Advanced Genetic Selection

Genetic improvement is crucial for optimizing fish performance within the specific conditions of RAS. Selective breeding for desirable traits such as enhanced growth rates and improved feed conversion ratios (FCRs) directly increases economic viability of the system by reducing grow-out time. For instance, selective breeding in rainbow trout has demonstrated an increase from 37.8 g to 552.2 g after 317 days in RAS [190]. In a direct comparison of selected versus non-selected rainbow trout from the same base population, selected fish (G10) showed 61% higher body weight (1688 g vs. 1038 g at 374 days) and a better feed conversion ratio (0.99 vs. 1.19) than non-selected fish (G0) [191]. In contrast, genetically improved tilapia (G5) reached commercial weight 57 days faster than the base population [192]. Genomic selection can be tailored to enhance the performance under specific conditions, such as adapting barramundi for diverse environments or selecting European sea bass strains that perform well on plant-based diets, eventually reducing reliance on marine ingredients [193] which will be important for balancing genetic gains with ecological and welfare considerations. Progressing genetic improvement from RRL 5 to RRL 7 requires multi-site validation across commercial facilities, full-cycle performance data, economic quantification of improved FCR and survival, and systematic assessment of genotype–environment interactions under variable RAS conditions.

8.3. Integrated and Specialized System Designs

Moving beyond traditional RAS, future systems will focus on integration and resource optimization, such as the use of decoupled aquaponics systems (DAPS), which separate the aquaculture and hydroponic units, helping to overcome the major nutrient limitation for plants found in traditional coupled systems. This allows for independent optimization and nutrient supplementation (e.g., for K, Fe, Ca), while also ensuring optimal growth for both fish and plants and creating a more resilient closed-loop system [186]. Innovations like Marine Aquaponics (Maraponics) and Inland Saline Aquaponics (Haloponics) leverage saltwater environments to conserve freshwater and cultivate marine species. Future work will focus on optimizing optimal salt-tolerant fish–plant combinations (e.g., halophytes, broccoli) and further defining stocking densities and nutrient management protocols for these environments. Elevating integrated systems from RRL 5–6 to RRL 7–8 requires commercial-scale DAPS demonstrations with sustained operational validation, comprehensive nutrient mass balances demonstrating substantial recovery efficiency, rigorous techno-economic analysis encompassing fish and plant revenues, and systematic optimization of fish–plant pairings for saline-tolerant systems.

8.4. Next-Generation Water Treatment Technologies

Addressing the limitations of conventional filtration is critical for removing a broader spectrum of contaminants. Emerging technologies are well focused on removing micropollutants, including taste and odor (T&O) compounds like geosmin and methylisoborneol (MIB), endocrine-disrupting hormones that impair fish reproduction and stress responses even at ng/L concentrations, and pharmaceuticals that can accumulate, pose toxicity risks, and are responsible for deteriorating water quality. The electro-oxidation (EO) process has been validated as a novel strategy to remove off-flavors from water without interrupting feeding, avoiding production losses of up to ∼15% associated with purging [155,194]. However, further advancements in the system are required to mitigate the adverse effects of off-flavors and hormones on fish health and water quality. For marine RAS, where denitrification is challenging, the use of solid carbon sources (e.g., corn cobs, peanut shells) in heterotrophic processes shows promise [167]. All new treatment methods must be tested at pilot scale to understand their long-term effects on fish health and the microbial community of biofilters [17]. Transitioning emerging treatments from RRL 4–6 to RRL 7 requires PHB systems to demonstrate economically viable production through sustained pilot validation, electro-oxidation to validate off-flavor removal without purging with quantified energy demand, and solid carbon denitrification to generate long-term performance data under marine RAS conditions.

8.5. Waste-to-Value and Circular Economy

Transforming waste streams into valuable products is key to improving sustainability and economics. The integration of microalgae into RAS provides a sustainable method for nutrient recovery from effluents, simultaneously improving oxygenation and consuming CO₂. Furthermore, technologies like Hydrothermal Liquefaction (HTL) can convert aquaculture waste into high-value products, promoting a circular economy [4]. Pioneering sustainable aquafeeds through alternative plant sources and genetic engineering will reduce dependence on fishmeal and lower the environmental footprint of feed production, which is a major contributor to the overall GHG emissions of aquaculture [175]. Advancing waste valorization from RRL 4–5 to RRL 6–7 demands cost-effective microalgae harvesting with continuous operation, HTL validation using real RAS sludge with positive net energy balance, and multi-site anaerobic digestion validation across species and feed formulations.

8.6. Enhancing Biosecurity and Fish Health

Proactive health management will reduce reliance on chemicals and antibiotics. The administration of probiotics via feed or water can strengthen the fish immune system, helping to improve water quality and suppressing pathogen activity. Strategic manipulation of the gut and system microbiome offers a sustainable path to enhancing fish health and welfare [195]. Moving biosecurity innovations from RRL 4–6 to RRL 7 requires probiotics and phage therapy to demonstrate substantial mortality reduction across commercial facilities, validated dose–response models for disinfection under variable water quality, and field-validated eDNA-based pathogen detection with rapid turnaround.

8.7. Improving Economic and Environmental Footprint

Future developments must address the high costs and climate impact of RAS. As RAS often has a lower carbon footprint per ton of fish produced than open systems due to higher feed efficiency, the energy-intensive nature of operations remains a challenge. Future applications must focus on improving energy efficiency and integrating renewable energy sources to further reduce GHG emissions from the system [174,175]. For RAS technology to reach its full potential, improved industry-wide collaboration and knowledge sharing between producers, suppliers, researchers, and consultants is essential. Overcoming the current lack of governance and inadequate cooperation is critical for continuous improvement and widespread adoption [13]. Elevating economic sustainability from RRL 5–6 to RRL 7–8 requires extended renewable energy integration data from multiple commercial facilities demonstrating significant cost reduction with acceptable ROI, standardized LCA frameworks enabling cross-facility comparison, and predictive energy–water optimization models validated to achieve measurable operational savings.

9. Conclusions

The evolution of RAS from a niche concept to a cornerstone of modern sustainable aquaculture is a direct response to the increasing pressure on wild fish stocks and the environmental limitations of traditional, extractive aquaculture models. By transitioning from open, flow-through systems to closed-loop, intensive production, RAS represents a paradigm shift towards a precision-controlled approach that prioritizes environmental stewardship. The core principles of RAS water treatment, reuse, and waste valorization are not merely alternatives but represent a refined and essential strategy for stable, environmentally controlled food production.

RAS offers significant advantages, including a drastic reduction in water consumption, precise control over water quality parameters, and the bioremediation of wastewater. This controlled environment prevents the release of pathogens, parasites, and metabolic wastes such as ammonia, carbon dioxide, and organic compounds into natural water bodies. The integration of deep learning and intelligent systems further enhances this sustainability, optimizing productivity and efficiency to create a smart, recirculating aquaculture framework.

Technological advancements continue to improve the environmental and economic viability of RAS. Key developments include advanced treatment technologies like membrane bioreactors, denitrification reactors, and ozone treatment, which collectively reduce water and energy use. The adoption of integrated multi-trophic aquaculture (IMTA) principles, alongside novel methods employing lignocellulosic materials and specialized bacterial consortia (e.g., Flavobacterium), enhances the removal of nitrogen, phosphorus, and organic matter. Furthermore, complementary systems such as constructed wetlands and microalgae-based treatments provide effective remediation for suspended solids, nutrients, and heavy metals.

A major frontier in RAS development is the integration of automation, the Internet of Things (IoT), and artificial intelligence (AI). These technologies enable precise, data-driven decision-making for optimizing water quality, aeration, environmental monitoring, and feeding regimes. This not only improves fish welfare and operational efficiency but also bolsters overall system sustainability. The potential of RAS can be further augmented by integrating biopolymer production, such as polyhydroxybutyrate (PHB), with electrochemical water treatments and energy-efficient designs, offering a comprehensive solution for future sustainability.

While RAS represent a foundation for sustainable aquaculture, their transformative potential remains constrained. This review introduces the RAS Readiness Level (RRL) framework, a diagnostic tool that reveals a critical disparity: core engineering components (e.g., filtration, disinfection) are commercially mature, yet pivotal biological and digital innovations such as AI-driven management and advanced bioremediation remain in mid-TRL pilot validation. This RRL assessment identifies the central challenge as bridging the translational “valley of death” to commercial deployment.

Future progress therefore necessitates a deliberate pivot in research from novel functionality to deployment-oriented validation. Priorities must include generating longitudinal economic data from commercial-scale demonstrations, developing standardized benchmarking protocols, and creating integrated designs that overcome cost and operational barriers. By using the RRL framework to guide targeted maturation and de-risk key technologies, stakeholders can systematically realize the promise of intelligent, circular aquaculture to meet global seafood demand within planetary boundaries.

Looking forward, the continued evolution of sustainable RAS is vital to meeting escalating global seafood demand. Future advancements must converge on three critical frontiers to overcome existing limitations and unlock the full potential of this technology. First, the comprehensive digitalization of RAS, marked by a transition from manual oversight to autonomous control via IoT sensors and cloud analytics, is essential for real-time optimization of water quality, feeding regimes, and system controls, thereby enhancing both fish health and operational productivity. Second, targeted genetic improvements through selective breeding for traits like enhanced growth rates and superior feed conversion ratios will directly bolster economic viability and resource efficiency. Finally, a new paradigm of system integration and novel technologies will be crucial. This includes implementing sophisticated designs like decoupled aquaponics (DAPS) and salt-tolerant systems (e.g., Maraponics, Haloponics) for synergistic production, alongside the adoption of emerging technologies such as electro-oxidation for off-flavor removal, advanced denitrification techniques, probiotics for bolstered fish immunity, and Hydrothermal Liquefaction (HTL) for waste valorization. The concerted development of these areas represents a critical pathway toward achieving a truly sustainable, resilient, and productive recirculating aquaculture system.

The RRL diagnosis makes clear that the principal barrier to realizing this future is no longer the absence of promising innovations, but rather the persistent gap between pilot-scale validation (RRL 4–6) and commercial deployment (RRL 7–9). Bridging this gap hinges not on novel inventions but on extended commercial trials, validated ROI models, standardized protocols, and multi-site validation which are the precise advancement pathways required to achieve each subsequent RRL. Each proposed frontier, including digitalization, genetic selection, system integration, must be pursued with explicit attention to its readiness level and specific translational barriers. This translation imperative applies directly to core operational challenges; so, further research is needed on smart aquaculture management systems for maintaining aeration and proper feeding without disrupting fish, and improvements in water quality management are needed, potentially through advanced oxidation processes or beneficial bacteria to remove off-flavors, nitrates, and waste without causing harm, while biofilter technology continues to evolve.