The Dilemmas and Challenges of Tail Water Treatment Technology for Land-Based Marine Aquaculture in China: A Review

Abstract

In recent years, China’s land-based marine aquaculture industry has developed rapidly. Frequent water changes during the aquaculture process have resulted in a large amount of aquaculture tail water. The untreated tail water, containing organic waste, nutrients, and chemicals, is often discharged into the seawater, potentially causing serious environmental and ecological problems. Therefore, the tail water from land-based marine aquaculture should be treated before being reused for resource utilization or safely discharged into the environment. This can promote the sustainable development and circular economy of the marine aquaculture industry. Against this background, this article provides an in-depth understanding of the generation, composition, and hazards of aquaculture wastewater. It reviews the various technologies for marine aquaculture tail water treatment currently adopted by scholars, classifying them into three major categories: physical, chemical, and biological. The paper analyzes the advantages and disadvantages of each technology, as well as the challenges they face. Additionally, future research directions are proposed, and suggestions are provided for achieving the sustainable development of the marine aquaculture industry and transitioning to environmentally friendly aquaculture.

1. Introduction

According to the 2024 Statistical Yearbook of Food and Agriculture of the World, the total output of global fisheries and aquaculture increased by 47% from 2000 to 2022. Over the past three decades, aquaculture has been the main driver of growth in the output of fisheries and aquaculture, with an average annual increase of 5% from 2000 to 2022. In 2022, aquaculture reached a record high of 94.4 million tons, accounting for more than half (51%) of the global total production for the first time [1,2]. Among them, China is one of the most developed countries in aquaculture in the world. Both its aquaculture area and total output rank among the top in the world [3]. The “2024 China Fisheries Statistical Yearbook” shows that in 2023, the output value ratio of farmed products to caught products in China was 81.9:18.1, and the output ratio of marine products to freshwater products was 50.4:49.6 (please refer to Figure 1). In recent years, due to overfishing, environmental pollution and other reasons, China’s marine aquatic resources have decreased. The focus of the aquatic industry has shifted towards aquaculture, which has led to the rapid development of the marine aquaculture industry [4]. During this process, China’s marine economy has continued to grow, and the pace of transformation and upgrading of marine industries has accelerated, providing solid policy support and infrastructure guarantee for marine aquaculture. Seafood is rich in high-quality protein, abundant minerals and trace elements, etc., and has high nutritional benefits [5]. Under the guidance of the marine economy strategy, the marine aquaculture industry still has huge growth potential in the future and is expected to further become an important force in ensuring national food security and promoting the development of the blue economy.

Marine aquaculture in China is mainly distributed in Shandong, Liaoning, Hebei, Guangdong and other places [6]. The planning of marine aquaculture waters and tidal flats in various regions divides the marine aquaculture areas into marine aquaculture zones, tidal flats and terrestrial aquaculture zones [7]. Marine aquaculture encompasses nearshore and offshore cage culture, primarily for finfish (e.g., large yellow croaker and grouper); suspended culture (including raft and longline systems), mainly for shellfish (e.g., oysters) and seaweeds (e.g., kelp); as well as bottom-sowing for stock enhancement of shellfish (e.g., clams) and sea cucumbers. Coastal and land-based aquaculture, on the other hand, includes pond culture of fish, shrimp, and crabs; intensive factory-based facility aquaculture of species such as finfish and shrimp; and intertidal zone aquaculture of shellfish (e.g., razor clams) [8]. The Bulletin on the Status of China’s Marine Ecological Environment points out that the main over-standard indicators in the water bodies of key marine aquaculture areas (Liaodong Bay, Yangtze River Estuary, Hangzhou Bay and Pearl River Estuary and other coastal waters) are inorganic nitrogen and active phosphate [9]. During the breeding process, feed, fertilizer and chemicals are repeatedly input by humans to ensure the growth and health of the breeding products [10]. Marine aquaculture systems generate a large amount of wastewater. A considerable amount of organic waste, nutrients and chemical substances in the wastewater are discharged into the seawater, which is then transported in the water body and deposited on the seabed, affecting adjacent areas (nearshore) and more distant areas (the open sea) [11]. These processes may lead to serious environmental and ecological problems, such as low dissolved oxygen levels (i.e., hypoxia), water eutrophication, heavy metal pollution and antibiotic pollution, etc. [12,13,14]. However, marine aquaculture wastewater with high salinity usually has a relatively low organic matter concentration and C/N ratio [15]. Compared with freshwater aquaculture wastewater, marine aquaculture wastewater is more difficult to treat due to the influence of salinity [16]. Therefore, the process of treating marine aquaculture tail water through a series of physical, chemical and biological processes to effectively remove the above-mentioned pollutants in the wastewater and make it meet the standards for environmental discharge or reuse within the system is very crucial.

The rapid development of the marine aquaculture industry and the large-scale generation of aquaculture tail water have attracted widespread attention. Against the current background of sustainable development, there is an urgent need to analyze the sources, hazards and treatment technologies of marine aquaculture wastewater. This article will systematically explain these processes and discuss the main challenges and future prospects of common technologies for treating tail water from marine aquaculture in practical applications at the present stage, so as to achieve a more sustainable development of the marine aquaculture industry.

2. Marine Aquaculture Tail Water

Aquaculture can be classified by aquaculture methods into pond aquaculture, factory aquaculture and open sea aquaculture [17,18]. At present, most of the tail water from Marine pond aquaculture in China is discharged directly into the sea without treatment or after simple treatment [19]. Factory aquaculture is also an important part of marine aquaculture in China [20,21]. They often have a high stocking density, long water usage cycle and high nutrient content in the tail water [22]. According to the classification of aquaculture waters, it can be divided into land-based aquaculture, nearshore aquaculture on tidal flats, shallow sea aquaculture and deep-sea distant-water aquaculture [23], as shown in Figure 2. Among them, the pollution of aquaculture tail water from land-based pond aquaculture and factory aquaculture is the most common [24], and it is also the focus of current research on the treatment technology of marine aquaculture tail water. Therefore, this paper will focus on evaluating the application of tail water treatment technologies in pond aquaculture and factory aquaculture, as well as their advantages and disadvantages.

2.1. Aquaculture Tail Water Pollution

From the perspective of marine aquaculture waters, the water quality in marine aquaculture areas located in open regions with relatively fast flow rates is less likely to deteriorate. However, in marine aquaculture areas near the tidal flats and land-based marine aquaculture, due to their relatively poor water exchange capacity and relatively slow flow rates, the water quality is more prone to deterioration [25]. From the perspective of the types of marine aquaculture, fish and shrimp pond farming is a type of aquaculture that causes relatively serious pollution [26]. Algal farming does not require breeding. Most shellfish do not need to be bred or are based on algal farming, and seaweed has a great potential for carbon sequestration [27]. The impact of these two types of marine aquaculture on seawater quality is less than that of fish and shrimp farming. For example, oysters feed on algae, and their raft farming has not had adverse effects on water quality [28]. Therefore, land-based aquaculture and cage aquaculture near the shore of tidal flats are currently the focus of attention in the treatment technology of marine aquaculture tail water, among which the research on the treatment technology of aquaculture wastewater from fish and crustaceans is the most extensive.

At present, most of the land-based marine aquaculture ponds belong to extensive aquaculture, with the phenomenon of untreated tail water being directly discharged into the ocean. On average, land-based aquaculture ponds are cleaned 1 to 2 times a year. During this period, a large amount of aquaculture wastewater is discharged into the surrounding water environment at one time. Meanwhile, during the process of Marine shrimp pond farming, there is frequent water change. Depending on the season and the water quality in the pond, water needs to be changed approximately every 10 days. During the dry season, the temperature and salinity of the water will increase, and 10% to 15% of the water needs to be changed. When the shrimp pond is contaminated or diseased, 10% to 15% of the water should also be changed [29]. In the process of factory farming, the water consumption of the pond will increase over time. After the harvest, a large amount of wastewater is discharged into the surrounding environment, and clear water is pumped into the aquaculture ponds to wash away the sediment in the ponds. Then, this water is usually pumped back into the ocean, which seriously affects the habitat of the mangroves on the tidal flats [30]. The main pollutants in these marine aquaculture tail water can be roughly classified into four categories [31]: organic carbon (accumulation of sediment organic carbon burial fluxes ranging from 2.9 to 79.71 g·m⁻²·a⁻¹ [20]); nutrients (the phosphorus flux of marine aquaculture in Fujian is 2.2 times that of the Minjiang River, and the nitrogen flux caused by seaweed farming accounts for 69% of that of the Minjiang River [32]); suspended solids (concentrations can be highly variable, from 30 to over 500 mg/L.); and new pollutants (for example, the highest antibiotic concentration in the Liaodong Peninsula was 692.0 ng/L [33], etc.). During the period of replacement and discharge, aquaculture wastewater containing a large amount of pollutants is discharged into the surrounding water bodies without treatment, seriously polluting the surrounding fishery water environment. It is an important part of seawater non-point source pollution. The discharge of untreated aquaculture wastewater is very likely to cause serious environmental and social and economic problems.

2.2. The Environmental Impact of Marine Aquaculture Tail Water

The potential interaction and conflict between marine aquaculture activities and the Marine environment and ecosystem is a long-term concern [34]. Many scholars have also analyzed the pollution caused by the discharge of tail water from marine aquaculture. Price et al.’s research indicates that Marine cage culture operations are recognized as one of the sources of nitrogen and phosphorus pollutants. Their emission forms include particulate matter (uneaten food and feces containing undigested food, passing through the fish’s digestive tract) and dissolved metabolic wastes, including ammonia, urea, and dissolved phosphorus [35]. Bouwman et al. estimated through a model that only 36% of the nitrogen in the feed was retained in farmed salmon and trout, while 54% was lost as dissolved waste, 10% as particle loss, and the phosphorus absorption rate was only 33% [36]. Herbeck et al. discovered through isotope tracking that in large-scale coastal aquaculture areas such as China and Southeast Asia, the impact of active nitrogen from aquaculture sources on the performance of coastal ecosystems may be greater than previously thought [37]. The research by JianWei Yu et al. found that the large-scale feeding of feed during the breeding period of large yellow croakers in Zhoushan City was the main factor leading to a significant increase in the biological oxygen demand and total nitrogen concentration in the breeding water [38]. As shown in Figure 3, dissolved pollutants are released into the water through excreta and granular solids. The continuous accumulation of pollutants will lead to the formation of algal blooms, reducing the dissolved oxygen in the water body. Harmful algal blooms can kill or poison marine aquaculture products, significantly reducing the quality of aquatic products and causing incalculable economic losses. In severe cases, they can even harm human health [39]. Apart from dissolved waste, particulate and suspended solid waste can cause fish death by blocking gills. The significant increase in particulate phosphorus in sediments and harmful substances in water bodies and sediments also poses a threat to marine ecosystems [40]. With the accumulation of pollutants in marine aquaculture ponds, biological diseases occur frequently, eventually causing huge economic losses to the aquaculture department [41]. Qin et al.’s research indicates that the direct discharge of wastewater from shrimp Marine ponds has a significant acidification effect on mangrove sediments in the discharge area. The accumulation of NO₃^--N, NO₂^--N, and DIN in intertidal water increases with the discharge of shrimp farming wastewater [42]. The high-level ponds for marine aquaculture are usually filled with seawater by pumping water with pumps, providing a suitable aquaculture environment for fish, shrimp, oysters, etc. The research on groundwater salinization in Qinzhou Bay, Guangxi, by Hu Kaiyan et al. found that the groundwater salinity in Qinzhou Bay has been on the rise since 2013 [43]. The intensive pumping and the extensive expansion of marine aquaculture in this area are both important reasons for the increased salinization of water bodies.

3. The Dilemma of the Existing Tail Water Treatment Technology

As shown in Figure 4, to date, a variety of physical, chemical and biological technologies have been employed to treat the tail water from marine aquaculture. The selection and optimization of these methods not only focus on the efficiency of pollutant removal, but also increasingly emphasize coordination with the goals of a circular economy [44]. Physical methods remove suspended large particle substances through sedimentation, foam separation and filtration, etc. [45]. Advanced oxidation methods, electrochemical oxidation, and coagulation/flocculation are examples of chemical methods. The high-salt environment of seawater (up to 30‰) provides certain favorable conditions for chemical treatment processes [46]. Biological treatment technologies, represented by microalgae technology, fluidized bed reactors and biological contactors, are the main wastewater treatment technologies for low-cost and high-efficiency removal of ammonia and organic pollutants [47]. Classification is a method based on the separation of pollutants. Physical methods purely separate pollutants from their original forms, while chemical and biological methods remove pollutants through assimilation, degradation or transformation into other substances. The comprehensive assessment and systematic integration of these technologies are precisely the key to promoting the transformation of the aquaculture wastewater treatment model from end-of-pipe treatment to resource reuse and system sustainability.

3.1. Physical Technology

Physical methods refer to a category of approaches that utilize physical actions to separate or remove insoluble suspended pollutants from wastewater. They play a significant pretreatment role in treating marine aquaculture wastewater, including aeration, physical adsorption, sedimentation [48], filtration and ultraviolet technology [49], etc. [50,51]. For example, Ali et al. prepared polyethersulfone (PES) membranes and studied the feasibility of treating aquaculture wastewater. They found that the PES membranes could remove approximately 85.70% of total ammonia nitrogen and 96.49% of total phosphorus, respectively [52]. Zhang et al. used ultraviolet/persulfate compound disinfectant (UV/PDS) to inactivate Escherichia coli and Streptococcus agalactiae in marine aquaculture water. The results showed that UV/PDS had a better disinfection effect on marine aquaculture water [53]. Some examples of physical methods and their advantages and disadvantages are shown in

Table 1. Summary of Physical Techniques for Treating Tail water from marine aquaculture.

Treatment Method	Material/Strain	Treatment Efficiency	Advantages	Disadvantages	References
Siphon aeration	Electroactive bacteria-algae biofilm coupled with siphon aeration	COD: 83.49%, TP: 93.56%, NH3-N: 91.26%, NO3−-N: 96.54%, NO2−-N: 86.11%, TN: 92.03%	Aeration can enhance the activity of microorganisms. Siphonic aeration reduces energy loss and provides an alternating aerobic and anaerobic environment, ensuring oxidation-reducing reactions and improving the degradation efficiency of organic matter.	The operation requirements are high, aeration equipment may become clogged and the cost is relatively high.	[54]
Solid- liquid separation	A three-dimensional two-phase model of these hydraulic cyclone separators	Under the optimal conditions, the efficiency of solid–liquid separation can reach 83.16% to 100%	It provides a reference for the design and optimization of series hydrocyclones in pond aquaculture wastewater treatment. The established model can be used to predict the hydrodynamic behavior and separation performance of series hydrocyclones.	Adjusting the optimal conditions of the series hydrocyclone separator may result in excessive energy consumption.	[55]
Filtration	Continuous flow electro- coagulation (EC)–filtration system	COD: 48.99%, TN: 55.26%, NH3-N: 57.06%, NO3−-N: 34.09%, NO2−-N: 18.47%, Energy consumption: 26.25 ± 4.95 × 10−3 kWh/m3	Continuous flow electrocoagulation can significantly improve the processing efficiency of filtration equipment. Increasing the hydraulic retention time and reducing the pore size of the filter have a more obvious reinforcing effect on the filter.	The impact of the scale and energy consumption of subsequent water treatment units still requires further research.	[56]
Filtration	Integrated process of screen mesh, sand filtration and adsorption	COD: 46.00% BOD5: 35.25% TIN: 62.94% Active phosphate: 84.34% SS: 84.73%	The inclined plane hydraulic screen can remove over 90% of the total suspended solids without affecting the drainage rate. The continuous flow sand filter does not require the pool to be shut down for backwashing.	There may be problems such as filter clogging and adsorption saturation.	[57]
Sedimentation	Sedimentation, aeration and a biological multi-stage treatment process	The removal rate of pollutants in the sedimentation tank: TSS: 71.9%; TN:55.6%; TP: 35.3%.	The technology is simple, easy to promote and the cost of infrastructure construction is low.	The volume of aquaculture wastewater is prone to exceeding the maximum load of the process.	[19]
Membrane technology	Continuous ultrafiltration and ultraviolet technology	SS > 73% BOD5: 35.25% COD: 40.00% Colibacillus:100%	The technology is simple, the operating cost is low, and the water change cycle is extended, which is conducive to water conservation.	Membrane fouling problem.	[58]
Ultraviolet treatment	Ultraviolet and microfiltration	Microparticle numbers: 74%; micro particle surface area: 54%; COD: 34%; microbial activity:89%.	Cartridge filtration appeared to reduce the build-up of micro particles by directly removing bacteria and bacterial substrate. Ultraviolet rays reduce the number of particles by reducing the dissolved matrix.	Filtration with a 1 μm filter element is difficult to apply; high energy consumption.	[59]
Ultraviolet treatment	Tetracycline (TC) is treated by ultraviolet/peroxymonosulfate (UV/PS) process	When the dosage of UV/PS was 5 mg/L, the removal rate of TC reached 95.73% after 30 min.	The main contributor to TC degradation by UV/PS in mariculture wastewater was reactive bromine species, followed by free chlorine. UV/PS process proved to have great potential for the harmless treatment of mariculture wastewater.	Cl− in mariculture wastewater slightly inhibited TC degradation by scavenging free radicals.	[60]

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3.2. Chemical Technology

Compared with fresh water or brackish water, seawater has a higher ionic strength [61], and marine aquaculture wastewater is characterized by high salinity and a low C/N ratio [62]. Chemical methods refer to the techniques that transform, separate and remove pollutants by adding chemical agents or initiating chemical reactions. Therefore, this technology may be beneficial for treating wastewater from marine aquaculture [63,64]. However, it must be noted that the high concentration of chloride ions (Cl⁻) in seawater may also have significant adverse effects on the chemical treatment process, such as competing for the consumption of oxidants, generating harmful by-products (such as chlorinated organic compounds), intensifying equipment corrosion, and interfering with the coagulation and flocculation process [65]. The main methods include chemical adsorption and sedimentation [66,67], flocculation and coagulation [68,69], electrochemical oxidation [70] and advanced oxidation [71], etc. For instance, Zhang et al. studied the crystallization of struvite in seawater/brackish water to explore the dissolution and precipitation of phosphate in the tail water. The results showed that the naturally occurring magnesium in seawater and the ammonia and phosphate in the tail water promoted the precipitation of struvite, which led to a decrease in the phosphate concentration and ammonia nitrogen concentration in the effluent [72]. It is worth noting that flocculants can not only remove suspended particles but also effectively eliminate certain organic substances. [73]. Zhang et al. studied the removal efficiency of inorganic coagulants FeCl₃ and polymeric aluminum sulfate (PAS) on TSS, turbidity, TOC, etc., in the tail water of saltwater aquaculture. The results show that both inorganic coagulants have good removal effects on solids and phosphorus under salt-containing conditions, and FeCl₃ is a powerful coagulant for further concentrating the wastewater from marine fish farms [74]. Seawater is a natural electrolyte. Yakamercan et al. treated recirculating aquaculture wastewater by electrooxidation (EO) with boron-doped diamond (BDD) electrodes [75]. The results show that the EO treatment method successfully removed more than 93% of the pollutants, including NH₃–N, NO₃⁺–N, NO₂⁺–N and TDN. The removal rate of COD in aquaculture wastewater was 86.85%, and that of TDP was 79.41%. Romano et al. conducted kinetic analysis and modeling of amination by establishing electrochemical cells with a Ti/RuO₂ anode and a Ti cathode [76]. The results show that electrochemical-assisted chlorination is an effective method for removing ammonia nitrogen from marine aquaculture wastewater, and the mechanism therein is revealed. Advanced oxidation methods include the Fenton process, photocatalysis, ozonation and persulfate process [77,78,79], etc. Ozone, chlorine and sodium hypochlorite are commonly used oxidants [80,81]. Zhang et al. studied the degradation effect of the ultraviolet/persulfate (UV/PS) process on tetracycline (TC) in marine aquaculture wastewater [60]. The results revealed that 95.73% removal of TC with a 5 mg/L dosage was achieved after 30 min UV/PS treatment. Zhao et al. used an ultraviolet/hydrogen peroxide (UV/H₂O₂) advanced oxidation process to treat mariculture wastewater containing sulfonamide-resistant genes [71]. The results show that the UV/H₂O₂ advanced oxidation process can effectively remove antibiotic resistance genes. However, due to the fact that high concentrations of Cl⁻ can decompose SO₄⁻ and form relatively inactive active chlorine species, it further affects the removal efficiency of pollutants. Some examples of chemical methods and their advantages and disadvantages are shown in

Table 2. Summary of Chemical Technologies for Treating Tail Water from Marine Aquaculture.

Treatment Method	Material/Strain	Treatment Efficiency	Advantages	Disadvantages	References
Adsorption precipitation	Struvite (magnesium ammonium phosphate hexahydrate, MgNH4PO4·6H2O) precipitation	Struvite precipitation effectively reduces the ammonium concentration in marine aquaculture wastewater from 10 mg/L to less than 1 mg/L.	The use of waste phosphate can reduce costs. The precipitated products can be used as flame retardants and anti-corrosion coatings for steel structures.	Large-scale implementation poses challenges, such as the design and maintenance of pumping and filtration systems.	[82]
Chemical precipitation	Sulfuric acid, nitric acid and citric acid. Sodium hydroxide is used to regulate the pH of water.	Sulfuric acid, nitric acid and citric acid can achieve a recovery rate of 71–86% of phosphorus content in sludge. Phosphorus re-dissolution with citric acid requires the highest amount of acid per g dry matter.	The processing steps are simple, the properties of the additives are simple, and they are widely available on the global market. Compared with inorganic acids, citric acid is cost-effective.	An economic assessment is required to determine the type, quantity and pH of the acid in order to estimate the actual feasibility.	[83]
Flocculation and coagulation	FeCl3 and AlSO4 Fe: PO43−-P = 2.6:1; Al: PO43−-P = 5.7:1	The removal rates of TCOD, TSS and TP were all above 89%. When the ratio of Fe: PO43−-P was 2.6:1, the removal rate of PO43−-P was 90%.	Higher water ion strength has an interaction effect on the removal of P. Salinity also improves the interaction between Fe3+ and P.	Pre-sedimentation treatment is required before adding any coagulant.	[84]
Electrochemical oxidation	Flow-through electrochemical oxidation process	The removal rates of NH3-N and NO2−-N can reach 90%. Antibiotics such as sulfamethazine and norfloxacin can be completely removed, with an energy consumption of only 0.054 kWh/g.	It has a good application prospect in the removal and sterilization of ammonia nitrogen and nitrite nitrogen.	The removal efficiency of total phosphorus and COD is relatively low, and disinfection by-products are produced.	[16]
Electrochemical oxidation	A bipolar system with Ti/BTNAs as the anode and cathode	The removal rate of NH3-N reached 97.6%, the residual total inorganic nitrogen was only 0.63 mg/L, and the minimum energy consumption was 0.043 kWh/g.	It is a long-lasting and low-consumption comprehensive strategy for removing organic pollutants and inorganic nitrogen from marine aquaculture wastewater.	The removal of active phosphate pollutants was not taken into account, and the types of antibiotic removal were relatively few.	[85]
Electrochemical oxidation	The electro-Fenton synergistic electrocatalytic system with dual cathodes	The removal rate of COD was 100%, and the removal rates of NH3-N, NO3-N and TN were 100%, 100% and 99.83% respectively.	Under the optimal operating conditions, the treatment cost is low and the effluent meets the first-level discharge standard.	The service life and the removal of active phosphate pollutants remain to be further studied.	[61]
Electrochemistry	Electrochemical-assisted submerged fixed-bed bioreactor (E-SFBBR)	The TN removal rate of E-SFBBR under electrical stimulation was 2.95–3.43 g N m−2 d−1. When the current density was 0.10 mA m−2, the removal effect on TN was better.	The N removal pathways related to the transformation of S and Fe in E-SFBBR were proposed.	The interaction between the S cycle, autotrophic denitrification and N transformation remains to be studied.	[86]
Advanced oxidation	Ozone (O3) treatment and ozone/ultraviolet (O3/UV) treatment	After treatment, the concentration of dissolved organic carbon in the aquaculture water was reduced by 40%, and NH3-N and NO2−-N were completely removed.	The O3 and O3/UV treatments have significantly improved the water quality of recirculating aquaculture and can also promote the growth of farmed fish.	Highly saturated intermediate products induce antibiotic resistance genes, presenting potential biological risks.	[87]
Advanced oxidation	Chloride ions (Cl−) promote the peroxymonosulfate (PMS) process (Cl−/PMS)	Under the condition of pH 8.0, the degradation rates of NH3-N and TN by the Cl−/PMS process reached 100% and 97%, respectively, within 15 min.	The Cl−/PMS process has a good anti-interference ability for the substrate of marine aquaculture wastewater. The intermediate product is green and non-toxic.	The simulation experiment awaits entering the application stage.	[88]
Advanced oxidation	Graphitic carbon nitride composite polymetallic doping was used to produce the photocatalyst (Pt/RuO2/g-C3 N4)	1.0 g/L Pt/RuO2/g-C3N4 photocatalyst can degrade 81% NH3-N. The higher Cl− concentration in real wastewater promotes the removal of NH3-N and sterilization.	In real water bodies, Pt/RuO2/g-C3N4 has a good removal effect on NH3-N and bacteria, and has good economy and feasibility.	The sterilization effect on bacteria other than Escherichia coli remains to be further studied.	[89]

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3.3. Biotechnology

Biological treatment refers to the technology that utilizes microorganisms, plants or their metabolic activities to degrade, transform and remove organic matter and nutrients from water bodies. Compared with physical and chemical techniques, this method is generally regarded as more economical, effective and environmentally sustainable [90]. However, the high salinity environment of marine aquaculture wastewater can exert osmotic pressure stress on microorganisms, leading to cell water loss, inhibition of enzyme activity and energy metabolism disorders, which is seriously detrimental to their survival and growth [91]. Therefore, researchers have specifically explored various biological treatment methods based on the characteristics of marine aquaculture wastewater, including membrane bioreactor (MBR) [92], microalgae technology [93], constructed wetland (constructed wetland system, CWs) technology [94], heterotrophic nitrifying—aerobic denitrification (HNAD) technology [95] and aquaponics [96], etc.

MBR technology, as a type of biological treatment method, possesses dual characteristics of biological treatment and membrane separation, with biological degradation as the main approach and membrane separation as a secondary one. For example, membrane contamination is a bottleneck that affects the application of MBR technology in mariculture wastewater treatment [97,98]. Therefore, Liu et al. conducted a study on the mitigation mechanism of membrane fouling by microalgae biofilms [99]. The results show that microalgae biofilm formation can effectively reduce the protein-to-polysaccharide ratio of extracellular polymers, thereby reducing the possibility of biofilm contamination by protein substances (especially tryptophane-like proteins), and is more economical and effective than the suspended microalgae method when treating low C/N marine aquaculture wastewater. Böpple et al.’s research also indicates that microalgae cultivation can fully absorb nitrates and phosphates in the effluent of recirculating aquaculture systems [100]. Biofilm reactors can adapt to changes in nutrient concentration and salinity, generating economic value while treating tail water. Microalgae technology also plays a key role in improving the removal of pollutants in marine aquaculture wastewater [101]. Liu et al. used a natural hybrid microalgae to study the mechanism of microalgae biofilm formation and the treatment effect on marine aquaculture wastewater under different ammonia nitrogen loads [102]. The results showed that when the ammonia nitrogen concentration was 25 mg/L, the removal rates of nitrogen and phosphorus were the highest (NH₃-N: 98%, TN: 84%, PO₄³⁻-P: 68%). Compared with single microalgae technology, the advantage of combined microalgae technology lies in the strong adsorption capacity of microalgae, which enables them to effectively absorb nutrients from wastewater and convert them into biomass [103]. Therefore, Zhang et al. studied the removal mechanism of antibiotics and nutrients in marine aquaculture wastewater by microalgae [104]. The results showed that the average removal rates of DIN and DIP in the membrane photobioreactor were 81.2% and 100%, respectively, and the degradation rate and adsorption rate of chlorella for the antibiotic (florfenicol) are 89.74% and 3.72% respectively.

In recent years, CWs has been gradually applied to the treatment of mariculture tail water because of its low economic cost and good benefits [105]. For instance, Li et al. investigated the effects of ammonia nitrogen loading rate and salinity on the removal rate of nutrients and the activity of substrate enzymes in the micro-ecosystem of constructed wetlands [106]. The results show that compared with low salinity, high salinity levels increase the removal rate of nutrients. The possible reason for this result is that halophilic and salt-tolerant microorganisms have dominated in CWs. Fu et al. demonstrated this possibility by screening out a salt-tolerant aerobic denitrifying bacterium, Zobellella denitrificans strain A63, and investigated its effects on the denitrification efficiency of saline wastewater and substrate denitrification microbial community structure in a vertical-flow constructed wetland. [107]. The results showed that the removal rates of NH₃-N, NO₃⁻-N and total nitrogen by this strain reached 79.2%, 95.7% and 89.9%, respectively. The heterotrophic nitrification—aerobic denitrification technology is also constantly maturing. Ning et al. studied the performance of continuous operation of heterotrophic nitrifying—aerobic denitrification biofilm reactors in treating wastewater from seawater recirculating aquaculture systems and tail water from traditional seawater aquaculture ponds [108]. The results show that the system achieved simultaneous nitrification and denitrification on the 7th day. The removal rates of inorganic nitrogen were 87.37 ± 4.23% and 65.00 ± 8.35% respectively. In addition, autotrophic denitrification is also an alternative to heterotrophic denitrification that has emerged in recent years. It reduces NO₃⁻-N to gaseous nitrogen by using inorganic electron donors and inorganic carbon sources [109]. Liu et al. established a heterotrophic–sulfur autotrophic denitrification system and studied the effect of Fe₃O₄ on the nitrogen removal effect of seawater recirculating aquaculture wastewater [110]. The results show that Fe₃O₄ enriches sulfur autotrophic denitrifying bacteria, thereby improving the denitrification efficiency. When the optimal concentration of Fe₃O₄ was 50 mg/L, the nitrogen removal efficiency reached 100%.

Halophytes can produce economically valuable crops while removing the main nutrients contained in aquaculture wastewater [111]. Doncato et al. cultivated sea asparagus (Sarcocornia ambigua) using wastewater from saline shrimp farms. By analyzing their growth rate and biomass, they demonstrated that they could serve as an alternative to irrigating salt-containing wastewater from aquaculture on salt-affected soil for food production [112]. Some examples of biological methods and their advantages and disadvantages are shown in

Table 3. Summary of Biotechnologies for Treating Tail Water from Marine Aquaculture.

Treatment Method	Material/Strain	Treatment Efficiency	Advantages	Disadvantages	References
Membrane Bioreactor	Sequencing batch biofilm reactors started with sludge inoculated at different C/N ratios	When the C/N ratio was 30, the average removal rates of NH3-N and TN were 95% and 73%. The high C/N ratio promoted the secretion of tightly bound extracellular polymers.	Membrane bioreactors inoculated with a high C/N ratio significantly enhance the positive interactions among dominant groups and promote the relative abundance of dominant bacteria.	The technical requirements are high, and the C/N ratio of biofilm sludge must be strictly controlled.	[113]
Biofilm technology	An A2O system enhanced by biofilm with specific bacterial strains added	The removal efficiencies of CODMn, NH3-N, TN and TP in the A2O system were approximately 86.3–90.8%, 97.7–99.5%, 94.6–95.2% and 97.0–98.1%.	The biofilm-enhanced A2O system can independently provide organic carbon sources, has strong salt tolerance, occupies a small area and has high treatment efficiency.	The technical requirements are high, and the service life of the biofilm needs further research.	[114]
Membrane Bioreactor	A rotating algal biofilm (RAB) system attached to seawater Chlorella sp.	The removal rates of TOC, NH3-N and PO43− reached 80%, 96% and 99%, respectively. Prolonging the retention time of algal biofilms can enhance the removal efficiency.	Algal biofilms can secrete a large amount of extracellular polymers. These substances promote the efficient removal of pollutants and help protect the biofilms from toxic substances.	The operating energy consumption is relatively high and the hydraulic retention time is long.	[115]
Microalgae technology	The microalgae co-culture system of Chlorella sp. and Phaeodactylum tricornutum	Under the optimal operating conditions, the removal rates of NH3-N, TN and TP were 100%, 93.26% and 96.12%, respectively.	The microalgae co-culture system can increase the total biological yield and achieve efficient removal of pollutants through interspecific interactions.	In actual operation, the environmental complexity is high and the operation difficulty is relatively large.	[116]
Microalgae technology	Microalgae membrane photobioreactor (MPBR) and autotrophic denitrification—nitrification integrated constructed Wetland (ADNI-CW)	The removal rates of TN and TP by the ADNI-CW-MPBR system were 92.63% ± 2.8% and 77.46% ± 8.41%, respectively. The biomass of microalgae was 54.58 ± 6.8 mg/L/d.	It has solved the problem of low efficiency in treating marine aquaculture wastewater by constructed wetlands, and has high-value biomass production. The synergistic effect between bacteria and chlorella ensures the efficient denitrification and phosphorus removal of the MPBR system.	The optimal reaction conditions need to be strictly controlled. The interaction between microalgae and bacteria requires further study.	[117]
Microalgae technology	Utilize Fe2+ to promote the granulation of pure algal strains into microalgae-bacterial granular sludge (MBGS)	Fe2+ can induce the formation of Fe precipitates and promote the adhesion of microbial cells. The removal rates of DOC, PO43−-P and NH3-N reached 98%, 98% and 87%, respectively.	The accumulation of exogenous heterotrophic bacteria and the enhanced expression of functional genes related to N and P metabolism significantly promoted the improvement of the nutrient salt removal rate.	Both insufficient and excessive Fe2+ concentrations can lead to a decrease in the ability to remove nutrients.	[118]
Constructed wetland	Partially saturated vertical flow constructed wetlands (PS-VFCW) at different saturation zone depths (SZD)	When the SZD was 60 cm, the removal rate of TN was as high as 97.3%, and the removal rate of the antibiotic sulfamethoxazole (SMX) reached its highest at 70 cm. The change in SZD has a significant impact on the structure of the bacterial community.	This technology can enhance the denitrification technology. The abundances of ammonia-oxidizing bacteria, nitrite-oxidizing bacteria, nitrifying bacteria and SMX-degrading bacteria show an increasing trend along the depth.	The technical requirements are high, and it is necessary to add exogenous carbon sources to improve its removal rate.	[119]
Constructed wetland	Autotrophic denitrifying—nitrification integrated constructed wetland (ADNI-CW) for mangrove plant cultivation	Under different hydraulic retention times, the nitrification rate of the autotrophic nitrifying constructed wetland unit (AN-CW) exceeded 92%, and the COD removal rate exceeded 96%.	ADNI-CW has the potential to couple sulfate reduction and sulfide-driven autotrophic denitrification in one device. The growth of various microorganisms in the constructed wetland system can enhance the removal of N.	The increase in the concentration of NO3−-N in the influent will lead to a decrease in the amount of sulfides and also affect the efficiency of autotrophic denitrification.	[120]
Constructed wetland	Constructed wetland of Sesuvium portulacastrum enhanced by iron–carbon microelectrolysis method	The presence of iron–carbon and seahorse teeth can increase the denitrification efficiency by 20–30% and 15–30% respectively. And they can all significantly change and improve the microbial community structure.	In this system, plant absorption, nitrification, denitrification and anaerobic ammonium oxidation all participate in the denitrification process of wastewater, and it has a good denitrification effect on wastewater with insufficient carbon sources.	The accumulation of intermediate products in the early stage may increase the toxicity of the effluent.	[121]
Nitrification and denitrification	Pyrite-driven autotrophic denitrification (PAD) and pyrite/poly-3-hydroxybutyrate-co-hydroxyvalerate-driven mixotrophic denitrification (PPMD)	The PPMD reactor has a better removal effect on NO3−N, but a poorer removal effect on PO43--P. When the influent NO3−N concentration is 25 mg/L, the removal rates of the PAD and PPMD reactors are 69.8% ± 5.6% and 82.3% ± 3.7%, respectively.	PAD also plays a key role in the PPMD reactor. Compared with the single PAD bioreactor, the synergistic effect between heterotrophic and autotrophic denitrification in the mixed nutrient denitrification bioreactor can promote nitrate removal.	The concentration and temperature of the influent NO3−-N will significantly affect the removal performance of NO3−-N and PO43− P in the PAD and PPMD reactors.	[122]
Nitrification and denitrification	Heterotrophic nitrification and aerobic denitrification (HNAD) processes based on solid carbon sources (SCSs)	The performance of SCS’s corn cobs is superior to that of peanut shells. The maximum removal efficiencies of NO3−-N and TIN were 99.71% and 96.72%, respectively, and the content of NO2−-N was always lower than 0.3 mg/l.	Corn cobs have a good denitrification effect. Compared with other carbon sources, they are low in price and do not cause secondary pollution. The pores formed by carbon release are conducive to the adhesion and growth of microorganisms.	The service life of corn cobs as a solid carbon source awaits further research to determine the service life of this system.	[123,124]
Nitrification and denitrification	A coupled system of simultaneous nitrification and denitrification (SND)—sulfur autotrophic denitrification (SAD) with an added carbon source	Under the condition of a C/N ratio of 1.2, the removal rates of NO3--N, TN and NH3-N reached 93.48%, 95.06% and 95.06%, respectively, and remained stable at 99.00% during steady-state operation.	The addition of carbon sources enhances the denitrification effect by simultaneously strengthening the SND and SAD processes, making it a cost-effective and sustainable denitrification technology for low C/N wastewater.	Sulfur element needs to be added externally as an electron donor, and its practical value in large-scale water bodies is relatively low.	[125]
Aquaponics	The removal of inorganic components in wastewater and their accumulation in Sarcocornia neei by sand substrate systems and deep-water systems	Sarcocornia neei is a halophyte with strong salt tolerance and great growth potential under seawater irrigation conditions. It has the highest removal rates of ammonia nitrogen (0.68 ± 0.41 g/m2/day−1) and total phosphorus (0.44 ± 0.34 g/m2/day−1) in sand substrate treatment.	Sarcocornia neei achieves a 100% plant survival rate, and the contents of organic nitrogen and organophosphorus in the plant biomass increase significantly. Therefore, it is a very good biological filter for marine aquaculture wastewater.	Further research is needed on the effects of adding essential micronutrients and different planting densities on its growth rate and nutrient removal efficiency.	[111]

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4. The Main Challenges Faced in Tail Water Treatment

To sum up, with the growth of population, the increase in per capita consumption of seafood and the decrease in consumption of other protein sources [126], the continuously developing land-based marine aquaculture industry has also drawn people’s attention to the importance of treating marine aquaculture wastewater and protecting the environment from the impact of polluted wastewater [39,127]. In recent years, research on various ways and methods for treating marine aquaculture wastewater has been conducted, as described in Section 3. Although physical technology can quickly remove large pollutants without generating by-products, it often only achieves partial performance and also has disadvantages such as sludge generation, high energy consumption, and frequent equipment maintenance [35]. Chemical technology is simple to operate, occupies a small area and can efficiently remove pollutants based on the high ionic strength of seawater. However, the chemicals added cannot be recycled and may produce harmful by-products. Electrochemical and other technologies may be difficult to implement on a large scale [46]. Biotechnology is a sustainable, environmentally friendly treatment technology that conforms to ecological laws. In view of the characteristics of high salinity and low carbon-nitrogen ratio of marine aquaculture wastewater, more efficient biological treatment technologies need to be studied specifically [128]. The research and application of comprehensive treatment technology is the direction of future marine aquaculture wastewater treatment [129]. However, the current research and application mainly focus on a single technology and remain at the experimental stage. Its application in real aquaculture wastewater is limited. The existing comprehensive technologies often originate from freshwater aquaculture [130], and targeted research is still scarce. The economic costs, advantages and disadvantages of several marine aquaculture tail water treatment technologies are summarized in

Table 4. Economic cost, advantages and disadvantages of tail water treatment technology for mariculture.

Type of Technology	Economic Cost	Advantage	Dilemmas
Physical technology	A set of integrated equipment that integrates precipitation, filtration and other physical processes, its tail water treatment cost is about 0.4–0.5 yuan/ton.	Simple and direct, no chemical reagents or biological treatment media are required. The processing effect is stable and controllable. It is applicable to the pretreatment of large-scale aquaculture tail water treatment.	The processing cost is relatively high and requires energy consumption (such as pumping, filtration, etc.). It is unable to remove dissolved organic matter and certain pollutants in water. The equipment maintenance requirements are high, and it may cause secondary pollution to the environment.
Chemical technology	The cost for treating wastewater with a TAN concentration of 10 mgN/L is approximately 6.67 cents/m3 [131].	It can remove most of the toxic and harmful substances in water, especially dissolved substances (such as heavy metals, organic pollutants, etc.). The treatment process is highly efficient and can respond quickly to changes in pollutant concentrations.	Chemical reagents need to be used, which may cause secondary pollution. The treated waste (such as chemical precipitation) requires further processing. The high cost of chemicals and their corrosiveness to equipment [132].
Biotechnology	The cost is about 240–300 yuan/m3 of water, saving about 50% of the traditional factory’s circulating water system. The energy consumption is approximately 0.002–0.006 kW·h/m3.	Biodegradation can remove organic matter from water, with good treatment effects and environmental friendliness. It has strong selectivity and can handle complex pollutants. It is environmentally friendly and causes little secondary pollution.	The processing speed is slow and it may take a relatively long time. It has high requirements for environmental conditions (such as temperature, pH, etc.), is difficult to control, and is easily affected by changes in aquaculture water quality. It is necessary to cultivate a relatively stable biological population.
Comprehensive technology	The cost of a traditional factory recirculating aquaculture system is about 400–600 yuan/m3 of water.	The combination of multiple technologies can better comprehensively utilize their respective advantages and improve processing efficiency. It can be flexibly adjusted and used according to the specific water quality conditions, reducing the limitations of a single technology. It is expected to achieve relatively stable long-term effects.	The system is complex, and the construction and operation costs are relatively high. It is necessary to coordinate the workflow of different technologies, which may increase the management difficulty. System maintenance is rather complicated and requires regular inspection and adjustment.

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China’s various provinces have successively introduced stricter standards for the discharge of aquaculture tail water. For instance, in Fujian Province, the discharge limits for TP, TN and COD in aquaculture tail water of Marine receiving waters are 0.5–1, 3–7 and 10–20 mg/L, respectively. The gradually strengthened regulatory requirements have also brought huge challenges to the marine aquaculture industry [133]. A too low C/N ratio in marine aquaculture wastewater will affect the effectiveness of nitrogen and phosphorus removal. In the actual treatment process, external organic carbon sources need to be added. With the improvement of tail water discharge standards, the excessively high TN concentration in tail water and the lack of carbon sources have become the main bottlenecks in the treatment of marine aquaculture wastewater, similar to the advanced denitrification of secondary effluent in urban sewage treatment [134]. It should be noted that excessive carbon sources can easily lead to anaerobic reduction of sulfates, releasing toxic H₂S [135]. The recirculating water aquaculture system is the development direction of the future land-based marine aquaculture industry, but at present, there are relatively few research cases of large-scale application in China.

The main challenge of current tail water treatment technology for land-based mariculture is the development and integration of core technologies. There is an urgent need to develop low-cost, high-efficiency and salt-alkali-resistant core technologies for land-based tail water treatment, such as halophilic/salt-tolerant microbial agents, Anammox technology suitable for seawater and low-energy consumption nitrogen and phosphorus removal processes. At the same time, the Internet of Things, sensors and big data algorithms should be fully utilized to conduct real-time monitoring of water quality parameters and precise and intelligent regulation of treatment processes, so as to reduce the consumption of chemicals, electricity and labor costs. It is important to note that the resource utilization of tail water and sludge is also one of the main challenges. Nutrients such as nitrogen and phosphorus in the tail water should be regarded as “resources” rather than “wastes”, such as by cultivating economically valuable microalgae and recovering nutrients, to create additional economic value and form a more sustainable development of land-based marine aquaculture.

5. Countermeasures and Suggestions

The adoption of highly engineered recirculating aquaculture systems (RAS) has become the development trend of global marine aquaculture and an inevitable trend for the sustainable development of aquaculture. RAS can reduce the discharge of aquaculture wastewater into the near-sea area and enable marine aquaculture technology to gradually achieve resource recycling [136]. Meanwhile, RAS reduces water consumption by optimizing the use and recycling of water. Therefore, controlling the concentrations of ammonia nitrogen and nitrite nitrogen in aquaculture water bodies is the key to achieving water circulation [137,138]. Zhang et al. compared the nitrogen removal characteristics and microbial communities in freshwater and seawater recirculating aquaculture systems. The research results provide a theoretical and practical basis for improving the activation speed of nitrifying biofilms in high salinity [136]. In the RAS water treatment process, physical filtration is the most ideal method for pretreatment of marine aquaculture wastewater. Then, ecological methods can be used for the deep removal of nutrients. Microalgae technology, constructed wetlands and aquaponics can generate certain economic value while treating the tail water [96,139,140]. Combining a bioreactor with RAS can achieve high treatment efficiency for most pollutants and offers high potential for resource recovery. At present, RAS projects account for the highest proportion in Europe. Among them, Norway dominates the market with its technological advantages and large-scale salmon production, ranking first in the world in terms of the number of projects. The application of RAS in China and other countries is shown in

Table 5. Comparison of the application of RAS between China and foreign countries.

Dimension of Comparison	International (Mainly European and American)	China
Size of market	Europe accounted for 52.1% of the world, followed by North America.	China accounts for 7.5%, with a fast growth rate.
Technology Maturity	Highly automated, integrating biofiltration and energy recovery technologies.	The closed system is the main type (85%), and indoor applications account for 70%
The main breeding species	High-value fish such as salmon and trout.	Litopenaeus vannamei (accounting for 40%), salt field shrimp, oysters, etc.
Driven by environmental protection policies	The strict emission standards of the European Union drive technological upgrades.	Policies such as the comprehensive management of the Bohai Sea have promoted the popularization of tail water treatment.
Extension of the industrial chain	Combine renewable energy sources (such as wind energy and solar energy).	The “breeding and cultural tourism” model.

and Figure 5.

Integrated multi-trophic aquaculture system (IMTA) is also a promising approach that enhances the performance of aquaculture while minimizing environmental pollution. It offers an alternative pathway toward the long-term sustainability and profitability of the aquaculture industry. Li et al. utilized a novel RAS-IMTA system to test the efficiency of nutrient salt bioreaction and the production capacity of diatom communities in oyster farming. More than 96% of nitrogen and phosphorus in the system were removed [141].

Despite being recognized as an imperative direction for transitioning mariculture towards resource efficiency and environmental friendliness, the large-scale adoption of RAS and IMTA in China continues to face significant challenges. Most current research remains confined to laboratory or pilot-scale stages, often overemphasizing the removal efficiency of individual pollutants while lacking a comprehensive evaluation of system integration stability, long-term operational reliability, and the economic costs of practical large-scale application. Consequently, many research outcomes struggle to translate into commercially viable solutions. Technically, the biofilm stability and energy efficiency of RAS under high-salinity conditions still require enhancement [142], while the species combination and ecological regulation techniques for IMTA remain underdeveloped. Economically, the absence of life-cycle cost–benefit analysis means that high initial investment and operational costs present major barriers. From a policy perspective, bottlenecks such as the lack of targeted incentives for resource recycling persist.

To address these issues, future research must shift from purely technological optimization to integrated techno-economic assessment. Key economic indicators—such as energy consumption, equipment durability, and operational maintenance costs—should be incorporated into the technology R&D framework. Furthermore, industry–academia collaboration should be promoted to conduct demonstration projects under typical production scenarios and develop cost-effective, low-maintenance technological models suited to China’s context. At the institutional and market guidance levels, it is essential to improve the supporting policy framework [143]. This includes accelerating the establishment of technical standards for RAS and IMTA, streamlining project approval procedures, and establishing green channel mechanisms. Innovative mechanisms for environmental rights trading should also be introduced—quantifying the water-saving and emission-reduction benefits of RAS and integrating them into carbon trading or eco-compensation schemes—to explore pathways for converting “reduction into carbon quotas, and quotas into economic benefits” [144].

Ultimately, by establishing a synergistic promotion framework centered on “technological iteration, economic viability, and policy empowerment,” mariculture tail water treatment technologies are expected to overcome current bottlenecks and transition from experimental research to full-scale industrial application. This progression will ultimately drive the mariculture industry toward the unification of environmental and economic benefits, with the key pillars of sustainable mariculture illustrated in Figure 6.

6. Conclusions

In recent years, land-based marine aquaculture has developed rapidly due to its abundant resources, and its output has exceeded that of traditional fisheries to meet the increasing demand for high-quality fish protein. However, land-based marine aquaculture poses a threat to the marine environment due to the generation of wastewater and the excessive use of resources. This article introduces various technologies for treating marine aquaculture wastewater at home and abroad, and summarizes the latest research achievements, advantages and disadvantages of each technology. It is found that the current research on marine aquaculture tail water treatment technology in China focuses on improving the decontamination efficiency of a single method, and most of it remains at the laboratory stage or the small-scale trial stage. Limiting factors such as energy demand, cost and scale have a significant impact on the expected treatment effect of actual tail water. For the more sustainable and circular development of land-based marine aquaculture, China needs to conduct targeted research on comprehensive treatment technologies that are suitable for the characteristics of marine aquaculture tail water, with a focus on resource recovery (such as water reuse and nutrient recycling). The ultimate goals are to optimize and develop safer, more energy-efficient and more efficient technical methods, and to ensure a sustainable marine aquaculture industry that operates within a circular economy framework.