Parasite Control in Flowing Aquaculture: Breaking the Transmission Cycle Through Flow-Rate Optimization and UV Sterilization

Controlling parasites in flowing aquaculture is one of the most long-standing problems of producers of the global community, especially in the systems whose water flow is continuous, i.e., flow-through, semi-recirculating and hybrid RAS aquaculture systems design (Power et al., 2025). This unceasing flow of water is not only vital in oxygenation but also in the removal of waste, which also provides effective routes through which parasites spread to various tanks and production lines. Many parasites possess mobile infective stages adapted specifically to aquatic hydrodynamics, allowing them to exploit water currents as transport mechanisms to reach new hosts (Mouritsen, 2025). As aquaculture becomes increasingly industrialized, the consequences of even moderate parasitic infestations have grown more severe because stocking densities are higher, production schedules are tighter, and biological stress tolerance among cultured species can be easily exceeded (Madsen & Stauffer, 2024). These pressures have made engineering-based parasite control a necessity rather than an optional management strategy. Among the technology-driven solutions available, the combined use of flow-rate optimization and ultraviolet sterilization has emerged as one of the most effective ways to interrupt transmission cycles and stabilize health performance in flowing aquaculture environments (Li et al., 2023).

To get to know this integrated approach, the first step is to see the behavior of parasites in flowing water. Almost all parasites that cause severe production losses in aquaculture, including Ichthyophthirius multifiliis, Trichodina, Amyluodinium and monogeneans of genera such as Dactylogyrus and Gyrodactylus, have free-swimming larvae or trophont stages that can move temporarily on their own (Buchmann, 2022). These infective stages depend on hydrodynamic forces to spread between tanks. In a connected water system, tomites, theronts and oncomiracidia are blown downstream by the currents and are transported because of sharing drainage lines, distribution manifolds, head tanks, and intermediate waterways, significantly amplifying the transmission potential (FAO, 2024). As they drift, they encounter new hosts at a much higher frequency than they would in stagnant water, allowing populations to expand even when clinical symptoms remain undetectable. Research from freshwater and marine aquaculture systems consistently shows that flowing water accelerates the spread of nearly all protozoan, monogenean, and crustacean parasites (Buchmann, 2022). Without intervention, parasites rapidly establish cyclical reinfection loops, increasing the likelihood of chronic gill irritation, reduced feed uptake, compromised immunity, and elevated mortality.

Flow-rate optimization is an interruption to this dynamic, which causes the hydraulic retention time in each tank or raceway to change. Hydraulic retention time is the time a particle stays in a particular unit before it is forced out (Fan et al., 2023). The shortening of this retention time will allow farms to physically eliminate stages of infective parasites before attaching to fish. The research on monogenean larvae reveals that, they are the most perilous during the initial two hours of their hatching and the infectivity reduces drastically after four to eight hours (Hoai, 2020). In juvenile salmonid or marine finfish systems with retention times in the farms of between thirty and fifty minutes they significantly decrease the likelihood of encountering a host by the larvae. It is an engineering-based solution that is not based on chemicals or biological remedies but rather relies on the velocity of water to exceed the pathogen biological window of infectivity (Morro et al., 2022). When handling highly parasite sensitive species like Atlantic salmon, rainbow trout, cobia, and sea bass, flow-rate manipulation is particularly of particular concern.

The flow characteristics within the pipes and tank systems also determine the presence of parasites. The laminar water flow is slow and facilitates sedimentation, thus the eggs of parasites, protozoa, or larvae settle on the surfaces of the pipes. Such deposits create reservoirs that inject infective content into the system on a regular basis. Conversely, turbulent water flow, which is normally attained when Reynolds numbers are greater than four thousand, suspends particulate material long enough to undergo mechanical filtration and sterilization processes (Li et al., 2023). The turbulent conditions are often created by engineers in the sections of the hydraulic line to prevent the destruction of fish species that are sensitive to turbulent water, including tilapia, catfish, and Pangasius (FAO, 2020).Species-specific hydrodynamic methodology is used so that the fish are subjected to suitable flow conditions without interfering with the removal of parasites.

Biology of species is important to identify the best hydraulic strategy. Cold-water species, which include trout and salmon, tend to have a high turnover rate due to their parasites being able to live longer in cold water (Madsen & Stauffer, 2024). On the other hand, warm-water species may have a higher retention time limit because of the variation in metabolic stability and oxygen requirement. The marine finfish are groupers, snappers, and sea bass which enjoy greater flow velocities and more beneficial aeration that also improve water quality and interfere with parasite attachment behaviors such as Neobenedenia, a highly problematic monogenean (Abbas et al., 2023). Therefore, designing a parasite-resistant flowing aquaculture system requires a deep understanding of the interaction between hydrodynamics and species-specific biology.

Flow-rate optimization involves eliminating parasites prior to infection whereas ultraviolet sterilization ensures that they do not even enter the system. The UV-C light, usually with the wavelength of 254 nm, alters and breaks the nucleic acid in microorganisms, inhibiting the replication of a species(González et al., 2023). Properly used, UV-C destroys more than 99 percent of free-moving parasite larvae, protozoan stages, zooplankton, as well as bacterial pathogens. Research has shown that doses of 30 to 120 mJ/cm² are neutral to a broad spectrum of aquaculture parasites (Fernández-Boo et al., 2021). Sensitive organisms, like Ichthyophthirius tomites, can be activated by low-levels as low as 25 mJ of energy, and more resistant organisms such as some marine protozoans such as Amyluodinium ocellatum could survive as many as 105 mJ (RK2, 2025). UV sterilization then appears as a necessary preventative that will stop parasitic and microbial pollution in flowing aquaculture systems.

However, UV performance depends heavily on system design. Undersized sterilizers allow partial bypass, leaving incoming pathogens untreated (Summerfelt, 2003). UV efficiency drops significantly in water with turbidity greater than five NTU, suspended solids above 25 mg/L, or UV transmittance lower than 85% (Desmi, 2025). For this reason, large-scale operations typically place mechanical drum filtration before UV chambers to remove particulates that would otherwise block light penetration. Many commercial aquaculture facilities install redundant UV banks to ensure uninterrupted disinfection even when lamps require maintenance or experience unexpected failure (Li et al., 2023).

UV strategies are also determined by species and production models. Salmon smolt systems have high requirements of 60-120 mJ since they are prone to protozoans and monogeneans (RK2, 2025). Farms of tilapia, which must operate in warmer and frequently murkier water, use never-ending UV loops with moderate flow-rate modifications. To ensure that larvae are not threatened by zooplankton and bacterial infections, shrimp hatcheries rely on high-dose UV and ultrafine mechanical filtration (FAO, 2020). Twin UV sterilizers are commonly used in marine finfish farms to reduce parasite pressure during the initial stages of production.

One of the most effective engineering-based parasite control systems in contemporary aquaculture is the interaction between the optimization of flowrates and UV sterilization. UV neutralizes pathogens prior to their being introduced into the culture units and optimized flow eliminates internally produced infective stages before they can achieve their life cycles. The dual model prevents parasite populations to create self-sustaining cycles and increases survival, feed efficiency, and long-term biosecurity (González et al., 2023).

Table: Key UV and Flow Parameters for Parasite Control in Flowing Aquaculture

The combination of these parameters results in the formation of hydraulic environments in which parasites cannot reproduce successfully in farms. Even though the method presupposes constant observation and technical skills, its long-term advantages are reduced treatment costs, improved welfare, and better predictability of production. The only way to achieve sustainable aquaculture in an industry where outbreaks can disrupt the whole production cycle is through parasite suppression, which is an engineering concept.

At WOLIZE , we specialize in designing customized flow and UV sterilization systems for industrial aquaculture. We support producers in ensuring good growth performance, predictable survival and low parasite pressure in the problematic production environments by combining specific hydrodynamics of species with high technology disinfection engineering.

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