Introduction
Intensive production of African catfish, Clarias gariepinus, generates water containing suspended solids and dissolved nutrients, particularly nitrogen and phosphorus compounds. Although recirculating aquaculture systems reduce water consumption, nutrient removal and the management of accumulated organic matter remain important operational and environmental challenges. Duckweed, particularly species of the genus Lemna, grows rapidly on nutrient-rich water and can assimilate dissolved nutrients into protein-rich plant biomass.
This work presents an integrated production concept combining indoor catfish culture in a recirculating aquaculture system, outdoor catfish tanks and duckweed cultivation within a single water-recycling loop. The objectives are to improve water reuse, recover nutrients and produce a locally available feed supplement for juvenile catfish.
System Concept and Operation
Water discharged from the indoor and outdoor catfish production units first passes through mechanical filtration for the removal of floating and larger suspended solids. The mechanically treated water, which retains dissolved nitrogen, phosphorus and other metabolites, is subsequently directed to shallow duckweed cultivation ponds.
Duckweed assimilates dissolved nutrients during rapid vegetative growth and thereby acts as a biological polishing stage. The treated water is then returned to both catfish production units, closing the water-recycling loop and reducing the requirement for fresh water and nutrient discharge.
Under favourable temperature, nutrient and light conditions, expected duckweed productivity is approximately 10–30 g dry matter m-2 day-1, with harvesting carried out every one to two days. Regular harvesting prevents excessive surface density and continuously removes assimilated nutrients from the water.
Duckweed Processing and Nutritional Use
Harvested duckweed is washed, dried and milled before being incorporated into formulated feed for catfish larvae and fingerlings. Depending on species and cultivation conditions, duckweed may contain approximately 35–45% protein on a dry-matter basis. It also provides essential amino acids, vitamins, minerals and carotenoids.
The harvested biomass can therefore partially convert nutrients originating from fish production into a valuable feed ingredient. Further trials will determine suitable inclusion levels and evaluate effects on feed intake, growth, feed conversion, survival, fish health and water quality.
Discussion and Conclusion
The proposed system links intensive fish production, water treatment and feed-resource generation in a circular production process. Duckweed closes part of the nutrient loop by recovering dissolved nutrients that would otherwise be discharged or require additional treatment. Its integration may reduce water exchange, nutrient emissions and operating costs while providing a renewable supplementary feed material.
The concept is particularly relevant for intensive inland aquaculture facilities seeking to improve resource efficiency without substantially increasing the complexity of water treatment. Operational validation will focus on nutrient-removal efficiency, duckweed productivity, seasonal stability and the nutritional value of the harvested biomass. The integrated approach offers a practical pathway towards more circular, water-efficient and resilient catfish production.