Water Quality Regulation and Stocking-Ratio Assessment in a Black Seabream–Mussel–Seaweed Integrated Multi-Trophic Aquaculture System

Integrated multi-trophic aquaculture (IMTA) combines fed and extractive species, yet appropriate biomass allocation among trophic groups remains poorly defined. This study investigated water quality regulation and stocking biomass ratios in a black seabream (Acanthopagrus schlegelii)–mussel (Mytilus coruscus)–seaweed (Ulva lactuca) IMTA system. Sequential monoculture, two-species, and three-species experiments were conducted in 300 L aquaria over 15 days, with three replicates per treatment. Black seabream monoculture increased NH4+-N and PO43−-P accumulation relative to controls, whereas U. lactuca reduced dissolved nutrient concentrations. Fish–mussel co-culture did not prevent nutrient accumulation, while increasing fish biomass in fish–seaweed co-culture was associated with greater nutrient accumulation and lower seaweed RGR, which may have also been influenced by direct grazing by black seabream. Conversely, seaweed relative growth rate (RGR) increased with mussel biomass in mussel–seaweed co-culture. Among three-species treatments, FMU1 (80 g fish, 600 g mussels, and 150 g seaweed; initial biomass ratio 1:7.5:1.875) showed the best overall balance among the three tested fish biomass levels under the experimental conditions. After 15 days, NH4+-N decreased by 41.18% in FMU1 but increased by 15.91% and 61.65% in FMU2 and FMU3, respectively. These findings highlight the importance of balancing fish loading with extractive biomass to improve water quality regulation and sustain seaweed growth in coastal IMTA systems.