Aquaculture intensification increases the accumulation of solid waste derived from uneaten feed and fish excretion, which may reduce water quality and compromise production efficiency. At the same time, such waste may represent a recoverable resource within circular aquaculture systems. This study evaluated solid waste generation under different stocking densities in a hybrid grouper (Epinephelus fuscoguttatus × E. lanceolatus) nursery system and estimated its economic value using a willingness-to-pay (WTP) approach. The study was conducted from February to March 2025 using a completely randomized design with three stocking densities, namely 10, 30, and 50 fish per 0.5 m³ enclosure, with three replicates, over a 28-day culture period. Solid waste accumulation, water quality, biological performance, and economic valuation were analyzed. Solid waste production differed among treatments, with the highest accumulation recorded at the intermediate density, 926.39 g, whereas the highest density treatment produced lower solid waste, 87.35 g, due to early mortality associated with water-quality deterioration. The highest density also showed the highest ammonia, 1.76 mg L-1, and hydrogen sulfide, 2.34 mg L-1, indicating that the carrying capacity of the nursery system had been exceeded. The WTP survey involved 110 valid respondents and showed that the mean WTP for solid aquaculture waste was Rp4,527 kg-1, equivalent to USD0.25 kg-1, with observed values ranging from Rp0 to Rp23,000 kg-1, equivalent to USD0.00-1.28 kg-1. Bioeconomic evaluation indicates that the intermediate stocking density provides the most balanced outcome in terms of biomass retention, environmental stability, and recoverable waste value. These findings demonstrate that solid nursery waste can be quantified not only as an environmental burden but also as a potential bioeconomic resource, providing baseline information for circular aquaculture and Integrated Multi-Trophic Aquaculture development.
