Nutrient Use Efficiency and Sustainable Productivity in Tilapia–Catfish and Lettuce–Spinach Aquaponic Polyculture

This study evaluated productive performance, nutrient use efficiency, and nitrogen and phosphorus mass balance in an intensive aquaponic polyculture system combining Nile tilapia (Oreochromis niloticus), channel catfish (Ictalurus punctatus), lettuce (Lactuca sativa), and spinach (Spinacia oleracea) under high biomass density (40 kg m−3). Nine treatments were established through a 3 × 3 factorial combination of fish (tilapia:catfish = 75:25, 50:50, 25:75) and plant (lettuce:spinach = 75:25, 50:50, 25:75) species ratios and evaluated over three consecutive 60-day production cycles. Nitrogen and phosphorus use efficiencies differed significantly among treatments, reaching maximum values above 50% for NUE and 47% for PUE in catfish-dominant systems with higher spinach proportions, indicating improved nutrient recovery and reduced losses. These treatments also produced greater fish biomass, whereas lettuce-dominant combinations favored plant yield. Water quality remained within acceptable ranges, although higher catfish proportions were associated with lower dissolved oxygen and increased nitrogen availability. Overall, results demonstrate that optimizing fish–plant species ratios enhances nutrient retention and sustainable productivity in intensive aquaponic systems. Future research should explore adaptive species ratio management and economic feasibility to support large-scale implementation of polyculture aquaponics.