Stocking filter-feeder in fed fish aquaculture pond: Unexpected Aggravation of nitrous oxide emission

Intensive farming of fed fish could produce large amounts of uneaten feed and feces, potentially leading to increased nitrous oxide (N₂O) emissions. Filter-feeding fish can ingest residual feed and feces, but it is unclear whether introducing them into fed fish farming ponds could reduce N₂O emissions. This study employed monoculture of fed largemouth bass (Micropterus salmoides, LB) and polyculture of LB with filter-feeding silver carp (Hypophthalmichthys molitrix, SC) at density ratios of 18:1, 9:1 and 4.5:1 to compare the N₂O emission characteristics. The results showed that silver carp could indeed feed on largemouth bass feces, and isotope mixing model indicated that feces was the second largest contributor to the food of silver carp, reaching 14.75 %-15.56 %. However, polyculture of the two species did not or even increased N₂O emission flux at water-air interface and its release potential in sediment. Increased mineralization, nitrification and denitrification rates were observed in polyculture systems, particularly at high stocking densities of silver carp. Also, the higher NH₄⁺ accumulation were found across sediment-water interface within polyculture systems. Metagenome revealed that polyculture disturbed the microbial community structure and increased the abundance of Burkholderiales and Steroidobacteraceae. Moreover, polyculture increased the abundance of nitrogen-cycling functional genes, including gdhA, hao, nirB and norB, potentially contributing to the elevated N₂O emissions. Structural equation model highlighted that polyculture of largemouth bass and silver carp could drive N₂O emissions, mainly through increased sedimental NH₄⁺ concentration and microbial activity. These findings indicate that the introduction of extractive filter-feeding fish into fed fish farming ponds could not reduce N₂O emissions, implying the need for optimized management strategies to balance aquaculture productivity with environmental sustainability.