Uptake of farming wastes by silver carp Hypophthalmichthys molitrix in polyculture ponds of grass carp Ctenopharyngodon idella: Evidence from C and N stable isotopic analysis

Understanding and reducing the environmental impacts of organic waste derived from fish farming activities are essential for improving the sustainable development of aquaculture and optimizing intensive farming techniques. To evaluate the feasibility and capability of using filter-feeding silver carp Hypophthalmichthys molitrix as biofilters to remove the farming wastes, including feed residues and fish feces, derived from the aquaculture of grass carp Ctenopharyngodon idella, five mesocosmic enclosures (A, B, C, D, E) combining grass carp (GC) and silver carp (SC) with different proportions were developed as experimental enclosures. For controls, silver carp from the same population were simultaneously transplanted to reference enclosures (F) where the co-culture of grass carp was absent. After 5 month acclimation, samples of SC tissue, particulate organic matter (POM), fish feed and fish feces (GC and SC feces) were collected for measurements of carbon and nitrogen isotopic ratios. Enrichment of 13C and 15N in the tissues of SC from the experimental enclosures compared to those from the controls indicated the uptake and assimilation of isotopically heavier farming wastes, i.e., fish feed and fish feces. Based on the isotope mixing model, the proportions of SC biomass assimilated from POM, fish feed, GC feces and SC feces to SC dietary consumption were 62.0, 24.4, 10.7, and 2.9% in A, 61.5, 26.9, 9.0, and 2.6% in B, 65.3, 24.3, 6.2, and 4.2% in C, 70.9, 20.6, 4.8, and 3.7% in D, and 72.1, 20.6, 3.5, and 3.9% in E, respectively.