Sustainable Polyculture of Hybrid Yellow Catfish (Pelteobagrus fulvidraco ♀ × P. vachelli ♂) and Chinese Olive Mussel (Solenaia oleivora) Enhances Water Quality and Modulates Bacterial Community Assembly in Pond Ecosystems

Fish–mussel polyculture is a promising strategy for sustainable aquaculture. This study investigated hybrid yellow catfish (Pelteobagrus fulvidraco ♀ × P. vachelli ♂) and Chinese olive mussel (Solenaia oleivora) polyculture on water quality, bacterial community structure, and fish growth performance over a six-month production cycle. At harvest, polyculture fish had an 11.65% higher weight gain rate than those in monoculture. Polyculture reduced TN to 1.89–1.95 mg/L (vs. monoculture 2.74–3.44 mg/L) in July–October and kept TP at 0.29–0.73 mg/L (vs. monoculture 0.37–1.45 mg/L). The microbial α-diversity analysis revealed that the community richness and diversity of monoculture were reduced in July, whereas polyculture experienced decreased richness in October and diminished diversity in both July and October. Dominant bacterial phyla included Proteobacteria, Actinobacteriota, Bacteroidota, and Firmicutes, with Proteobacteria showing higher relative abundance in polyculture. Genus-level analysis revealed distinct successional patterns driven by season and cultivation mode. Notably, polyculture systems can effectively suppress potential pathogens. Redundancy analysis indicated that environmental factors played crucial roles in shaping the microbial community structure. More importantly, it provides scientific basis for optimizing freshwater polyculture models and offers practical technical support for promoting ecologically sustainable aquaculture through improved nutrient cycling and microbiome modulation.