Fish Assemblages Distinguish Eco-Friendly from Conventional Rice Paddies Through Abundance and Biomass Rather than Diversity

Although rice paddies are agricultural land, they also serve as artificial wetlands that deliver essential ecosystem services. Yet how farming systems affect their fish communities—and which metrics best capture those effects—remains poorly quantified in Asia. We compared fish assemblages between eco-friendly and conventional rice paddies in South Korea across the 2015 rice growing season (May–September), sampling 94 events by fyke net in Dangjin City (2704 individuals; 13 species) and analyzing the data with generalized linear mixed models and multivariate methods. Eco-friendly paddies supported 2.3-fold higher abundance and 2.6-fold higher biomass; differences were negligible in May (Cohen’s d ≈ 0) but large from June onward (d > 1.0), with a moderate-to-large pooled effect (d = 0.76–0.89). Community composition differed significantly (PERMANOVA), and eco-friendly assemblages were more consistent among fields (PERMDISP). Carassius auratus and Misgurnus anguillicaudatus were 10.8- and 2.7-fold more abundant under eco-friendly management, whereas α-diversity indices (richness, Shannon, Simpson, evenness) showed no differences, as three taxa comprised over 90% of the catch. In these species-poor, dominance-skewed systems, abundance-based and species-specific metrics detected management effects more sensitively than α-diversity indices, complementing rather than replacing them. Because these results derive from a single growing season in one region, however, they should be regarded as specific to comparable systems and require multi-year, multi-region validation before their use in biomonitoring and eco-friendly agricultural policy can be generalized.