Fish feeding groups, food selectivity, and diet shifts associated with environmental factors and prey availability along a large subtropical river, China

Understanding spatial variation in fish trophic structures along large river systems remains a challenge, and the influence of fish food selectivity and diet shifts on these structures remains unknown. In this study, the assemblage composition and stomach contents of fish in the subtropical East River of southern China were analyzed to determine their diet composition (DC) and identify prey-oriented feeding groups. Ten prey items were identified and used to cluster 106 fish species into 23 feeding groups. The number of groups increased longitudinally due to the accumulated emergence of site-specific prey sources in fish DC, although this number decreased sharply near the estuary due to the loss of insectivorous fish. The fish assemblages showed a longitudinal decrease in abundance and an increase in biomass, with higher values observed in the rainy season than the dry season. A downstream decrease in insectivores and epiphytivores and increase in detritivores and molluscivores represented the basic patterns observed along the river. Seven widespread fish species exhibited spatial dietary shifts, among which three generalist feeders with a high abundance notably influenced the fish trophic structures, and four specialist feeders with high food selectivity were significantly (P<0.05) correlated with prey availability. Multivariate data analysis showed that the flow velocity, water depth, riffle areas, and nutrient concentrations were the key environmental factors that determined the distribution of fish feeding groups, while hydrophytes, plant debris, Ephemeroptera and Odonata insects, and Atyidae shrimp were the key prey sources.