Abstract: The mackerel scad (Decapterus macarellus), a commercially important pelagic species in the South China Sea, is a primary target for the light falling-net fishery in that region. To elucidate the spatio-temporal variation in the resource density of D. macarellus in the central-southern South China Sea and identify the key environmental factors, we analyzed the fishery survey data collected during the spring seasons of 2017, 2020 and 2023. Eight oceanographic variables were considered, including sea surface salinity (SSS), sea surface temperature (SST), sea surface height (SSH), chlorophyll a (Chl a) concentration, ocean depth (Depth), offshore distance (DIS), temperature gradient (TGR), and mixed layer thickness (MLT). We assessed the relationship between the resource density of D. macarellus and these environmental factors by using Generalized Additive Model (GAM), and optimized the model by using Pearson correlation coefficients, variance inflation factor (VIF) analysis, and the Akaike information criterion (AIC). The results reveal significant regional differences in the spring distribution of D. macarellus fishing grounds in the central-southern South China Sea. SST and Chl a concentration were the primary factors influencing resource density. The optimal GAM model explained 47.1% of the total deviance. The results suggest that the spring distribution of D. macarellus in that region is mainly influenced by a combination of SST, Chl a concentration and ocean depth, with Chl a concentration exerting the most pronounced effect on the fishery catches.