Spring–Autumn Differences in Nekton Community Structure and Resource Density in the Coastal Waters of Fujian, China, in 2025

To compare spring–autumn differences in nekton composition, abundance density, and biomass density, bottom-trawl surveys were conducted at 81 matched stations off the coast of Fujian in April and September 2025. We recorded 455 species-level taxa: 330 in spring and 369 in autumn, with 244 shared (Jaccard dissimilarity, 0.464). Abundance-based diversity and evenness increased in autumn, whereas biomass-based indices remained similar. Community composition differed between surveys for both density matrices (PERMANOVA, both p = 0.001), with season explaining 7.3% and 7.0% of the total variation in abundance and biomass, respectively. Dominance shifted from Evynnis cardinalis and Trichiurus japonicus in spring to Pennahia macrocephalus and E. cardinalis in autumn. Total abundance density showed no significant difference, whereas mean biomass density reached 1372.06 kg km−2 in autumn, 1.90 times the spring value of 721.07 kg km−2 (p < 0.001). Pennahia macrocephalus, P. argentata, E. cardinalis, and Atrobucca nibe together accounted for 60.79% of the autumn-minus-spring biomass difference. Within each season, community composition was associated with depth, surface temperature, and surface salinity. These results demonstrate that similar biomass-based diversity values can accompany substantial compositional change and that the higher autumn biomass was associated primarily with a limited set of fishes. Monitoring programs should therefore integrate abundance, biomass, and composition rather than rely on a single aggregate indicator.