The Differences in the Responses of Pelagic Fish Distribution in the Northern South China Sea to Environmental Factors: A Case Study of Round Scad and Jack Mackerel in the Hainan Island Offshore Area

Round scad (Decapterus maruadsi) and jack mackerel (Trachurus japonicus) are economically significant pelagic species widely distributed in the northern South China Sea (SCS), with overlapping habitats and life history stages. To examine the distribution patterns of round scad and jack mackerel and their responses to environmental variables, we conducted a preliminary analysis using catch and environmental data from four seasonal surveys around Hainan Island. Three species distribution models—generalized linear models (GLM), generalized additive models (GAM), and random forests (RF)—were applied to quantify species–environment relationships. Explanatory variables included both biotic and abiotic factors: temperature, salinity, water depth, sea surface chlorophyll a concentration (SSC), phytoplankton abundance, and zooplankton abundance. The results revealed pronounced spatial heterogeneity in the high-density areas of both species. Among the models, GAM consistently explained a higher proportion of deviance in the observed distributions. Further analysis showed that round scad and jack mackerel responded differently to environmental gradients such as water depth and temperature, although their responses to varying plankton concentrations were largely consistent. Specifically, round scad are typically found in waters at depths ranging from 0 to 50 m, whereas jack mackerel tend to inhabit depths exceeding 100 m. In response to high plankton abundance, both species exhibit a notable increase in resource availability when plankton levels surpass 3. These findings indicate distinct spatial niches and suggest potential competition in feeding ecology between the two species. Overall, the study enhances understanding of the spatial dynamics of key commercial species in the northern SCS and provides valuable insights for sustainable fisheries management and conservation planning.