Climate Variability and Fish Community Dynamics: Impacts of La Niña Events on the Continental Shelf of the Northern South China Sea

This study investigates the impacts of climate variability, particularly La Niña events, on the fish community on the continental shelf of the northern South China Sea, a region highly sensitive to environmental fluctuations. Historical fishery survey data, collected from autumn 2019 to autumn 2022, were used to analyze changes in species composition, diversity indices, and community structure during La Niña and non-La Niña periods. The results show that La Niña significantly altered the fish community dynamics. During La Niña, cold-water conditions expanded the range of suitable habitats for cold-water species, leading to increased dominance of the Japanese scad (Decapterus maruadsi), with its index of relative importance (IRI) reaching 1795.9 and 1320.2 in autumn 2021 and 2022, respectively. In contrast, warm-water species experienced a reduction in suitable habitats. During La Niña, Margalef’s richness index (D’) peaked at 23.18 in autumn 2021 but decreased to 20.69 by spring 2022. The Shannon–Wiener diversity index (H’) dropped from 2.597 during a non-La Niña period (spring 2020) to 2.406 during La Niña (spring 2022); similarly, Pielou’s evenness index fell from 0.4749 to 0.4396, indicating an increase in ecological imbalance. As La Niña conditions weakened, the fish community began to recover. By autumn 2022, D’ had risen to 22.73 and H’ to 2.573, reflecting a gradual return to fish community conditions before the La Niña event. Species distribution models incorporating key environmental variables (i.e., sea surface temperature, salinity, and dissolved oxygen) demonstrated that the habitat of D. maruadsi expanded significantly during La Niña and contracted during post-event periods. Our findings highlight the ecological sensitivity of fish communities to climate variability and underscore the importance of adaptive resource management strategies to mitigate the impacts of climate change on marine ecosystems. This research provides valuable insights for sustaining regional fishery resources under changing environmental conditions.