Evaluation of ecological quality in the northeastern waters of the Beibu Gulf based on the fish index of biotic integrity

In this study, 29 sampling stations were established in the northeastern Beibu Gulf. Environmental DNA (eDNA) metabarcoding was applied to analyze fish community structures, and the fish index of biotic integrity (F-IBI) was used to evaluate ecological quality. A total of 70 fish species were detected, belonging to 11 orders, 31 families, and 59 genera, dominated by Perciformes. The dominant species included Trachinotus ovatus, Sardinella gibbosa, and Nematalosa japonica. Redundancy analysis indicated that NO3-N, Pb, Dep, pH, Cu, TPH, and Chl a were significant driving factors of fish community patterns. The F-IBI results suggested that the overall ecological status was relatively good, with 45% of stations rated as “good” or above, and 55% rated as “fair” or below, while no stations were classified as “poor”. Spatially, stations with “good or above” conditions were mainly distributed in the western and offshore areas, “fair” conditions in the central–southern offshore areas, and “poor” conditions in the northern and eastern nearshore areas, which overlapped with regions of intensive human activities such as port shipping. This study demonstrates that combining eDNA with F-IBI provides a feasible and promising approach for ecological quality assessment, offering a scientific basis for bay ecosystem conservation and restoration.