To reveal the current status of fish resources and the changing trend of trophic levels in the western Xiamen Bay, and to provide a scientific basis for the management of fishery resources. Based on the results of bottom trawl surveys conducted in the western Xiamen Bay during 8 sampling cruises from 2023 to 2024, we analyzed the fish species composition, community structure, and carbon and nitrogen stable isotope characteristics of dominant species in this area. Results of diversity indices show that the Margalef's species richness index (D) ranged from 1.00 to 5.23, with a mean of 2.70±0.47; the Shannon-Wiener diversity index (H') ranged from 0.65 to 2.48, with a mean of 1.77±0.18; and Pielou’s evenness index (J') ranged from 0.23 to 0.99, with a mean of 0.67±0.05. The values of D, H', and J' were generally higher during the wet season (Spring and summer) than during the dry season (Autumn and winter). Cluster analysis (CLUSTER) and non-metric multidimensional scaling (NMDS) indicate that the fish community in the western waters of Xiamen Bay could be broadly divided into five groups, with distinct characteristics between the wet and dry seasons. Stable isotope analysis of carbon and nitrogen in dominant fish species indicates that the current trophic levels of these species are relatively concentrated. Compared with the levels observed in the 2010s, the average trophic level has declined, suggesting a shift in the community structure toward lower-trophic-level groups. This shift may be attributed to the replacement of high-trophic-level dominant species or the miniaturization of individuals. The findings provide stable isotopic evidence for the degradation of food-web trophic structure driven by fishing pressure and other human activities.

Lipids in animal muscle tissue influence the determination results of stable isotopes. Given the high body fat content in cetaceans, this effect might be more pronounced, potentially leading to misinterpretations in their dietary source analysis. To investigate the impacts of lipid removal treatment on carbon and nitrogen stable isotope analysis in cetacean muscle tissue, we applied a methanol-chloroform lipid extraction method. We treated the muscle tissues from Indo-Pacific humpback dolphins (Sousa chinensis) and East Asian finless porpoises (Neophocaena asiaeorientalis sunameri) with lipid removal across six different time gradients, and fitted curves to track temporal shifts in carbon stable isotope values (δ¹³C). The results show that there were significant differences in δ¹³C levels, but not in δ¹⁵N levels, in muscle tissue before and after lipid removal. Furthermore, δ¹³C values in both species increased initially with longer extraction time but stabilized after approximately 6 h. Based on these findings, this study determined the optimal lipid removal duration for small cetaceans that balances accuracy and efficiency, providing a reference for subsequent related research. Finally, we performed a comparative analysis of carbon and nitrogen stable isotopes in muscle tissues of S. chinensis and N. asiaeorientalis sunameri for optimal lipid removal duration, and investigated niche differentiation between the two species, thereby further elucidating their trophic relationship. The results indicate a high degree of trophic niche overlap between the two species in the western Taiwan Strait, with differentiation potentially occurring along other dimensions, such as habitat utilization.