Trophic ecology of moonfish in southwest Taiwan: Insights from stable isotopes and stomach content approach in connection to marine environmental factors.

Moonfish (Mene maculata) has been recognized as a prospective commercial species in Taiwanese waters; however, its diet and trophic ecology remain poorly understood. It was hypothesized that diet composition and trophic position (TP) vary with body size and monsoonal environmental changes. This study presents the first investigation of its feeding ecology in southwest Taiwan (SWT), focusing on seasonal and ontogenetic variation. A total of 2329 individuals collected from September 2021 to May 2024 were analyzed using stomach content analysis (SCA) and stable isotope analysis (SIA). Eight zooplankton prey groups were identified, with feeding intensity found to be higher during the northeast (NE) monsoon, likely for energy restoration during gametogenesis, and reduced during the southwest (SW) monsoon, coinciding with spawning. Immature individuals exhibited a more diverse diet, particularly during the NE monsoon (e.g., Decapoda: 58.6 % IRI; Euphausiida: 14.2 % IRI), while Decapoda dominated (>90 %) across all size classes during the SW monsoon. PERMANOVA confirmed significant effects of size, season, and their interaction on diet (R2 = 93.17 %, p < 0.001). Isotopic values ranged from -18.0 ‰ to -16.24 ‰ for δ13C and 6.57 ‰-11.92 ‰ for δ15N, confirming a mid-level carnivorous role. Generalized additive models (GAMs) revealed that dissolved silicate, nitrate, and phosphate were key environmental predictors of TP. Broader isotopic niches were observed during the NE monsoon, suggesting greater dietary plasticity. These findings illustrate how nutrient-driven environmental variability influences trophic dynamics and support ecosystem-based fisheries management (EBFM) strategies in Taiwan's coastal ecosystems. By elucidating the feeding adaptability of moonfish, this study contributes valuable insights for predicting how climate-induced shifts in monsoonal regimes may affect food web stability and fishery sustainability.