Ecological Niche Changes of Two Sympatric Species ( <i>Pennahia argentata</i> and <i>Larimichthys polyactis</i> ) Revealed by Bulk and Compound Specific Isotope Analyses

Sympatric species in marine ecosystems often share habitats and food resources, leading to niche competition. We performed ecological niche width estimation based on bulk carbon (δ¹³C) and nitrogen (δ¹⁵N) isotopes and trophic position estimation via compound-specific isotope analysis (CSIA) of amino acids to compare ecological interspecific competition between two sympatric fish species, <i>Pennahia argentata</i> and <i>Larimichthys polyactis</i> in Gwangyang Bay, South Korea. Moreover, ontogenetic niche changes were investigated by size classification for each species. The δ¹³C and δ¹⁵N values in <i>P. argentata</i> showed no significant size-related differences (<i>p</i> > 0.1), while <i>L. polyactis</i> exhibited significant variation (<i>p</i> < 0.001), indicating different ontogenetic niche shifts between two sympatric species. Trophic positions (TP_Glu/Phe) estimates based on nitrogen isotopic composition of glutamic acid (δ¹⁵N_Glu) and that of phenylalanine (δ¹⁵N_Phe) were consistent across groups (3.53-3.62), indicating similar diet consumption. However, we found lower δ¹⁵N_Phe values in large <i>L. polyactis</i> suggesting assimilated prey from distant marine areas before migrating into the study site. These findings demonstrate the utility of combining bulk stable isotope analysis and amino acid-specific CSIA to disentangle dietary patterns and habitat use in mobile fish species. This study provides fundamental information for more targeted and ecological management strategies under variable environmental conditions by contrasting ontogenetic niche shifts and habitat-driven isotopic variation between two sympatric species.