Trophic niche shifts in Japanese anchovy, Engraulis japonica, during ontogenetic migration in a temperate continental shelf system

Understanding the mechanisms driving ontogenetic niche shifts in fishes has predominantly focused on dietary changes in large predatory migrants. This study investigates trophic niche dynamics in a small, pelagic migratory fish, the Japanese anchovy (Engraulis japonicus). By mapping migratory paths across varied oceanographic regions on a temperate continental shelf of Korean waters, we quantified changes in the trophic niche using δ¹³C and δ¹⁵N values across different ontogenetic stages (juvenile, subadult, and adult). We hypothesized that biological and ecological processes associated with fish ontogeny and migration cause trophic niche shifts. Results showed that juvenile anchovies had a narrow trophic niche, which expanded as they matured, indicating a shift from specialist to generalist feeding habits, unlike many large predatory fishes. Limited isotopic niche overlap among life stages suggested niche partitioning within inshore habitats where all stages coexist. Morphological and behavioral traits related to life history likely drive these feeding modes and prey use changes. Isotopic profiles varied with migratory routes, reflecting spatial variations in zooplankton isotope baselines. Adults showed higher δ¹³C in the southern offshore (Yangtze River Diluted Water area) and lower δ¹⁵N in the eastern offshore (Tsushima Warm Current area) compared with inshore (South Korea Coastal Water area), resulting in distinct isotopic niches. This isotopic variation is likely due to differing lower trophic-level processes and zooplankton assemblages, as supported by our dietary mixing model. Our findings highlight that ontogenetic changes in feeding habits and adaptive responses to prey availability along their migratory paths facilitate trophic niche shifts in Japanese anchovy.