Determining spawning ground of cryptic mullet (Mugil cephalus) juvenile off Taiwan with otolith micro-structure and micro-chemical signal

Grey mullet, Mugil cephalus, is an important fisheries resource in Taiwan, comprising three cryptic species (NWP1-3) coexisting in the Northwest Pacific. However, studies of species-specific biological characteristics remain limited. This study employed rapid screening technique to identify species and conducted otolith microstructure and microchemical analysis on juvenile mullet collected from four estuaries around Taiwan to determine their spawning grounds with focused on NWP2. Otolith daily increments showed no significant differences in NWP1 drifting durations among estuaries, whereas NWP2 exhibited significant differences in partial months, with notably shorter durations observed in the northeastern and southwestern estuaries. These findings suggest that NWP1 lacks a geographic gradient, while NWP2 exhibits a non-unidirectional geographic gradient in drifting durations, with both patterns supporting the hypothesis of multiple spawning grounds around Taiwan. Robust Linear Discriminant analysis (R-LDA) showed that the overall successful classification rate for the spawning sources of NWP2 among estuaries was 65.2 %, with higher classification rates observed between the two farthest estuaries. Cluster analysis further indicated that NWP2 may originate from five spawning sources. Spatial variability in elemental signatures of five spawning sources likely reflects the combined influence of physiological factors (e.g., species-specific sensitivity) and environmental conditions (e.g., freshwater input and water mass characteristics), providing additional evidence for the location of multiple spawning grounds. Overall, this study provides important insights into the life history and spawning behavior of different cryptic mullet species, which is crucial for effective fisheries management and conservation strategies in Taiwan.