Tracking the Migratory Life History of Brown Croaker (Miichthys miiuy) Through Otolith Microchemistry in the East China Sea

Brown croaker (<i>Miichthys miiuy</i>) is an economically, ecologically, and culturally important species in the East China Sea (ECS); however, populations of <i>M. miiuy</i> have declined in recent years due to climate change and high fishing intensity. Our limited understanding of wild <i>M. miiuy</i>'s migratory life history hampers effective population conservation. To meet this need, and to elucidate the migratory life history of wild <i>M. miiuy</i>, we quantified the elemental composition of otolith samples using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). This approach, combined with analysis of otolith microstructure, was used to evaluate the feasibility of using the Mg:Ca of otoliths chemical clock for <i>M. miiuy</i>. Using cluster analysis alongside bivariate time series analysis, we identified natal sources and reconstructed migratory histories. The results showed that consistent, periodic fluctuation of Mg:Ca ratios in otolith profiles can be used as a chemical index to indicate the age and life history stage of <i>M. miiuy</i>. Natal sources of <i>M. miiuy</i> originated from three distinct water environments: estuary (14.2%), coastal mixed waters (57.3%), and coastal reef waters (28.5%). A diverse migratory life history of <i>M. miiuy</i> was observed based on Sr:Ba thresholds, and ultimately, we identified four migratory life histories of the species, including an estuary-coastal migratory type, a coastal resident type, a coastal-offshore migratory type and an estuary-coastal-offshore migratory type. This study provides a scientific basis for the protection of key habitats and seasonal management of <i>M. miiuy</i> in the ECS.