Fine-scale variability in otolith chemistry: Application to the life history analysis of Pacific cod (Gadus macrocephalus) in the Yellow Sea

The Yellow Sea is the southwestern limit of the distribution of Pacific cod (Gadus macrocephalus), a typical cold water and commercially important fish in the North Pacific. The catch of Pacific cod in the Yellow Sea has increased gradually in recent years, and has received substantial attention. To explore the use of otolith chemistry in the characterization of different spawning grounds and crucial life history events, a total of 60 specimens of Pacific cod were obtained from October 2016 to August 2017. The contents of barium (Ba) and strontium (Sr) in otoliths from the core to the edge along the ventral side were determined using laser ablation-inductively coupled plasma-mass spectrometry. Based on hierarchical cluster analysis (HCA), the specimens were assigned to two groups based on the ratio of Ba:Ca at the core. The various patterns of core-to-edge Ba:Ca inferred using a linear mixed-effect (LMM) model suggested differences in the spawning ground environments for Pacific cod in the Yellow Sea, and step changes found in the Sr:Ca ratio, and a corresponding check observed in otolith images suggest that the settlement of Pacific cod occurred approximately 80–120 days after hatching. The nature of the data prevents firm conclusions being drawn from the data available at present, but the significant differences in certain elements among individuals and the association with crucial life history events in Pacific cod are noteworthy. •Differences in Ba:Ca ratios at the otolith core is useful to distinguish spawning grounds of Pacific cod in the Yellow Sea.•There may be at least two different types of spawning ground environment for Pacific cod in the Yellow Sea.•Step change of Sr:Ca ratios in the otolith reflects the settlement of Pacific cod.