Abstract: Pacific cod (Gadus macrocephalus) and walleye pollock (Gadus chalcogrammus) are commercially important species in bottom trawl fisheries in Japanese waters. However, their first-year life history processes and whether current management units align with the scale of their biological processes remain unclear. To investigate this, we conducted otolith macrostructural analysis on age-1 Pacific cod and walleye pollock collected from the Pacific Ocean off Hokkaido, Aomori Prefecture, and Sendai Bay (Honshu). Stable isotope analysis (δ¹³C, δ¹⁸O) was performed on both the inside and outside of the otolith check using isotope ratio mass spectrometry (IRMS) and secondary ion mass spectrometry (SIMS). The δ¹⁸O shift across the check, combined with higher δ¹³C and δ¹⁸O values on the outside of the check compared to the inside, suggests that both species undergo a habitat transition during their first-year settlement. Stock discrimination was more successful for individuals from Hokkaido and Sendai Bay than for those from Aomori Prefecture. When isotope data from the two southern regions (Aomori Prefecture and Sendai Bay) were combined into a single Honshu stock, the overall classification success increased from 71 to 76%, with Cohen’s Kappa (κ) rising significantly from 0.31 to 0.60. These results indicate that managing stocks based on geographic distribution is both scientifically valid and practical though inter-stock admixture should be taken into account. Overall, otolith stable isotope analysis proves to be a valuable tool for identifying key life history events, stock structure, and migration patterns in these gadoid species.