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.

In order to understand the spatial and temporal changes in suitable habitats of Clupeidae and Gadidae in Arctic and sub-Arctic waters under climate change, and compare the difference between the suitable habitats changes of Clupeidae and Gadidae in different waters, we constructed the species distribution models of 6 typical fish species inhabiting Arctic and sub-Arctic waters in this study, which included polar cod (Boreogadus saida), Arctic cod (Arctogadus glacialis), Atlantic cod (Gadus morhua), Atlantic herring (Clupea harengus), Alaska pollock (Gadus chalcogrammus) and Pacific herring (Clupea pallasii), to project their suitable habitats in current and under two different climate change scenarios in future (2010—2099). The research showed that (1) true skill statistics (TSS) and the area under the curve (AUC) of receiver operating characteristic (ROC) values were all higher than 0.80, which indicated a good prediction performance; and (2) the main environmental variables affecting the distribution of all studied fish species were different, and sea surface temperature was an important environmental variable affecting the distribution of almost all studied fish species. In the future, the suitable habitats of the studied fish species may move to higher latitudes, and the range of suitable habitats for Atlantic cod may vary the most. The suitable habitats of polar cod and Arctic cod in Arctic sea area will shrink in the SSP5-8.5 scenario compared with the SSP2-4.5 scenario. In the future, the Atlantic cod and Atlantic herring mainly distributed in Arctic Atlantic side will further enter the Barents Sea while Alaska pollock and Pacific herring mainly distributed in Arctic Pacific side will not cross Bering Strait to Chukchi Sea in a large scale. In this study, we predicted the potential habitats of Clupeidae and Gadidae in the east and west sides of Arctic and sub-Arctic waters (Atlantic and Pacific) under climate change, which is important for understanding the response of Clupeidae and Gadidae to climate change in Arctic and sub-Arctic waters.