Determining horizontal and vertical distributions of polar cod (Boreogadus saida) in the Bering Sea and western Arctic Ocean using traditional sampling and environmental DNA metabarcoding

Boreogadus saida, commonly known as polar cod, is a key species in Arctic marine ecosystems that is also an indicator of environmental change. This study examined the spatial distribution of B. saida, which typically inhabits circumpolar waters ranging in temperature from −2 to 8°C. Age classification based on size was performed on fish samples collected using ring nets, frame trawl nets, and hand nets. The results demonstrated that most individuals collected by ring and frame trawl nets in the southern Chukchi Sea (SCS) and East Siberian Sea (ESS) during late July and early August were age-0, while those collected in mid-August were age-1 and age-2. The individuals collected by hand net in the surface of the ESS and the northern Chukchi Sea (NCS) were all in age-1 and age-2 + . Smaller larvae collected in the ESS suggested local spawning grounds. Environmental factors, particularly temperature and prey availability, were found to influence habitat selection. To complement traditional sampling methods, environmental DNA (eDNA) metabarcoding using MiFish primers was conducted at various horizontal and vertical sites across the Bering Sea (BS), Chukchi Sea (CS), and ESS. B. saida eDNA was detected from surface waters to a maximum depth of 1804 m at temperatures ranging from −1.6 to 4.4°C, which demonstrated the utility of eDNA for mapping both horizontal and vertical distributions. These findings highlight the value of integrating eDNA metabarcoding with conventional techniques to clarify B. saida’s distribution and life history. They also emphasize the broader potential of eDNA monitoring as a sensitive, non-invasive framework for Arctic biodiversity assessment. Taken together, our results underscore not only the current distribution of B. saida but also the promise of eDNA as a foundation for long-term, ecosystem-wide monitoring in a rapidly changing Arctic.