Species-specific detection of polar cod (Boreogadus saida) eDNA from the Bering and Chukchi seas in the MR20-05C research cruise

To provide an overview of the distribution of polar cod (Boreogadus saida) across the Bering and Chukchi Seas, environmental DNA (eDNA) of polar cod was analyzed using a novel species-specific assay. eDNA samples were collected on the cruise (MR20-05C) conducted by the 'R/V Mirai' of the Japan Agency for Marine-Earth Science and Technology during the open-water season of 2020. For eDNA sampling, surface seawater was collected and then filtered through a cartridge filter (Sterivex HV, 0.45 m pore size; Millipore, Burlington, MA, USA). Additional eDNA samples were collected from the middle or bottom layer in the same manner. We developed a species-specific quantitative real-time PCR (qPCR) TaqMan assay to exclusively detect polar cod eDNA by targeting the D-loop region of mitochondrial DNA. Polar cod eDNA was detected and quantified in the samples using the assay. To investigate the relationship between eDNA detection and environmental conditions, temperature (degrees Celsius), salinity, chlorophyll-a fluorescence (RFU), and bottom depth (m) were obtained from the ship's continuous sea surface water monitoring system. Water masses were classified using the SIMPROF test with Eucledean distance and Ward linkage based on environmental parameters (water temperature, salinity, and log-transformed chlorophyll a fluorescence). The topology was classified as shelf (bottom depth < 200m) or basin (bottom depth >= 200m). The distance to the sea ice edge was defined as the distance between sampling sites and the nearest sea ice edge, which was defined as the contour of sea ice concentration = 0.15.