Research methods

　On June 29-30, 2019, water samples and net samples were collected by the training ship Oshoro-Maru attached to the Faculty of Fisheries Sciences, Hokkaido University at two stations on the east coast of Shiretoko Peninsula in the southern Sea of Okhotsk (Figure 1). Samples were fixed and brought back, and microplankton species and sizes were measured under a fluorescence microscope. Net sampling was conducted by day and night vertical segmented sampling (p1. Figure) in three layers between 0 and 1,000 m depth using a 63 µm mesh VMPS^＊１, and horizontal towed sampling using a mid-water trawl. VMPS samples were divided on board, live zooplankton were sorted by species and developmental stage, reared in filtered seawater under in situ temperature conditions, and faecal pallets excreted were fixed. The remaining VMPS and MOHT^*2 samples were also fixed and brought back to the laboratory. The zooplankton that appeared in the samples were counted by species and developmental stage, and their sizes were measured and expressed in carbon. For faecal pallets that appeared in large numbers in the VMPS samples, image data were acquired using the ZooScan^*3 imaging scanner (see figure), and the number and size of the faecal pallets were quantified and expressed as carbon content.