Case Report: Oceanic migratory behavior of chum salmon in the Bering Sea during summer using a pop-up satellite archival tag

Chum salmon (Oncorhynchus keta) is one of the most abundant Pacific salmon species and spends much of its life in the open ocean. Due to its offshore distribution, knowledge of the migration ecology of chum salmon, including vertical habitat use and experienced temperature, remains limited, particularly during the immature stage. The Bering Sea is a key foraging habitat for chum salmon. To obtain preliminary information on its vertical and horizontal behavior in this area, we deployed a pop-up satellite archival tag (PSAT) on a chum salmon (fork length: 530 mm; ocean age: 3) captured and released in the central Bering Sea in July (56.0°N, 174.9°W). The tag was programmed to record depth, temperature, light and acceleration data at intervals of 3, 5, 15, and 0.1 s, respectively, for 75 days, and to resample the data into 10-minute time-series before transmission to satellites. Seventy-five days later, the PSAT began transmitting data along the Aleutian Island chain, just south of Amchitka Island (50.8°N, 179.3°E). From 10 to 2 days before transmission, the PSAT recorded ambient temperatures above 20 °C, implying predation by an endothermic fish. The estimated track, based on light-level geolocation, indicated a relatively slow southward movement within the Bering Sea compared with the migration speed of maturing Pacific salmon returning to the coastal areas reported in previous studies, suggesting continued foraging migration. The fish predominantly occupied surface waters (above 10 m) but occasionally dived below the thermocline (deeper than 40 m). No consistent diel vertical migration was observed; however, activity increased during twilight and during daytime and twilight dives, potentially linked to foraging behavior. This pilot represents the first deployment of a PSAT on a chum salmon and documents its migration behavior in the Bering Sea during summer over two months. The PSAT data provide new insights into the detailed open-ocean behavior of chum salmon, including diving behavior, horizontal movements, and crepuscular activity. However, further tagging studies with additional individuals are required to assess the consistency and variability of these behaviors.