The long-distance migrations by marine fishes are difficult to track by field observation. Here, we propose a new method to track such migrations using stable nitrogen isotopic composition at the base of the food web (δ¹⁵N_Base), which allows for direct comparison of isotope ratios between proxy organisms of the isoscape and the target migratory animal. We initially constructed a δ¹⁵N_Base isoscape in the North Pacific by bulk and compound-specific isotope analyses of copepods (n = 360 and 24, respectively). We then determined retrospective δ¹⁵N_Base values of spawning chum salmon (Oncorhynchus keta) from their vertebral centra (10 sections from each of two salmon), and estimated their migration routes by using a state-space model. Our isotope tracking method successfully reproduced a known chum salmon migration route between the Okhotsk and Bering seas, and our findings suggest the presence of a migration route to the Bering Sea Shelf during a later growth stage.