Tracking long-distance migration of marine fishes using compound-specific stable isotope analysis of amino acids

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 (δ15NBase), which allows for direct comparison of isotope ratios between proxy organisms of the isoscape and the target migratory animal. We initially constructed a δ15NBase isoscape in the North Pacific by bulk and compound-specific isotope analyses of copepods (n = 360 and 24, respectively). We then determined retrospective δ15NBase 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.