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 (δ¹⁵ N_Base ), which can be estimated by using compound-specific isotope analysis. δ¹⁵ N_Base exclusively reflects the δ¹⁵ N of nitrate in the ocean at a regional scale and is not affected by the trophic position of sampled organisms. In other words, δ¹⁵ N_Base allows for direct comparison of isotope ratios between proxy organisms of the isoscape and the target migratory animal. We initially constructed a δ¹⁵ N_Base isoscape in the northern North Pacific by bulk and compound-specific isotope analyses of copepods (n = 360 and 24, respectively), and then we determined retrospective δ¹⁵ N_Base values of spawning chum salmon (Oncorhynchus keta) from their vertebral centra (10 sections from each of two salmon). We then estimated the migration routes of chum salmon during their skeletal growth 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 new migration route to the Bering Sea Shelf during a later growth stage.