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

Long-distance migrations by marine fish have long fascinated scientists, but are difficult to track by visual surveys. Here, we propose a new method to easily and precisely track such migrations using stable nitrogen isotopic composition at the base of the food web (δ15NBase), which can be estimated by using compound-specific isotope analysis. δ15NBase exclusively reflects the δ15N of nitrate in the ocean at a regional scale and is not affected by the trophic position of sampled organisms. We initially constructed a δ15NBase isoscape in the northern North Pacific, and determined retrospective δ15NBase values of chum salmon (Oncorhynchus keta) from their vertebral centra. Then, we 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 indicates the presence of a novel migration route to the eastern Bering Sea Shelf during a later growth stage.