Potential usefulness of isotope tracking for the study of marine animal migration
Our results demonstrate that an isotope tracking method using copepod-based δ 15NBase isoscapes coupled with retrospective isotope analysis of vertebral sections has the potential to provide new insights into the migration routes of highly migratory marine fish. So far, several studies have published marine isoscapes of light elements such asδ 13C, δ 15N, andδ 34S (McMahon et al . 2013; St. John Glewet al . 2018). However, bulk tissue analysis of these elements is generally unsuitable for migration tracking. This is because trophic discrimination factors between diet and consumer sometimes differ among species or tissues (Caut et al . 2010), making it difficult to calibrate isotopic values across these categories. This uncertainty in trophic discrimination factors is an important source of error in migration route or habitat estimates calculated from bulk tissue analysis of several species.δ 15NBase, on the other hand, sidesteps this issue because it exclusively reflects site-specificδ 15N values independently of the diet and species of target organisms.
In addition to the use of isoscapes ofδ 15NBase, retrospective isotope analysis also greatly improves the precision of migration route estimates generated by isotope tracking when target animals move into an isotopically distinct area. One advantage using the sequential analysis of vertebral sections is that it can provide a larger number of samples needed for multi-isotope analysis than otolith analysis. One disadvantage of using vertebral sections, however, is that isotopic records are only available for periods of skeletal growth. This is why no vertebral isotopic records were available for chum salmon migrating back to Japan from the Bering Sea Shelf, since chum salmon do not exhibit substantial skeletal growth after sexual maturation. Similarly, our isotope tracking did not detect winter migrations into the Gulf of Alaska, which are described in previous studies (Urawa et al . 2005; Urawa et al . 2009). Several studies have revealed significantly lower growth hormone levels in chum salmon in the Gulf of Alaska than in the Bering Sea (Onuma et al . 2010), suggesting that chum salmon do not exhibit substantial somatic growth in the Gulf of Alaska. Consequently, our results suggest that isotope tracking using bone collagen is a sensitive method for detecting foraging migrations, but may overlook other migration types such as winter and homing migrations.
Because the NP is a hotspot for highly-migratory marine animals (Blocket al . 2011), our isoscape in this area can be a powerful tool for investigating long-distance ocean migration for other animals. One advantage of the isotope tracking method is that it can reveal migration routes of individual animals over a prolonged timescale (the timescale of bone growth). Although the spatial resolution of the method depends on the spatial scale of isotopic gradients among habitats, the use of additional isoscapes based on radiocarbon content (Larsen et al . 2018) or neodymium stable isotope ratios (Saitoh et al . 2018) could help enable finer-scale determinations of fish migration routes.