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.