[A-OS15] Marine ecosystems and biogeochemical cycles: theory, observation and modeling

[AOS15-05] Reconstruction of Migration from Chub Mackerel Eye Lens δ¹⁵N and Δ¹⁴C Using Isoscapes State-Space Model

*Kohei Sakamoto^1,2, Chisato Yoshikawa², Nanako O. Ogawa², Yutaka Osada⁴, Naoto F. Ishikawa², Shin-ichi Ito³, Yosuke Miyairi³, Yusuke Yokoyama³, Naohiko Ohkouchi² (1.The University of Tokyo, 2.Japan Agency for Marine-Earth Science and Technology, 3.Atmosphere and Ocean Research Institute, The University of Tokyo, 4.Tohoku University)

Keywords:Isologging, Nitrogen stable isotope, Radiocarbon, State-Space model, Kuroshio Current, Fish Migration

The method known as “iso-logging”, which utilizes the stable and radioisotope composition of animal tissues, has recently attracted attention for its potential to reconstruct migratory histories and natal origins based on differences in isotope ratios between regions. In fishes, body tissues such as otoliths, vertebrae, and eye lenses record the isotopic and trace element composition of their diet and ambient seawater during development, making them useful for reconstructing migratory histories. In this study, we analyzed bulk and amino acid nitrogen isotope ratios (δ¹⁵N_Bulk and δ¹⁵N_AAs) in samples obtained by separating the eye lens of an adult chub mackerel (Scomber japonicus), collected in August 2020 from Otsuchi Bay, Iwate Prefecture, into 34 layers (Harada et al. 2022). For each layer, we corrected δ¹⁵N_Bulk for trophic level effects using δ¹⁵N_AAs to estimate the δ¹⁵N value of primary producers (δ¹⁵N_Base). We also measured Δ¹⁴C in the same eye lens samples. To represent the δ¹⁵N_Base isoscape from March to July—corresponding to the growth period of the fish—we adopted the dataset from Yoshikawa et al. (2024). By constructing a state-space model that incorporating both the estimated δ¹⁵N_Base values of the lens and this δ¹⁵N_Base isoscape, and by further integrating a physical model reflecting surface currents during the same period, we inferred the migratory route of the chub mackerel. The results suggest that the fish migrated southward under the influence of the Kuroshio Extension until about 30 days after hatching, and then migrated northward to higher latitudes in the later growth stages. In addition, δ¹⁵N_Base in the lens remained nearly constant at approximately +2‰ from about 50 days after hatching, which caused a sharp decline in the accuracy of the model prediction. On the other hand, the newly measured Δ¹⁴C values in the lens changed dramatically from +40‰ to −40‰ between 50 and 100 days after hatching. By incorporating the Δ¹⁴C of dissolved inorganic carbon in the Pacific near the Kuroshio region into a new observation model, and thereby imposing additional constraints on the nitrogen-based state-space model, it was suggested that the accuracy of migration route estimates beyond 50 days after hatching could be improved. In this presentation, we discuss the potential of combining multiple isotope ratios with physical ocean current data in a state-space model to achieve more accurate reconstructions of migratory routes in pelagic fishes.