[H-TT16] Development and application of environmental traceability methods

[HTT16-09] Nitrogen isotopic relationships in diets, eye lenses, and muscle of chub mackerel revealed through a laboratory rearing experiment

*Chisato Yoshikawa¹, Nanako O. Ogawa¹, Naoto F. Ishikawa¹, Michio Yoneda², Ryuji Yukami², Shin-ichi Ito³, Naohiko Ohkouchi¹ (1.JAMSTEC, 2.FRA, 3.AORI)

Keywords:Nitrogen isotope ratio, Iso-logging, Chub mackerel

The isotopic composition of animal tissues and local environments can be used as a natural tag to track an animal’s movements through isotopically distinct habitats, called iso-logging. In the ocean, the nitrogen isotope composition (δ¹⁵N) of phytoplankton significantly varies reflecting the biogeochemical processes such as nitrogen fixation, denitrification, and nitrate utilization (Yoshikawa et al., 2024). The δ¹⁵N value of fish tissues inherits the δ¹⁵N value of phytoplankton in regions where they grew up with ¹⁵N enrichment of ~3‰ per trophic position (Minagawa and Wada, 1984). Recently, the δ¹⁵N analysis of incremental growth tissues, such as eye lenses and vertebral bones, has proven promising for reconstructing migration routes and feeding environments over the fish's life (Matsubayashi et al., 2017, 2020; Vecchio and Peebles, 2020; Harada et al., 2022). However, no studies have been conducted yet to investigate how the δ¹⁵N value of the diet is recorded in growth layers of fish eye lenses through a laboratory rearing experiment, which hampers a precise reconstruction of fish migration. In this study, we conducted a laboratory rearing experiment of chub mackerel with isotopically distinct diets from hatching to larval and juvenile stages for 100 days. The diets given were rotifer (0‰) on Day 2-16, artemia (11‰) on Day 10-19, and commercial dry pellet with low δ¹⁵N value (5‰) on Day 20-59. After Day 60, the mackerel were divided into two treatments, one feeding on pellets with a low δ¹⁵N value (5‰) and another with a high δ¹⁵N value (14‰). Eye lenses and muscle were sampled on Day 3, Day 10, Day 20, Day 60, Day 71, Day 80, Day 90, and Day 100 and their δ¹⁵N values of bulk nitrogen for all samples and the δ¹⁵N values of individual amino acids for selected samples were measured. Although the metabolic turnover rate decreased with growth, the eye lenses responded to the change in the δ¹⁵N value of diet quicker than muscle at all stages during the experiment. The δ¹⁵N values of the growth layers of eye lenses sampled on Day 100 are consistent with the δ¹⁵N value of each outer layer sampled on Day 20-90. These findings suggest that the δ¹⁵N values of growth layers of eye lenses are suitable for an iso-logging study, especially for a juvenile stage of fish. In this talk, we will also report the δ¹⁵N values of individual amino acids in eye lenses.