Abstract

The N and C isotopic compositions (δ15N and δ13C) of phytoplankton vary due to biogeochemical processes. The δ15N and δ13C values of fish tissue are inherited from the phytoplankton in the regions where the fish lived. The δ15N and δ13C values of fish eye lenses, which are an incrementally growing tissue, have shown promise for reconstructing migration routes and feeding environments over the entire life of a fish. We conducted a laboratory rearing experiment of chub mackerel fed isotopically different diets from hatching to the juvenile stage for 100 d. The nascent fiber cells in the eye lenses were sampled at each growth stage, and mature fiber cells in the lenses at ages of 60 and 100 d were delaminated. The δ15N and δ13C values of these tissues and the δ15N values of individual amino acids were measured. The temporal changes in the δ15N and δ13C values of laminae in mature fiber cells in mackerel eye lenses were consistent with those of the nascent fiber cells at each age, and with the diet values corrected by trophic discrimination factors (TDFs). The TDFs for N and C in the mackerel eye lenses were 2.8–3.4‰ and 2.1–2.6‰, respectively. A model simulation revealed that it requires 191 d for the eye lenses in fish that switched diets at 60 d of age to reach the observed TDF for N of 3.1‰, and that the eye lenses of the adults take longer to reach isotopic equilibrium than the eye lenses of actively growing juveniles. These results suggest that the δ15N values of phytoplankton in the regions where chub mackerel grew can be reconstructed with about six months of time integration from the δ15N values of laminae in eye lenses based on the TDF correction for N in eye lenses. In addition, the TDFs for phenylalanine of − 0.8‰ to 0.2‰ suggest that the δ15N values of phytoplankton in the mackerel nursery areas can be approximated from the δ15N values of phenylalanine in mackerel eye lenses. The establishment of an iso-logging method using eye lenses has the potential to reconstruct the migration routes of all species of eyed organisms on Earth.