RATIONALE: Nitrogen and carbon stable isotope ratios (δ15N and δ13C values) of carbonate-bound organic materials in otoliths can provide information to address the biological and ecological functions of fish. Correct interpretation of otolith δ15N and δ13C profiles requires knowledge of the metabolic routes of nitrogen and carbon isotopes. However, the isotopic assimilation of δ15N and δ13C compositions from diets to otoliths has rarely been investigated. METHODS: This study traced the daily nitrogen and carbon isotopic assimilation between diets and otoliths using nanoscale secondary ion mass spectrometry (NanoSIMS). Isotopically labeled algae (Tetraselmis chui) were fed to tilapia (Oreochromis niloticus) for 14–17 days. NanoSIMS and conventional isotope ratio mass spectrometry were used to measure δ15N and δ13C variations in the otoliths and fish muscle, respectively. RESULTS: Otolith δ15N values abruptly surged from natural abundance levels by 1000–2300‰ after the fish ate 15N-spiked algae with δ15N values of approximately 2200‰. However, the δ15N values of fish muscle increased to only approximately 500‰ at the end of the feeding experiment. Much higher δ15N values (3700–14 000‰) and moderate δ13C values (60–200‰) were detected in the otoliths after the tilapia ate 15N- and 13C-spiked algae with a δ15N value of 36667‰ and a δ13C value of 272‰. Mapping analysis showed sub-micrometer-scale distribution of 15N embedded in the otolith growth increments with a low-to-high δ15N signal after the tilapia shifted diets from non-spiked to 15N-labeled algae. CONCLUSIONS: These results suggest that otolith nitrogen and carbon isotopes from food were directly assimilated on the same day. Food is the major and in some cases only source of otolith nitrogen isotopes but makes only a partial contribution to otolith carbon isotopes. Therefore, the δ15N values recorded in the sclerochronological layers of the otoliths can be used to determine the trophic levels, food sources and diet changes of fish.
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