Assimilation of nitrogen and carbon isotopes from fish diets to otoliths as measured by nanoscale secondary ion mass spectrometry

Rationale: Nitrogen and carbon stable isotope ratios (δ¹⁵ N and δ¹³ C values) of carbonate-bound organic materials in otoliths can provide information to address the biological and ecological functions of fish. Correct interpretation of otolith δ¹⁵ N and δ¹³ C profiles requires knowledge of the metabolic routes of nitrogen and carbon isotopes. However, the isotopic assimilation of δ¹⁵ N and δ¹³ C 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 δ¹⁵ N and δ¹³ C variations in the otoliths and fish muscle, respectively.

Results: Otolith δ¹⁵ N values abruptly surged from natural abundance levels by 1000-2300‰ after the fish ate ¹⁵ N-spiked algae with δ¹⁵ N values of approximately 2200‰. However, the δ¹⁵ N values of fish muscle increased to only approximately 500‰ at the end of the feeding experiment. Much higher δ¹⁵ N values (3700-14 000‰) and moderate δ¹³ C values (60-200‰) were detected in the otoliths after the tilapia ate ¹⁵ N- and ¹³ C-spiked algae with a δ¹⁵ N value of 36667‰ and a δ¹³ C value of 272‰. Mapping analysis showed sub-micrometer-scale distribution of ¹⁵ N embedded in the otolith growth increments with a low-to-high δ¹⁵ N signal after the tilapia shifted diets from non-spiked to ¹⁵ N-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 δ¹⁵ N values recorded in the sclerochronological layers of the otoliths can be used to determine the trophic levels, food sources and diet changes of fish.