Fractionation of otolith nitrogen stable isotopes measured by peroxodisulfate oxidation-bacterial conversion and isotope ratio mass spectrometry

© 2018 John Wiley & Sons, Ltd. Rationale: Otoliths are usually used to estimate the age of fish and the chemical composition such as nitrogen stable isotope ratios (δ 15 N values) may record environmental information and ecological role of the fish. However, the isotopic fractionation of δ 15 N values between diets and otoliths has rarely been investigated and remains unclear. Methods: Nitrogen isotopic fractionation between five different diets (δ 15 N diet values) and otoliths (δ 15 N oto values) were elucidated in tilapia Oreochromis mossambica reared in controlled feeding experiments. The otoliths were dissolved with hydrogen chloride and peroxodisulfate was used to oxidize the total organic materials to nitrate, which was further converted into N 2 O gas by denitrification bacteria before the measurement of δ 15 N oto values by isotope ratio mass spectrometry. The δ 15 N values of muscles, gills, scales and livers of the tilapias were also measured by isotope ratio mass spectrometry. Results: The peroxodisulfate oxidation-bacterial conversion method reduced the minimum mass of the otoliths required for analysis to as low as 2 mg, unlike past methods, which have required masses of 8–155 mg. The δ 15 N oto values were not significantly different from the δ 15 N diet values of the five diets. Furthermore, the somatic growth rate had no effect on the δ 15 N oto values. Nevertheless, the δ 15 N values of metabolically active tissues were significantly different from each other and higher than the δ 15 N diet values, due to the deamination of these tissues. Conclusions: These results suggest that diet was the main source of amino acids for the otolith organic matrix and there was no biochemical transamination during the assimilation of dietary amino acids to otoliths. The δ 15 N oto value can be used as a proxy of nitrogen sources of fishes and may have potential application in ecological studies such as the detection of diet shift, migration, trophic levels and environmental changes experienced by the fish population.