Rationale
Otoliths are usually used to estimate the age of fish and the chemical composition such as nitrogen stable isotope ratios (δ15N values) may record environmental information and ecological role of the fish. However, the isotopic fractionation of δ15N values between diets and otoliths has rarely been investigated and remains unclear.
Methods
Nitrogen isotopic fractionation between five different diets (δ15Ndiet values) and otoliths (δ15Noto 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 N2O gas by denitrification bacteria before the measurement of δ15Noto values by isotope ratio mass spectrometry. The δ15N 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 δ15Noto values were not significantly different from the δ15Ndiet values of the five diets. Furthermore, the somatic growth rate had no effect on the δ15Noto values. Nevertheless, the δ15N values of metabolically active tissues were significantly different from each other and higher than the δ15Ndiet 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 δ15Noto 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.