The use of fish otoliths of river bass in studies focusing on its trophic position in the fish community using SIA is possible. For analyzes relying on carbon, their use is even possible without treatment of otolith samples, as has been confirmed in the literature in the past. However, when it comes to nitrogen analysis, it is a slightly more complicated process. The amount of nitrogen in otoliths is not very high so it is completely undetectable with a mass spectrometer without a preliminary treatment of the samples. We managed to extract nitrogen from the otoliths, which is locked in the structure thanks to the proteins conducting its crystallization. After this extraction its content was sufficient for analysis, but only for the fraction that is made up of osoluble proteins. It was possible to get results for the insoluble fraction only for carbon, that is the reason why results of this fraction were not used. The process of nitrogen extraction is not very complicated, and even when using perch otoliths, which are relatively small, it is possible to get results. However, the small size and weight of perch otoliths will preclude the use of individual otoliths from individual fish. For that reason, summary samples were used for analysis. For this procedure, it is necessary to have a sample of 100..
