RESUMEN
The extraction of lipids by the Folch method from the muscles of all the fish studied led to statistically significant differences in the values of δ15N. At the same time, lipid extraction led to a statistically significant increase in δ13C in pike and roach, and to a statistically insignificant decrease in δ13C in perch and bream. Thus, lipid extraction cannot serve as a universal method of sample preparation for the analysis of the isotopic composition of carbon (13C/12C) and nitrogen (15N/14N) in fish muscles. The differences between the δ13C values in the samples before and after lipid extraction were statistically investigated by different models. It is shown that mathematical correction method models can be used, but the results are depending on the fish types.