Fish sexual characteristics are determined by measuring the concentration in fish serum, plasma or whole blood of one or more peptide hormones of the transforming growth factor-beta superfamily (TGF-β superfamily). The disclosed method and an accompanying field test kit may be used in sturgeon aquaculture to cull out young male fish so that increased time and resources may be devoted to the further rearing of female fish for caviar production. The method and test kit may also be used for wild fish life history studies.