Environmental context. Crystal violet (CV), also known as gentian violet, is a triphenylmethane dye used as a veterinary drug for the treatment of ornamental fish infected with the parasitic protozoa Ichthyophthirius multifiliis. CV is a suspected carcinogen not registered for use with food-producing animals in the European Union, Australia, the US and some other countries. The results from this study now demonstrate that residues of leuco CV, the main metabolite of CV, may occur as an organic contaminant in samples of wild eels caused by discharges of municipal sewage effluents.

Abstract. Crystal violet (CV) and leucocrystal violet (LCV) were found in the tissues of wild eels caught from surface waters under the influence of effluents discharged by municipal sewage treatment plants (STPs). LCV was the dominant residue detected with total concentrations up to 6.7 μg kg^–1 fresh weight in tissues of eels caught from lakes, rivers and a canal in Berlin, Germany. The occurrence of the residues found in 35 out of 45 samples could directly be linked to the presence of discharges by municipal STPs into the receiving surface waters. CV is a multiple-use compound that is also used as a biological stain and to colour materials such as textiles, paints and printing inks. Thus, it seems very likely that the residues of CV found in the eel samples originate from such uses and from legal uses of CV as a veterinary drug for the treatment of ornamental fish. The results obtained from this study are the first report of background contamination by CV found in samples of fish not intentionally treated with this agent. Although the residues of CV and LCV measured in the tissues of the eels were very low, an oral exposure to residues of CV or LCV should be avoided with regard to their mutagenic and carcinogenic potential. According to European Union law, zero tolerance applies to all residues of CV and LCV found in food for human consumption, as CV is not registered for use as a veterinary drug.