Disappearance of malachite green residues in fry of rainbow trout (Oncorhynchus mykiss) after treatment of eggs at the hatching stage

The disappearance of malachite green (MG) residues was determined in fry of rainbow trout (Oncorhynchus mykiss) after six repeated treatments of the eggs at the hatching stage with MG oxalate at exposure levels of 1, 3 and 6 mg l− 1 for 30 min. Fry samples were taken from newly hatched fry (0 days post-hatch, d.p.h.) and at regular time intervals at 16, 31, 43, 57 and 96 d.p.h. The residues of MG and its major metabolite, leucomalachite green (LMG), were found to accumulate in the fry after MG treatments of eggs, with the highest residue levels being determined in the newly hatched fry. After exposures of 3 mg l− 1 MG, mean concentrations of 1170 ± 106 µg kg− 1 and 276 ± 38.6 µg kg− 1 (n = 3) were found in fry for LMG and MG, respectively. However, the disappearance of residues occurred rapidly in the fry, such that by 43 d.p.h. only low levels of LMG could be determined. To confirm the elimination of residues, determinations were made also in fry muscle at 96 d.p.h. but no residues were detected. The residues of MG in fry were determined by liquid chromatography-tandem mass spectrometric (LC-MS/MS) analysis with a limit of detection (LOD) of 0.5 µg kg− 1 and a limit of quantification (LOQ) of 1.0 µg kg− 1. The accumulation as well as the elimination of residues correlated well with the level of exposure. During the study, the fry increased their weight, such that at the end of the study, their mean body weight was about 150 times greater than the mean body weight of the newly hatched fry. As the disappearance of residues occurred in conjunction with the growth of fry, the present results indicate that no residues of MG will remain in the fish intended for human consumption, if MG treatment takes place at the hatching stage under controlled conditions.