As represented in Table 3, the occurrence of LMG in trout products is higher than MG. It is due to the longer half-life of LMG comparing to MG in fish muscle tissue (Bauer et al., 1988). In a previous study (Máchová et al., 1996), rainbow trout was exposed to 0.2 mg/l of MG in a water bath for 6 days. After 11 weeks, MG was no longer detected in the muscle; but LMG persisted in the treated fish for 10 months.
Despite the fact that MG is prohibited as a veterinary medicine in many countries, it is still used in aquaculture industry. Regarding the potential adverse effects of MG residues on human health, monitoring and inspection programs on the MG residues in various aquaculture products should be conducted in a regular manner to provide information for better risk assessment.