Tissue distribution and elimination of radiolabelled methyltestosterone fed to sexually undifferentiated blue tilapia
Populations of monosex male Oreochromis aureus can result when the synthetic androgen 17α-methyltestosterone (MT) is fed to sexually undifferentiated fish; however, concerns exist over residues of the androgen remaining in fish destined for human consumption. Radioactivity in the carcass and viscera was evaluated in juvenile fish fed steroid-incorporated diet (tritium and carbon-14 labelled MT and 30 μg unlabelled MT/g feed) for 21 days, and depletion was monitored for 21 days after return to an untreated diet. Radioactivity was detected in the carcass within 1 h after initial feeding and reached highest levels by 6 h. Most of the radioactivity (> 90%) was in the viscera during the 21 days the radio-labelled diet was being fed. Radioactivity was eliminated exponentially, decreasing by 90% within 24 h after the last feeding. After 21 days of feeding untreated diet, < 1% of the original radioactivity remained (representing about 5 ng MT/g of tissue), and was evenly distributed between carcass and viscera. The observed low levels of residual radioactivity at conclusion of the sex reversal period, as well as anticipated dilution through growth during the culture of fish to marketable size, support the conclusion that no potential health hazard exists for people who eat fish that have been fed MT as juveniles.