EFFECT OF 17-ALPHA-METHYLTESTOSTERONE ON THE GROWTH OF THE EURYHALINE TILAPIA, OREOCHROMIS-MOSSAMBICUS, IN FRESH-WATER AND IN SEA-WATER

These studies were aimed at determining whether: (1) rearing salinity influenced the growth of the tilapia, Oreochromis mossambicus, and (2) long-term 17alpha-methyltestosterone (MT) treatment of tilapia enhanced growth beyond that produced by masculinization alone. These studies showed that tilapia raised in sea water (34-36 ppt) grew to twice the size attained by freshwater fish under the conditions of our experiment (P<0.01). To evaluate the optimal pattern of MT treatment, four treatments identified as control, continuous, early, and delayed were employed. Early and delayed MT-treated fish grew significantly faster than controls in both salinities (P<0.001). The growth of tilapia continuously treated with MT exceeded that of all other treatments in both fresh water and in sea water and was three times higher than that of the controls (P< 0.001). Most importantly, the growth of continuously treated tilapia exceeded the growth of early treatment tilapia by 30-50% despite the fact that both treatments induced nearly complete sex reversal (approximately 99%). These findings suggest that continuous treatment with MT offers considerable advantage over sex reversal alone in the commercial culture of the tilapia. Overall, the greatest growth was observed in seawater-reared tilapia which were continuously treated with MT and, these fish were 5-7-fold larger than freshwater controls, The practical applicability of the findings from our tank investigations was evaluated in full-scale pond studies in which tilapia were cultivated in freshwater polyculture with common carp (Cyprinus carpio), grass carp (Ctenopharyngdon idellus), mud carp (Cirrhina molitorella), silver carp (Hypophthalmichthys molitrix), and bighead carp (Aristichthys nobilis). The administration of 17alpha-methyltestosterone consistently and significantly augmented the growth of titapia (22%) and the carp species as well.