The purpose of this study was to develop a method for the hormonal, masculinization of sea bass (Dicentrarchus labrax L.) reared under natural conditions of photoperiod and temperature. Sexually undifferentiated sea bass were fed 17 alpha-methyltestosterone (MT) at 10 mg kg(-1) food during three distinct periods of 100 days each: Period I, 126-226 days post fertilization (DPF) (July-November); Period II, 226-326 DPF (November-February); Period III, 326-426 DPF (February-May), and their combinations. Phenotypic sex ratios after hormone treatment were determined by histological examination of the gonads. MT significantly increased the proportion of juvenile males from controls (79%), to 93-100% in groups for which treatment included Period I. In contrast, no significant alterations in the phenotypic sex ratio were observed when MT treatment started after this period. In groups treated with MT for more than 100 days, including Period I, growth was significantly depressed. Furthermore, when MT was given for 300 days (126-426 DPF), 7% of the fish were intersex. The effects of MT on the developing gonads were confirmed by a significant reduction of the gonadosomatic index (GSI) when treatment included Period I regardless of treatment duration. The visceral fat index (VFI) decreased with increasing durations of MT administration, denoting a lipolytic effect. However, 200 or mote days after MT withdrawal, the mean VFI of MT-treated groups showed no differences with respect to that of the controls, indicating that the effects of MT on visceral fat mobilization were transitory. Similarly, reduction of the hepatosomatic index (HSI) and increase of the carcass index were found to be related to the length of MT administration, but values similar to those of controls were obtained after suppression of hormonal treatments, Survival of the treated groups was not compromised (average 76% vs. 78% in controls), and no teratological phenomena were observed. No treatments starting before Period I or of less than 100 days of duration were tested in this study; however, the acquisition of complete and functional sex reversal without any deleterious effects after MT administration during this period and the lower effectiveness of MT treatment in progressively more advanced stages of development, shows that Period I is effective for the complete and permanent masculinization of sea bass.
