DEVELOPMENT OF SEX CONTROL TECHNIQUES FOR EUROPEAN SEA BASS (DICENTRARCHUS-LABRAX L) AQUACULTURE - EFFECTS OF DIETARY 17-ALPHA-METHYLTESTOSTERONE PRIOR TO SEX-DIFFERENTIATION

The purpose of this study was to develop a method for the hormonal, ma sculinization of sea bass (Dicentrarchus labrax L.) reared under natur al conditions of photoperiod and temperature. Sexually undifferentiate d sea bass were fed 17 alpha-methyltestosterone (MT) at 10 mg kg(-1) f ood 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 dete rmined by histological examination of the gonads. MT significantly inc reased the proportion of juvenile males from controls (79%), to 93-100 % in groups for which treatment included Period I. In contrast, no sig nificant alterations in the phenotypic sex ratio were observed when MT treatment started after this period. In groups treated with MT for mo re than 100 days, including Period I, growth was significantly depress ed. Furthermore, when MT was given for 300 days (126-426 DPF), 7% of t he 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. Th e visceral fat index (VFI) decreased with increasing durations of MT a dministration, denoting a lipolytic effect. However, 200 or mote days after MT withdrawal, the mean VFI of MT-treated groups showed no diffe rences with respect to that of the controls, indicating that the effec ts of MT on visceral fat mobilization were transitory. Similarly, redu ction of the hepatosomatic index (HSI) and increase of the carcass ind ex were found to be related to the length of MT administration, but va lues similar to those of controls were obtained after suppression of h ormonal treatments, Survival of the treated groups was not compromised (average 76% vs. 78% in controls), and no teratological phenomena wer e 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 effect s after MT administration during this period and the lower effectivene ss of MT treatment in progressively more advanced stages of developmen t, shows that Period I is effective for the complete and permanent mas culinization of sea bass.