Abstract :
To masculinize Danio rerio, Norethindrone acetate (NE) was administered through discrete immersion (2 hrs each) from the 18th to the 25th day after hatching (dph) at selected concentrations (100, 200, 400, 800, 1,000 and 1,200 μg•l^-1). The immersion at a concentration of 1,000 μg•l^-1 ensured 100 % masculinization and the highest survival (97 %) after treatment and at the age of puberty. Barring this concentration group, the NE acted as a growth suppressant and the magnitude of its suppression was stronger in females. During the 240 day experiment, sexual maturity in females was postponed from 120th dph in the control to 270th dph in those treated at 1,200 μg•l^-1, and reduced spawning frequency (15 - 3 times) and cumulative fecundity (1,168 - 111 eggs) by reducing the number of vitellogenic eggs. However, in the treated males, sperm motility was reduced (105 – 73 sec); consequently, fertilizability of the milt drawn from these males was also reduced from 91 % to 78 %. Progeny testing showed that the cross between males treated at 800 μg•l^-1 and normal females generated the presumed ‘homogametic’ males, which however, could induce the females to spawn fewer eggs than that of the presumed ‘heterogametic’ males.
