Sex control of the Japanese eel by an estrogen (DES-Na) in feed

The sex of the Japanese eel could be controlled by the oral administration of DES-Na (sodium diethylstilbestrol) which also promoted growth. Elvers were reared from glass eels for more than 500 days with a commercial feed which contained DES-Na (0, 0.5, 0.75, 1.0ppm). All the eels were dissected at the end of the experiment to check the sex by both external appearance and histological examinations of gonads. The sex groups were divided into male, male?, female, female? and unknown by their external appearance, and into male, female, hermaphrodite, and non-sex by histo-logical examination. The largest discrepancy (27.5%) in the two examinations was found in the male: the histological examination was found to be essential. The female ratio increased along with an increase in DES-Na. It was 7.7% in the control and 23.2-35.4% in DES-Na groups. The male ratio was 64.1% in the control and 32.0-40.4% in DES-Na groups. The hermaphrodite ratio was 1.1-4.6%, and the non-sexual ratio was 23.8-38.0%. Females were the largest of all sex groups. The growth promoting effect was found to vary with the growth stage.