Sex control of the Japanese eel by an estrogen (DES-Na) in feed.
Elvers were reared from glass eels for >500 days with a commercial diet (controls) or the diet plus sodium diethylstilboestrol (DES-Na; 0.5, 0.75 or 1.0 p.p.m.). At the end of the experiment, the sex was assessed by external appearance and histological examination of gonads. The results showed that determination of sex from external appearance was unreliable. Histological examination showed that the percentage of ♀♀ increased from 7.7 in controls to 35.4 for the highest level of DES-Na; the percentage of ♂♂ was 64.1 and 32.0 resp. Corresponding values for hermaphrodites were 1.7 and 1.1%, and for fish without defined gonads 26.5 and 31.5%.