EFFECTS OF ESTRADIOL-ENRICHED DIET AND OF FEEDING WITH PORCINE TESTICULAR TISSUE ON MACROSCOPIC GONADAL SEX IN EUROPEAN EELS

Oestradiol-enriched food (25 mg kg(-1) oestradiol) fed for 91 days to small eels (2 or 7 g) with undifferentiated gonads (UD) significantly Favoured development of gonads with the macroscopic appearance of ovar ies (female-gonads) during the next 8 months. Fifty-four and 51% of co ntrol eels and 75 and 78% of oestradiol-treated eels developed female- gonads. Minced porcine testicular tissue fed to 2-g eels for up to 371 days had effects similar to oestradiol-enriched food (68% with female -gonads), whereas feeding for only 91 days had no effect during the ne xt 8 months. Oestradiol-enriched food was also fed to larger eels with already macroscopically differentiated gonads (30-70 g; 95% with male -gonads, 5% with female-gonads). After 27 days significantly more eels with a female-gonad or with a mixture of ovary-like and testis-like r egions in the gonads (male+female-gonads) were found. After 96 days th ere were 44% with female-gonads, 40% with male+female-gonads and 16% w ith male-gonads. Oestradiol thus had a feminizing effect, not only on morphologically undifferentiated gonads but also on morphologically di fferentiated male-gonads. The presence of sex steroid hormones or thei r precursors in porcine testicular tissue may also exert a feminizing influence. In all experiments the hormone-fed groups showed a tendency (not significant) towards increased growth rate. In small eels early rapid growth and differentiation of female-gonads were clearly correla ted, both in hormone treated and in control eels. Otherwise no correla tion was found between growth rate and gonadal sex. (C) 1996 The Fishe ries Society of the British Isles