Morphological sex change upon treatment by endocrine modulators in meiogynogenetic tench (Tinca tinca L.)

Exogenous steroids alter sex differentiation in fish substantially. In the present study we have evaluated the effects of 17α-methyltestosterone (MT) and oestrogen receptor antagonist Tamoxifen (TA) on gonadal development and skewness of the sex ratio in all-female tench juveniles. In the first two experiments, sexually undifferentiated juveniles were orally treated with three doses of MT and TA (50, 100 and 150 mg kg⁻¹). Both the treatments resulted in a moderate dose-dependent masculinization, with neomale production ranging from 17% (50 mg kg⁻¹) to 26% (150 mg kg⁻¹) for the MT only treatment, and from 0% (50 mg kg⁻¹) to 27% (100 mg kg⁻¹) only for the TA treatment respectively. In the third experiment treatment of sexually differentiated tench females with single steroid treatments or combinations of the two resulted in populations composed of females and intersex individuals. The significantly highest occurrence of intersex individuals (45.5%) was found in the group subjected to combine treatment of MT+TA (150+200 mg kg⁻¹). No masculinization effect of the single or the combined treatment occurred. It can be concluded that oral treatment with MT or TA only slightly modifies the normal process of sex differentiation in gynogenetic tench juvenile, but treatment with the above-mentioned combinations has a highly significant potential to skew the sex ratio in sexually differentiated tench females. However, from an applied point of view, the treatment procedure will need optimization before use at a commercial level.