Hormonal sex reversal in Atlantic cod, Gadus morhua

Pre-harvest sexual maturation, a major constraint to aquaculture operations, commonly occurs in both sexes of Atlantic cod (Gadus morhua). Production of all-female triploids would alleviate this problem because they are sterile. The objective of this project was to develop effective endocrine manipulations to produce monosex stocks of cod, as the first step towards producing all-female triploids, using dietary 17β-estradiol (E2) and 17α-methyldihydrotestosterone (MDHT) fed during the labile period of gonadal differentiation. In the first experiment, triplicate treatments of E2 at 5, 10 and 20 mg/kg feed and MDHT at 0.67, 2 and 6 mg/kg feed were fed to fish during the growth interval from 17 ± 2 to 43 ± 4 mm standard length (SL). The only treatment which affected sex ratio was the highest MDHT dose, with a significant reduction in proportion of females compared to controls. Based on these results and a thorough histological examination of gonadal development, a follow-up experiment used duplicate treatments of E2 at 20, 40, 80 and 120 mg/kg and MDHT at 3, 6, 12 and 18 mg/kg fed to fish during the growth interval from 7.8 ± 1.2 to 45 ± 2 mm SL. This resulted in a dose-dependent shift in sex ratio in both estrogen and androgen treated groups, with no females observed in the groups fed the two highest doses of MDHT and a significantly reduced male ratio in E2 groups at the two highest doses. These results indicate that successful sex reversal can be achieved through dietary hormonal manipulation when the treatment starts at around 8 mm SL.

► Dietary steroid treatment resulted in dose-dependent sex reversal in Atlantic cod.
► Optimum treatment period was during growth interval from 8 to 45 mm standard length.
► Treatment with 12–18 ppm MDHT can be used to produce neomales.