Establishment of monosex female production of sablefish (Anoplopoma fimbria) through direct and indirect sex control

Methods are needed for monosex production of sablefish (Anoplopoma fimbria) to capitalize on superior growth of females relative to males. This study aimed to characterize the system of sex determination and establish methods for both direct and indirect feminization of sablefish. Juvenile sablefish ~ 40 mm fork length were fed diets containing 17β-estradiol (E2), 17α-methyltestosterone (MT), or ethanol vehicle alone (control) for 2 months and then divided into subgroups either shifted to the control diet (E2- and MT-short (S) duration treatments) or maintained on steroid-treated diets for an additional 2 months (E2- and MT-long (L) duration treatments). Fish were subsampled for gonadal histology at the midpoint and end of the dietary treatments and all remaining fish were individually tagged to monitor subsequent growth and gonadal development for 36 months (post-treatment time points of 0, 4, 16, 24, and 36 months). Control fish exhibited balanced (1:1) sex ratios. E2 treatment regardless of duration induced ovarian differentiation in 100% of the genotypic males (i.e., direct feminization), while only the MT-L treatment induced testicular differentiation in 100% of the genotypic females (i.e., generating “neomales”). A high proportion (89%) of MT-S genotypic females possessed intersex gonads, indicating short-term MT exposure was insufficient to induce complete sex reversal. Significant differences in sex-specific growth were apparent by 16 months post-treatment and associated with gonadal phenotype; sablefish with ovaries outgrew those with testes, regardless of genotypic sex. Fish from the E2-S and -L and MT-L treatment groups maintained their respective feminized and masculinized states after steroid withdrawal, although MT-S treated genotypic females tended to show further masculinization post-treatment. Upon reaching sexual maturity, control males and neomales were crossed with wild-caught female broodstock. Resultant embryos from control crosses were 54.6 ± 8.9% (mean ± SD) genotypic female, while those from neomale-female crosses were 100 ± 0.0% genotypic female. Following sex differentiation, gonadal histology demonstrated that offspring from control crosses had a balanced phenotypic sex ratio, whereas all neomale offspring developed normal ovaries. This study establishes that sablefish utilize an XX/XY system of sex determination and methods for both direct and indirect monosex female production.