Methods for sex control are needed to establish monosex aquaculture of sablefish (Anoplopoma fimbria).
Here we conducted the first characterization of sex differentiation by histology and hormonal sex reversal
experiment in sablefish. Ovarian differentiation was first discernible at ~80 mm fork length (FL) and
characterized by development of lamellar structures and onset of meiosis. Testes exhibited a dual-lobe
appearance over much of their length and remained non-meiotic until males were ≥520 mm FL (2 years
post-fertilization). Juveniles with undifferentiated gonads were provided diets containing 0 (control), 5 or
50 mg 17α-methyltestosterone (MT)/kg for 2 months. Following treatment, controls possessed either
ovaries or non-meiotic testes, whereas MT-treated fish exhibited meiotic testes (60 % of the fish), intersex
gonads (~30 %), or gonads that appeared sterile (~10 %). A genetic sex marker revealed that all intersex
fish were genetic females, although other females appeared to be completely sex reversed (i.e., neomales).
One year after treatment, MT-treated fish possessed non-meiotic testes similar to control males or intersex
gonads with reduced ovarian features, presumably due to atresia following MT withdrawal. Milt collected
from neomales and genetic males 3 years post-treatment permitted sperm motility analyses; however, neomale
sperm were virtually immotile. These results demonstrated that sablefish are differentiated gonochorists
and that MT treatment from 76 to 196 mm FL induced permanent masculinization of a portion of the
genetic females, but acquisition of sperm motility was impaired. Earlier administration of MT may be
necessary to sex reverse a higher proportion of genetic females and reduce negative effects on fertility.