Effects of immersion of larvae in oestradiol-17β on feminization, structural changes of gonad and growth performance in the Far Eastern catfish Silurus asotus (Linnaeus)

Immersion of 1-day-old Silurus asotus (Linnaeus) larvae in 50, 100 or 200 mug L-1 oestradiol-17 beta for 15 days altered gonad differentiation and gonadal morphology. The survival of the group immersed at the highest dose (280 mug L-1) was reduced significantly (86%), whereas survival of the groups immersed at the lower doses was as high as that of untreated controls (94%). The sex ratio of the untreated group was close to 1:1 (female-male): however, the incidence of females was significantly higher in groups immersed in doses of 50, 100 and 200 mug L-1, i.e. 86%, 92% and 99.4%, respectively. Ovarian morphology of feminized individuals was different to that of the controls: the ovary was morphologically rather similar to testis although it contained fully matured oocytes. Morphological changes in genital organs were also noted in fish exposed to oestradiol-17 beta; the shape and size of genital papilla were intermediate between those of normal male and female. During a 4-month growth trial after the exposure, the feminized fish skewed significantly improved growth performance or er normal genetic males (P < 0.05), although the growth was not equal to that of normal genetic females (P < 0.05).