Effects of estradiol-17β implantation on ovarian growth, sex steroid levels and vitellogenin proxies in previtellogenic sturgeon Huso huso

Sexual development in female great sturgeon (Huso huso) is arrested at the previtellogenic stage for many years. The present study investigated the effects of different levels of estradiol-17β (E2) on gonadal development, levels of sex steroids and proxies of vitellogenin in 3-year-old cultured previtellogenic great sturgeon. Fish were intraperitoneally implanted every 1.5 months over a 6-month period from January to July with capsules filled with 0, 3, 6 or 12 mg E2/kg body mass as control, low, mid and high experimental groups, respectively. Blood sampling was performed at the start of experimentation and 3 weeks after each implantation for quantification of sex steroid levels and of vitellogenin-associated variables (triacylglycerol, cholesterol, calcium, phosphorus). Gonad biopsy samples were taken at the beginning and the end of the experiment in order to determine the gonad stage and oocyte morphometrical measures were taken to evaluate treatment effects. E2 implants produced a significant elevation in serum concentrations of E2, calcium, triacylglycerol, cholesterol and phosphorus. A rapid significant decrease was observed in serum testosterone levels in a dose-independent manner, so that the highest testosterone concentrations were observed in control fish throughout the experiment. There were no significant differences in oocyte stage or morphometric end points among the treated fish. We conclude that E2 implants do not stimulate ovarian growth, and hence, E2 implants alone are insufficient to reducing the time until onset of sexual maturation in previtellogenic great sturgeon.