Oocyte maturation and changes in plasma steroid levels in snapper Pagrus (=Chrysophrys) auratus (Sparidae) following treatment with human chorionic gonadotropin

Abstract

Snapper Pagrus (= Chrysophrys) auratus were captured by long-lining from the Hauraki Gulf, NE New Zealand, and returned to the laboratory. Eighteen female fish were injected with either saline or 1000 U kg −1 human chorionic gonadotropin (hCG). Blood samples were taken at the time of injection and subsequently at 1, 6 and 24 h post-injection (pi). Plasma levels of 17β-estradiol (E 2), 17,20β-dihydroxy-4-pregnen-3-one (17, 20βP), and testosterone (T) were measured by RIA. Seven out of the eleven hCG-injected fish ovulated within 24 h, whereas no control fish ovulated. In fish injected with hCG, plasma E 2 levels were elevated at 24 h pi, 17,20βP levels at 6 h pi, and T at 6 and 24 h pi, relative to controls. In a second experiment, twelve fish were injected with 1000 U kg −1 hCG as before and bled at 0, 4, 7, 13, 17 and 26 h pi. At each sample time a small piece of ovarian tissue was taken from each fish using a 12 G syringe needle introduced through the body wall. Aspirated follicles were measured and then cleared to determine the position of the germinal vesicle. Follicles from six of the fish underwent a significant increase in diameter, accompanied by germinal vesicle breakdown (GVBD) and oocyte hydration. Plasma E 2 and T levels were elevated in fish with maturing oocytes from 13 h pi. 17,20βP showed a peak at 4 h pi, but only in the group of fish in which oocyte maturation did not occur. There was no difference in plasma cortisol levels between fish that showed oocyte maturation and those that did not, suggesting that proximate stress factors were not responsible for the differential response to hCG treatment observed in this experiment.