Gonadotrophin-releasing hormone agonist raises plasma concentrations of progestogens and enhances milt fluidity in male Atlantic halibut (Hippoglossus hippoglossus)

In two separate spawning seasons, spermiating male Atlantic halibut were implanted with pellets containing gonadotrophin-releasing hormone agonist (GnRHa). Males were bled repeatedly, and milt samples were collected. Blood samples were assayed for free and conjugated steroids: testosterone, 11-ketotestosterone, 17,20 alpha-dihydroxy-4-pregnen-3-one (17,20 alpha-P), 17,20 beta-dihydroxy-4-pregnen-3-one (17,20 beta-P), 17,20 beta,21-trihydroxy-4-pregnen-3-one and steroids with a 17,20 beta configuration. Towards the end of the first season, pellets were implanted into three wild-caught and three hatchery-reared males. No control fish were available. The major progestogen in plasma was identified as sulphated 5 beta-pregnane-3 beta,17,20 beta-triol (3 beta,17,20 beta-P-5 beta-S). Concentrations of this steroid were stimulated by the GnRHa. Sulphated 17,20 beta-P was also identified in the plasma, but at 10-fold lower concentrations than 3 beta,17,20 beta-P-5 beta-S. In the middle of the second season, pellets were implanted into five hatchery-reared males; five unimplanted males were used as controls. Levels of androgens fell following GnRHa treatment, levels of progestogens rose briefly, and there was a significant increase in the fluidity of the milt. Of all the measured steroids, free and sulphated 17,20 alpha-P showed the best correlation with milt fluidity.