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 im planted with pellets containing gonadotrophin-releasing hormone agonist (Gn RHa). Males were bled repeatedly, and milt samples were collected. Blood sa mples were assayed for free and conjugated steroids: testosterone, 11-ketot estosterone, 17,20 alpha-dihydroxy-4-pregnen-3-one (17,20 alpha-P), 17,20 b eta-dihydroxy-4-pregnen-3-one (17,20 beta-P), 17,20 beta,21-trihydroxy-4-pr egnen-3-one and steroids with a 17,20 beta configuration. Towards the end o f the first season, pellets were implanted into three wild-caught and three hatchery-reared males. No control fish were available. The major progestog en 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 s timulated by the GnRHa. Sulphated 17,20 beta-P was also identified in the p lasma, 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 hat chery-reared males; five unimplanted males were used as controls. Levels of androgens fell following GnRHa treatment, levels of progestogens rose brie fly, and there was a significant increase in the fluidity of the milt. Of a ll the measured steroids, free and sulphated 17,20 alpha-P showed the best correlation with milt fluidity.