Quantitative studies of steroid bioconversions in the seminal vesicles of spawning male African catfish, Clarias gariepinus (Burchell), under natural conditions, and of non-spawning catfish under natural and fish farm conditions.

1. Steroid bioconversions in the seminal vesicles of Clarias gariepinus were studied quantitatively in vitro by tissue incubations with [3H]pregnenolone, [3H]androstenedione and [14C]11 beta-hydroxyandrostenedione, respectively. 2. Spawning and non-spawning catfish, collected in the Hula nature reserve in northern Israel during the spawning period, and non-spawning animals, collected from a fish pond in the same region during the same period, were studied. 3.Spawning animals showed a significantly higher production of 17 alpha-hydroxyprogesterone, 5 beta-pregnan-17 alpha-ol-3,20-dione and 5 beta-reduced androgens than non-spawning feral and pond catfish, as a result of a significantly increased contribution of the enzymes 5 beta-reductase and 3 alpha-hydroxysteroid dehydrogenase (HSD). 4. In spawning catfish the concentration of gonadotropin in blood plasma were also significantly higher than in the plasma of non-spawning feral and pond catfish. This increase in gonadotropin level might have induced the rise in enzyme activity of 5 beta-reductase and 3 alpha-HSD. 5. It is concluded that the absence of a shift in steroidogenesis towards the production of 5 beta-reduced steroids may be among the factors preventing spontaneous spawning in male African catfish under husbandry conditions.