Temperature effects along the reproductive axis during spawning induction of grass carp (Ctenopharyngodon idella)

For grass carp (Ctenopharyngodon idella) raised in the Ivory Coast (with water temperatures of 26-31°C), induced spawning is obligatory for fry production. However, ovulation rates following hormonal treatment are often low. We hypothesized that high temperatures are an inhibiting factor for the reproductive axis (brain-pituitary-gonad) in these conditions. By in vivo and in vitro experiments, we tried to determine the thermosensitive steps during spawning induction. We compared gonadotropin and maturation-inducing steroid (MIS) profiles during a spawning induction at controlled temperatures of 24 and 28°C in relation to ovulation success. We performed pituitary cell cultures and ovarian fragment incubations at controlled temperatures. The ovulation rate was lower at 28°C (10%) than at 24°C (36%). At the pituitary level, we found only minor thermal impacts on GnRH-stimulated LH release, but our data suggest an increase of the dopaminergic inhibition by high temperatures. The main effects were found at the ovary level, where ovary responsiveness to gonadotropin by MIS synthesis was disturbed, as well as oocyte responsiveness to MIS triggering final maturation, and probably ovulation. These results show the importance of regulating temperature during spawning induction to ensure a high rate of ovulation. (Résumé d'auteur