Effects of acute versus sustained administration of GnRHa on GtH release and ovulation in the rainbow trout, Oncorhynchus mykiss

The effects of different forms of GnRHa (sustained vs. acute) on the stimulation of gonadotropin secretion and the rate of ovulation were investigated in the rainbow trout, Oncorhynchus mykiss. A biodegradable sustained release form of DTrp 6 LH-RH induced a progressive rise in the gonadotropin plasma levels in a dose-dependent manner. The stimulation of gonadotropin secretion was maintained for 3 weeks. All fish ovulated within 5 days after injection using a high dosage (50 μg/kg body weight) whereas with the lower dosages (25 and 12.5 μg/kg body weight) 100% ovulation was reached after 8 days. In contrast, acute injections of GnRHa (20 μg/kg body weight) induced a transient stimulation of gonadotropin secretion. Ovulation rates were first accelerated within 8–10 days and then remained stable but they did not reach 100% 26 days after the injection. At that time the rate of ovulation in the control group had increased to 70%. These results are discussed, along with a comparison of variability of egg quality with treatments.