The effects of the GnRH agonist, azagly-nafarelin (Gonazon™), on ovulation and egg viability in the European grayling ( Thymallus thymallus L.)

In 2005–2007, four separate experiments in three different hatcheries were performed on the effects of azagly-nafarelin, a commercially-available (Gonazon™) GnRH agonist (pGlu-His-Trp-Ser-Tyr-[ d-Nal(2)]-Leu-Arg-Pro-[Aza-Gly]) on ovulation and egg viability of the European grayling ( Thymallus thymallus L.). The experiments were performed on cultured as well as captured wild grayling broodstock. Gonazon was injected intraperitoneally at doses of 16, 32 or 48 μg kg − 1 body weight. Control fish were injected with phosphate-buffered saline (PBS). In the first experiment performed on cultured 2-year-old females (first reproduction), there was no significant effect of Gonazon on ovulation rate between the control and the 16 μg kg − 1 Gonazon groups. In three other experiments performed on older females (3 or more years old), Gonazon at all doses (especially 16 μg kg − 1 ) significantly accelerated and synchronized ovulation (90–100% of ovulation occurred 10–14 days post injection) in comparison with the control group (only 50–60% ovulated during the same period). No differences in mortality rate were observed between the control and Gonazon groups. In two experiments, the egg viability was monitored during the whole incubation period. There were no significant differences in the percentage of fertilized eggs between Gonazon and control fish; however, there was a 4 to 5% higher egg mortality from Gonazon fish during the whole incubation period. It is concluded that Gonazon (especially at a dose of 16 μg kg − 1 ), given at the beginning or in the middle of the spawning period, is highly effective and a safe inducer of ovulation in cultured and wild European grayling.