More effective induction of spawning with long-acting gnrh agonist in the shi drum, Umbrina cirrosa L. (Sciaenidae, Teleostei), a valuable canditate for Mediterranean mariculture

Three experiments of spawning induction in shi drum were performed in six different commercial Italian hatcheries from May to August (water temperatures: 19-29°C; salinity: 21-37 ppt). In the 1st experiment, 119 females (1-4.7 kg), subdivided into 29 lots, were injected with a single dose (2, 5, 8, 10, 15 and 20 µg×kg-1 body weight) of short-acting GnRH agonist (GnRHa-S), des-Gly10,[D-Ala6]-LH-RH ethylamide. In the other two experiments, 85 females (0.7-5.8 kg), subdivided into 22 and 4 lots, were treated with one (40 or 80 µg×kg-1) or three doses (40 µg×kg-1) of long- acting GnRHa (GnRHa-L), respectively. GnRHa-S stimulated spawning in 69% of the 29 treated lots; the number of laid eggs reached a maximum of 130,000 and a weighted mean of 29,200 total eggs×kg-1. GnRHa-L elicited a spawning response in 95% of the 22 one-dose treated lots; the number of laid eggs was higher than with GnRHa-S, reaching a maximum of 213,100 and a weighted mean of 59,400 total eggs×kg-1. The yield of developing embryos in 67% of the single GnRHa-L treatments was higher (sometimes up to three times) than with GnRHa-S. Triple treatments of the 4 lots of females with GnRHa-L resulted always in spawning responses; the best result corresponded to a number of total laid eggs of 358,900 eggs×kg-1 with a yield of 177,300 developing embryos.