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

Three experiments of spawning induction in shi drum, Umbrina cirrosa L., were performed in six different commercial Italian hatcheries from May to August (water temperatures: 19–29 °C; salinity: 21–37 p.p.t.). In the first experiment, 119 females (1–4.7 kg), subdivided into 29 lots, were injected with a single dose (2, 5, 8, 10, 15 and 20  μ g kg −1 body weight) of short‐acting gonadotropin‐ releasing hormone 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 four lots, were treated with one (40 or 80  μ g kg −1 ) or three doses (40  μ g kg −1 ) of long‐acting GnRHa (GnRHa‐L), respectively. GnRHa‐S stimulated spawning in 69% of the 29 treated lots; the number of eggs laid 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 four lots of females with GnRHa‐L always resulted 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.