Reproductive biology of the shi drum (Umrina cirrosa) in captivity and induction of spawning using GNRHA

Abstract

The reproductive biology of the shi drum (Umbrina cirrosa) in culture was histologically examined and sperm quality was monitored during an entire reproductive period. Already in April, the ovary contained oocytes in all stages of maturation, from primary oocytes to full vitellogenesis, as expected from a group-synchronous multiple-batch spawning fish. Vitellogenesis of the first batch of oocytes occurred very rapidly and their mean diameter (500 μm) did not increase significantly (p>0.05) as the reproductive period proceeded. The spermiation index peaked in May-June, but fish never produced copious amounts of milt upon abdominal pressure. The spermatozoa motility percentage remained unchanged throughout the spawning season (80%) and a significant percentage (40%) maintained viability after overnight storage at 4°C. Sperm density and motility duration increased during the reproductive period and varied 13-26 × 109spermatozoa/ml and 26-40 s, respectively. Spontaneous spawning was not observed during the two-year study. Injection of post-vitellogenic females with an agonist of gonadotropin-releasing hormone (GnRHa) was successful in inducing a single spawning after two days, with fertilization, hatching and 4-day larval survival rates of 65%, 42-76% and 46-80%, respectively. The results underline the failure of female shi drum in culture to undergo final oocyte maturation and, although GnRHa injection was effective in inducing spawning of viable eggs, multiple treatments did not induce multiple spawns, as was expected from fish with multiple-batch group-synchronous ovarian biology.