Hormone-induced spawning of the Australian freshwater fish Murray cod, Maccullochella peeli (Mitchell) (Percichthyidae)

Abstract

Murray cod, Maccullochella peeli, originally captured from the wild, underwent normal gonadal development in earthen ponds. Handling of broodfish in the 3 months before a breeding season caused atresia and resorption of oocytes in most females. Cod were removed from the ponds when the water temperature reached 20°C during spring, and final oocyte maturation and ovulation were induced in mature females by injecting 1000 or 2000 IU/kg human chorionic gonadotrophin (HCG) or 2–5 mg/kg of a preparation of the pituitary gland from common carp (CPG). Control treatments and dosages of 100–750 IU/kg HCG did not induce ovulation. Broodfish were held at 21 ± 1°C in 2000-l tanks after injection. The time of stripping and fertilization of Murray cod eggs was an important factor determining their hatchability. There was generally high post-fertilization mortality of eggs stripped within 1 h or between 4 and 6 h of ovulation, but high hatchability of eggs stripped 2–3 h after ovulation. The mean hatchability of eggs stripped 48.5–49.5 h after the injection of 1000 IU/kg HCG was 79.8%, but there were significantly lower mean hatchabilities of eggs stripped after 46–48 h and 50–52 h, as well as after the injection of 2000 IU/kg HCG. Results using CPG were variable. Possible reasons for the high post-fertilization mortality of Murray cod eggs are discussed, and techniques for broodfish handling, injection, stripping and the fertilization and incubation of eggs are presented.