| | Snapper are functional gonochorists, that is, during its life each fish may spawn either as a male or a female, but not both. Protogynous (female to male) sex inversion (change) occurs in only some of the population. Prior to becoming sexually mature, juvenile snapper may undergo sex inversion to become functional males, those individuals which do not undergo this sex inversion, will mature to become functional females. The age at which sex inversion occurs varies between populations (Francis & Pankhurst 1988). In Australia, snapper have been reported to spawn when the water temperature reaches 18 o C, although in Japan they may spawn at water temperatures between 15 o C and 22 o C (Battaglene & Talbot 1992, Foscarini 1988, Smith & Hataya 1982). Snapper spawn as early as June/July in the warmer waters of Shark Bay, Western Australia. At lower latitudes, the more southerly populations spawn later in the year as the water temperature increases at around September or December (Moran pers com). Natural spawning occurs at dusk and evenings (Foscarini 1988, Smith & Hataya 1982). Females spawn repeatedly over a 3 to 4 month spawning season. In a spawning season, a 3 year old female may produce 250,000 eggs, a 1.5 kg female may produce 300,000 to 400,000 eggs and a 10 year old female may produce 5 million eggs (Smith & Hataya 1982, Kafuku & Ikenoue 1983). Fish can mature after 2 years. Mature broodstock fish are usually held in spawning tanks or floating net cages, at a sex ratio of 1:1 and a density between 0.5 and 1.5 kg/m³ (Foscarini 1988, Smith & Hataya 1982). Females with oocytes greater than 0.9 mm in diameter can be readily induced to ovulate using hormone induction (Battaglene & Talbot 1992). Induced spawning techniques using human chorionic gonadotropin (HCG) at 1,000 IU/kg are well developed, however, natural spawning produces better quality eggs (Battaglene & Talbot 1992, Foscarini 1988). Fertilized eggs are between 0.9 and 1.0 mm in diameter. Positively buoyant eggs have a higher hatching rate than do negatively buoyant eggs, which are generally unfertilized, immature or overripe. Consequently, eggs are often collected from tank overflows. Egg quality is related to broodstock nutritional state and the age of the females (Foscarini 1988, Smith & Hataya 1982). Eggs are incubated in the dark either in tanks with volumes generally ranging from 50 to 100 m³ at a density of 20,000 to 30,000 eggs/m³, or more intensively in smaller tanks with a volume of 0.5 m³ and a density of 40,000 eggs/m³ (Foscarini 1988). Although egg development is closely related to water temperature, eggs generally hatch into 3.1 mm yolksac larvae approximately 28 hours after fertilization (Battaglene & Talbot 1992). After hatching, larvae are grown at a density of 10,000 to 15,000 per m³. Higher densities result in cannibalism (Foscarini 1988, Battaglene & Talbot 1992). Failure of the swim bladder to inflate at around days 7 - 11 has been a major problem in rearing red sea bream larvae (Battaglene & Talbot 1992). At days 30 - 40 after hatching the juveniles are between 12 and 17 mm in length and may be transferred to cages for growout at a density between 2,000 and 2,500 fish/m³ (Foscarini 1988, Smith & Hataya 1982). Although hatchery work has been undertaken in Western Australia and New Zealand, these projects have until recently been are reliant upon obtaining fertilized eggs from public aquaria or collection of ripe broodstock from the wild. Closing of the life cycle has only recently been achieved in Western Australia. | |