Snapper may grow to a maximum length of 1.3 m and weigh 19.5 kg (Hutchins & Thompson 1983). Wild fish may take 3 to 5 years to reach a market size of 250 mm. However, in NSW captive snapper have shown an increased growth rate and have been grown to market size within 21 months (Bell et al 1991). Where water temperatures do not drop below 14 o C, cultured snapper may grow to between 600 and 700 gm in 18 months and 1.2 to 1.5 kg in 2½ years. This growth rate is approximately twice that of wild fish. There is, however, a wide variation in growth rates due to varying environmental conditions between farms, particularly water temperature (Foscarini 1988). The growth and development of reproductively isolated strains of snapper from Japan, New Zealand and Australia are similar. However, the influence of genetic factors upon spawning, maturation and growout between these strains has not been investigated. It is believed that growth differences may be due to variations, not only in strains but also in habitats (Francis & Winstanley 1989). Consequently, the importance of maintaining the integrity of local stocks is fundamental to any future aquaculture breeding program. In Japan selective breeding of snapper over 5 generations has produced a 40% increase in growth rate of cultured snapper compared with wild populations. This has halved the required culture time of snapper from 3 years to 18 months. This genetic improvement is reported to save the Japanese farmers 30% in culture costs. However, the genetic improvements obtained overseas will not be achievable in Western Australia until the life cycle of snapper is closed and the controlled spawning of this species is achieved within this state. Snapper Health Issues As the aquaculture of snapper has yet to reach commercial production in Western Australia, the disease status is unknown. However, snapper in Western Australia are known to have several parasites including the didmyozoid trematode Gonapodasmius williamsoni and the gonad nematode Philometra spp (M Hine pers com, Williams et al 1993). In addition, pathogens commonly experienced in Japan and reported from New Zealand, such as Vibrio spp and Cryptocaryon irritans , are also likely to present problems within Australia (Hine 1982, Foscarini 1988). Both diseases are, however, generally only a problem in stressed fish and therefore can be controlled by husbandry practices. In hatcheries, mortalities of over 40% have been reported due to a vertebral malformation known as lordosis (Foscarini 1988). Most health problems experienced with adults have been attributed to either handling stress or poor diet (Hine 1982, Smith & Hataya 1982). |