Stimulating investment in pearl farming in ... - World Fish Center

Suitability of Habitats for Growth of Black-Lip and Silver-Lip Pearl Oysters Final, June 2008and that temperatures greater than 35 °C were lethal for larvae. Survival of larvae wasgreatest, but growth was slow, close to 20 °C, the lowest temperature examined.Yukihira et al. (2000) examined the effects of temperature on suspension feeding and energybudgets of black-lip and silver-lip pearl oysters. They found that the clearance rate,absorption efficiency, amount of energy absorbed, respiration rate, excretion and henceScope for Growth (i.e. energy available to sexually mature animals for growth andreproduction) of both species were much lower at 19 °C than between 23 °C and 32 °C. Theclearance rate of and amount of energy absorbed by black-lip oysters declined between 28°C and 32 °C, but that of silver-lip oysters remained fairly constant. The increase inabsorption efficiency of black-lip pearl oysters was less marked that of silver-lip pearloysters between 19 °C and 32 °C. The respiration rate of black-lip pearl oysters increasedmarkedly between 28 °C and 32 °C, whereas that of silver-lip pearl oysters only increasedslightly over this range. Black-lip oysters excreted less energy than silver-lip oysters overthis temperature range. High temperatures had a marked effect on the rate of ammoniaexcretion in both species. The Scope for Growth of black-lip pearl oysters was adverselyaffected by both low (19 °C) and high (32 °C) temperatures, whereas that of silver-lip oysterswas affected mostly by low temperatures. On the basis of these results, Yukihira et al. (2000)concluded that the optimum temperature ranges for black-lip and silver-lip oysters were 23 -28 °C and 23 -32 °C, respectively.In contrast, to the results from the GBR, temperature appears to have no effect on thephysiology of black-lip oysters in Takapoto Atoll, probably because the temperature range isonly 26 ºC to 30 ºC (Pouvreau et al. 1999a). The feeding responses of black-lip oysters,however, may differ in Polynesian atolls with wider annual temperature ranges (e.g. 22 – 32°C) or larger quantities of POM Pouvreau et al. (2000a).2.4 SalinityPearl oysters prefer full salinity seawater, but can tolerate a wide range of salinities (Gervisand Sims 1992). Taylor et al. (1997) found that exposure of juvenile silver-lip oysters toreduced (25-30 parts per thousand, ‰) or elevated (40-45 ‰) salinities resulted in greaterdetachment from the substratum than in ambient (34 ‰) seawater. Subsequent studies onthe effects of prolonged (20 day) exposure to salinities ranging from 25-45 ‰ have shownthat the survival of spat did not vary across this range, but growth was significantly reducedat 25‰, 40 ‰ and 45 ‰ (Taylor et al. 2004). Growth was also found to be significantlygreater at 30 ‰ than at ambient salinity, suggesting that the optimal salinity range forrearing of spat may be between 30 ‰and 34 ‰. Dourodi et al. (1999) found that normaldevelopment of black-lip oyster embryos and optimal survival and growth of larvae tookplace between 25 ‰ and 35 ‰. These studies suggest that the reductions in salinity to levelsas low as 25 ‰ that occur during the rainy season in northern Australia may have anadverse effect on the growth rate of silver-lip spats.2.5 HabitatBlack-lip pearl oysters are most abundant around the low tide mark, but extend to depths of40 m in the Torres Strait and Polynesia (Gervis and Sims 1992). This species is typicallyfound in oligotrophic coral reef and atoll water, where primary productivity and turbiditylevels are often low. In the Solomon Islands, black-lip pearl oysters are typically found inshallow water (1-2 m) (G Tiroba pers. comm. in Richards et al. 1994).The Ecology Lab Pty Ltd – Marine and Freshwater Studies Page 4