CONDITION INDEX AND CHEMICAL-COMPOSITION OF MEATS OF SYDNEY ROCK OYSTERS (SACCOSTREA-COMMERCIALIS) AND PACIFIC OYSTERS (CRASSOSTREA-GIGAS) AT 4 SITES IN PORT-STEPHENS, NSW

Adult Sydney rack oysters (Saccostrea commercialis) and Pacific oyster s (Crassostrea gigas) were kept on commercial oyster leases at three i ntertidal sites in Port Stephens, New South Wales, and subtidally unde r an experimental raft at a fourth site between Jury 1988 and Septembe r 1989. Oysters were sampled from each site at approximately monthly i ntervals for chemical and histological analysis. Condition index and p ercentage glycogen of Pacific oysters were higher than those of Sydney rock oysters during winter and spring but tended to be lower during s ummer and autumn. Gonads of Pacific oysters matured two months earlier than those of Sydney rock oysters, with spawning being observed at al l sites in October. Sydney rock oysters spawned later during December- January and did not lose as much condition after spawning as Pacific o ysters. The absolute amount of glycogen in the meats of both species d ropped at the expense of protein and lipid as the oysters became fully ripe. For both species, general condition of the oysters was best whe n they were grown subtidally under the raft, although both species wer e badly affected by invasion of the protistan parasite Mikrocytos roug hleyi at this site. Poorest overall condition for both species occurre d at a site (Karuah River) that experienced decreased salinities and i ncreased turbidity after rain. Highest condition indices were found in Sydney rock oysters, at the site most dominated by coastal conditions (Corrie island).