GROWTH AND BIOCHEMICAL-COMPOSITION OF CRASSOSTREA-GIGAS (THUNBERG) AT3 FISHFARM EARTHEN PONDS

Growth and survival of Crassostrea gigas (Thunberg) juveniles, from na tural spatfall, were compared at three earthen fishponds in the northw estern Portuguese coast, from June 1990 to October 1991. Station Marin ha was located at Ria de Aveiro, a seawater lagoon with little incomin g freshwater. Stations INIP and Gil, located at Mondego River estuary, experienced an important influence from freshwater, particularly Stat ion Gil. Oysters were sampled at 45-day intervals for their growth, co ndition index, and biochemical composition. Temperature, salinity, and particulate organic and inorganic matter in the water at the rest sit es were monitored at the same time. Juveniles initially with 0.4 g rea ched the end of the experimental period with an average of 53, 50, and 38 g at Stations Marinha, INIP and Gil, respectively. In general, oys ters from Station Gil showed the worst condition, especially during th e second growing season. Oysters from both Mondego stations presented shells infested by Polydora sp. Apparently, the high mortalities obser ved at the same stations had nothing to do with the worm infestation. Mortality at Station Marinha was negligible. Variations observed in dr y meat weight and lipid and carbohydrate levels are probably associate d with gonad growth and spawning activity at Stations Marinha and INIP . Several factors that map have been responsible for the observed diff erences in oyster performance are discussed. In particular, the water movement is thought to have played a key role in dictating oyster perf ormance. Fish farm earthen ponds, with controlled water circulation, s eem to be suitable sites for intermediary oysters growth.