Growth and biochemical composition of Crassostrea gigas (Thunberg) and Ostrea edulis (Linne) in two estuaries from the North of Portugal

Oysters C. gigas and O. edulis were grown in hanging cultures at river Lima estuary and Ria of Aveiro, in the north of Portugal. Growth and biochemica l composition were recorded between May 1990 and October 1991. Temperature, salinity, and particulate matter in the water, at both stations, were moni tored at the same time. No differences were found in the environmental para meters between stations, although the oysters presented some differences, m ainly in biochemical composition. C. gigas at both locations showed some sh ell thickening, evaluated through the shell thickness index (STI). The pres ence of butyltin compounds in the water seems to be the most reasonable exp lanation for the anomalous thickening and the highest constraint to the dev elopment of commercial oyster farms. Both stations have good trophic condit ions for culturing the two species.