The growth of oysters Ostrea edulis and Crassostrea gigas in Emsworth Harbour

Abstract

To investigate the growth, mortality and condition of European and Japanese oysters ( Ostrea edulis and Crassostrea gigas) in Emsworth Harbour, fixed trays were placed at two sites in the harbour. Oysters from the White Fish Authority Hatchery at Conway were placed in these trays at an initial density of 6 kg/m 2. At monthly intervals from October 1970 to October 1971 the oysters were weighed and counted and the stock thinned to 6 kg/m 2. The condition index of a sample of 25 oysters from each group was determined. Temperature, salinity, turbidity and plant pigments were monitored over the same period, and the phytoplankton was examined qualitatively. A batch of hard clams ( Mercenaria mercenaria) laid in a separate tray at each site became smothered by silt and suffered almost total mortality. This part of the experiment was discontinued. Both species of oyster showed good growth rates through the summer and C. gigas also grew appreciably during the winter. A decline in condition index during the summer was found in both species, indicating spawning activity. Among the oyster spat found in the harbour during the spring of 1972 were two speciments of C. gigas.