Abstract
Growth and mortality of the Pacific oyster, Crassostrea gigas Thunberg, introduced into Oualidia lagoon (Morocco), were studied during the period 1981–1983. Environmental parameters of the lagoon were found favourable for a rapid growth of spats and adult oysters when reared at subtidal level. Cultch-free spats grew from 6 g (live weight) to 43 g in 1 year. Free adult oysters having an average weight of 12 g, 31 g, 46 g, 72 g, and 89 g attained 80 g, 114 g, 125 g, 162 g and 184 g, respectively, within 1 year. Oyster spats kept attached to cultch materials and reared at intertidal level showed only a 15 g weight gain in 10 months. Seasonal growth rates of oysters and spats exhibited the same seasonal trend as those observed for oysters reared in other culturing grounds from temperate regions. Survival of oysters in Oualidia seemed to be affected by the elevated water temperature in summer and autumn and by its wider diurnal variation during these periods. The spats showed mortality rates of 70–77%, the rates decreasing with growth. Mortality rates of adult oysters varied from 13 to 54% depending upon the size of the animal.
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