The effect of mesh covers on the survival and growth of Crassostrea gigas Thunberg grown on the sea bed

Growth and mortality were measured in samples of Crassostrea gigas planted in trays on the beach at 4.5, 7.5 and 20% exposure. The oysters were graded by live weight into < 1, 1 2, 3–4, 5 6 and 8–10-g groups at planting. Three levels of protection were provided: none, 36-mm and 12.5-mm mesh. Five separate trials of 41–108 days covered about a year.Mortality decreased with increasing size of oyster at planting and with increasing degree of cover. On average, oysters of < 1 g and without cover showed 50% survival over 30 days, while oysters of 3–4 g protected with a 12.5-mm mesh showed 97% survival. Neither seasonal variation in survival nor variation related to exposure was detected. Unprotected oysters grew to a smaller size than protected oysters and those at 20% exposure grew less than those at 7.5 or 4.5% exposure. It is probable that the two sizes of mesh provided protection against interference and predation by the shore crab Carcinus maenas.The combined results of the trials indicated that the standing stock at the end of about 6 months, which included the summer, obtained from laying 100, 0.6 g oysters at 4.5% exposure would be 564 g and 99 g with 12.5-mm mesh and no protection, respectively.