EFFECTS OF STOCKING DENSITY ON JUVENILE SYDNEY ROCK OYSTERS, SACCOSTREA-COMMERCIALIS (IREDALE AND ROUGHLEY), IN CYLINDERS

Two size grades ( 1 3. 5 and 18.2 mm average shell length spat-1) of juvenile Sydney rock oysters, (Saccostrea commercialis Iredale & Roughley) were grown at a range of stocking densities (0.5 to 6.0 1 of oysters cylinder-1 ) in PVC cylinders for 3 months. Cylinders were deployed on horizontal hardwood frames on an intertidal lease, where they rotated in response to changes in tidal level. For each grade, spat mortality was similar (P > 0.05) for stocking densities of 0.5 to 4.0 1 cylinder-1 (mean +/- s.e. 11.7 +/- 1.1% and 22.5 +/- 2.3%, respectively) but higher for the larger grade (33.3 +/- 1.7%; P < 0.05) at the highest stocking density. For both grades, weight gain and length increase declined as stocking density increased (P < 0.001), although differences for initial densities above 3.0 and 4.0 1 of oysters cylinder-1, for the smaller and larger grades respectively, were not significant (P > 0.05). Biomass gain and volume of oysters both increased with increasing stocking density (P < 0.05), although increases above initial densities of 2. 0 and 3.0 1 of oysters cylinder-1, for the smaller and larger grades, respectively, were not significant (P > 0.05). For both grades, coefficient of variation for weight gain and shell length increase data increased (P < 0.001) with density. For maximum growth and minimum coefficient of variation for weight gain and shell length increase, 0.2 and 0.4 g spat should be stocked at low densities of 0.5 or 1.01 cylinder-1. To optimise biomass gain, while minimising growth reductions and size variation, 0.2 and 0.4 g spat should be stocked at densities of 2.0 and 3.0 1 of oysters cylinder-1, respectively.