2. Materials and methods
2.1. Mortalities at fiÕe growing leÕels on the Georges RiÕer Space on a commercial oyster lease in Woolooware Bay, Georges River, NSW
Ž
X X
. 33
859 S, 151812 E was provided by an oyster farmer. The tarred wooden rails used to Ž
. support trayed oysters in the traditional growing method Malcolm, 1987 were modified
to provide five growing levels at height intervals of 150 mm, centred on the intertidal Ž
growing level normally used by oyster farmers in Woolooware Bay slightly below .
mid-tide within the maximum range of 2 m . Oyster sticks, when laid out by oyster farmers in Woolooware Bay, are set at the second highest level used in this experiment,
i.e., normal tray level plus 150 mm. Commercial 1.8 = 0.9 m, tarred timber and galvanised wire mesh oyster trays were
divided into two equal compartments. Two trays at each level provided four replicate compartments at each of the five growing levels.
Ž Three-year-old S. glomerata of approximately ‘plate’ grade 45 to 50 g whole
. weight were obtained from local oyster farmers, and were randomly mixed before
allocation to tray compartments at a stocking rate of 190 oysters per compartment Ž
2
. 235rm . The trays were covered with a light plastic netting to minimise oyster losses
resulting from wave action. Ž
. Ž
The experiment ran from 23 April 1986 mid-autumn to 12 January 1987 mid- .
summer . Oysters that died at the start as a result of culling damage were replaced with live oysters held for this purpose in trays adjacent to the experimental oysters.
Mortalities were determined monthly by counting and removing dead shells; a sample of six oysters from each replicate was weighed, the condition index of the pooled meats
determined and analysed for glycogen content. Glycogen content was assayed by the Ž
. Ž
. enzymatic method of Kepler and Decker 1974 . The condition index CI was calcu-
lated as: dry weight of oyster meat g = 100
Ž .
CI s shell cavity volume ml
Ž .
Cavity volumes were determined by subtracting the weight in-air of the oyster’s shell Ž
. from the weight in-air of the whole oyster Lawrence and Scott, 1982 . This method is
valid because the effective density of cavity contents is close to 1 grml, providing the oysters have not gaped and lost cavity fluid. Meat weights were determined by drying
them to a constant weight at 90 8C.
2.2. Growing method and leÕel on diploid and triploid S. glomerata at Merimbula Juvenile sibling diploid and triploid S. glomerata were obtained from the same batch
Ž .
used in the commercialisation trials of Hand et al. 1998a . Parent stock had been spawned at the Port Stephens Research Centre’s Mollusc Hatchery in February, 1994.
Triploidy was then induced by blocking meiosis 2 polar body extrusion with cyto- Ž
. Ž
chalasin B Nell et al., 1996 resulting in 88 triploidy measured using the flow .
cytometry method on spat .
Ž .
Spat were reared in mesh cylinders Holliday et al., 1993 , in Tilligerry Creek, Port Stephens to a size suitable for the experiment. Triploid and diploid oysters for the
Ž experiment were dry graded through 18 mm and retained on 14-mm mesh diagonal
. measurement .
Ž
X
The experiment used commercial oyster racks at Merimbula Lake, NSW 36 855 S,
X
. 149
855 E from June 1994 to October 1996. Racks were located at the high salinity, seaward end of the oyster growing area, where exposure to M. roughleyi is high, and the
experiment was serviced at approximately 3-month intervals to remove fouling organ- isms.
Ž .
Treatments comprised two growing systems stick and tray , two growing levels and two ploidy types, providing eight treatment combinations, each replicated four times.
Each replicate was stocked with 400 oysters. The ‘stick’ growing system used standard tarred hardwood oyster sticks 25-mm
square section = 2-m long in frames of five sticks nailed at 130-mm intervals to half Ž
. stick cross members near each end Nell, 1993 . The sticks were used to support oyster
spat glued to the sides at 50-mm intervals using a non-acid curing silicone plumbers’ sealant. This system models closely the traditional commercial stick growing method of
Ž .
wild oyster spat caught on tarred sticks Nell, 1993 with the exception that oysters were glued at regular spacings rather than randomly settled from the wild.
The ‘tray’ growing system used standard single seed trays of 1.8 = 0.9-m tarred hardwood timber frames, subdivided into 12 compartments, and the bottom covered with
Ž .
9-mm diagonal polyethylene mesh. A second tray was used as a lid. Half the tray, six compartments, were stocked with oysters for the experiment, the remaining six were
stocked with oysters used to provide samples to test for the presence of the M. roughleyi parasite.
Frames of sticks and trays were supported on a commercial oyster growing rack of Ž
. tarred hardwood posts and rails Nell, 1993 . Two growing level treatments were used.
Standard growing level, at a little under half tide was the same as other commercial oyster leases in the area. The second growing level was set up adjacent to the first but
Ž .
300 mm below the standard level to increase exposure to winter mortality Wolf, 1976 . Ž
. Shell height umbo to ventral valve margin and whole weight were measured for 400
oysters randomly taken from each ploidy type before the sticks and trays were stocked at the start of the experiment. At termination of the experiment in October, 1996, shell
height and whole weight were measured for 50 oysters from each replicate. Mortality levels during the final winter were assessed by counting live and dead oysters in each
replicate at the termination of the experiment in October 1996. Percentage mortalities
Ž .
were calculated as number of deadr number of dead q number of alive = 100. In June and October 1996, oyster samples were taken and forwarded to Brisbane for
Ž M. roughleyi diagnosis by the Hemacolor stained imprint method Adlard and Lester,
. 1995 .
2.3. Statistical methods 2.3.1. Mortality at fiÕe growing leÕels on the Georges RiÕer
Ž .
Homogeneity of variance was confirmed using the Cochran test Winer, 1971 . The data for each of these experiments were then subjected to a single factor analysis of
Ž .
variance Sokal and Rolf, 1981 in the statistical software package,
STATGRAPHICS
, Ž
. version 5 Statistical Graphics, Rockville, MD .
2.3.2. Growing method and leÕel on diploid and triploid S. glomerata at Merimbula All data were analysed using multiple ANOVA in the Statgraphics package above.
For the measurements on samples of spat taken at the start of the experiment, homogeneity of variances was confirmed for shell height for triploids and diploids using
Ž .
Cochran’s test C s 0.507, P 0.05 . Weights were log transformed to satisfy require- Ž
. ments for homogeneity C s 0.501, P 0.05 .
Ž Weight data were expressed as weight increase, final mean whole weight for each
.
0.5
replicate-initial mean weight . Percentage mortality data were arcsin x transformed.
Ž Homogeneity was confirmed for shell height and whole weight C s 0.354, C s 0.256,
. respectively: P 0.05 . Mortalities remained slightly heterogeneous following transfor-
Ž .
mation C s 0.467, P s 0.029 . This is, however, a minor departure from homogeneity Ž
. and ANOVA is sufficiently robust to overcome such departures Underwood, 1981 .
3. Results
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