THE LIPID-CONTENT, SPATFALL AND SUBSEQUENT GROWTH OF EARLY AND LATE SETTLING HATCHERY-REARED PACIFIC OYSTER, CRASSOSTREA-GIGAS THUNBERG, LARVAE

Six experimental batches of,Pacific oyster larvae, Crassostrea gigas T hunberg, were reared on standard diet in the hatchery. After 8-12 days from fertilization, the larvae were separated into two to three size groups. These groups of larvae were then reared separately through to settlement. A sample was taken from each group when the larvae had rea ched the stage at which they were ready to settle for estimation of nu mber, size and weight, and analysis of total lipid. All the spat that settled from each group were removed daily during the settlement perio d of 10-14 days. Selected samples of 1000 or more spat, collected from a single (day) settlement, were reared in standard hatchery upwelling systems for a further 30 days. Spatfall, as a percentage of the numbe r of larvae in the group available to settle, was similar for all thre e groups of larvae, although it varied between experiments, and was po sitively correlated with the amount of lipid ih the eyed larvae. The a verage spatfall, for all groups, was 44%. The dry and organic weights of spat that settled from the group of larvae that grew fastest were s ignificantly greater than that for spat from the group with the slowes t growth rate. Lipid content of the spat in the three groups was simil ar (9.3-12.4%). The spat that settled from the group of larvae that gr ew fastest also tended to be biggest after rearing in the hatchery for a further 30 days, but this result was not statistically significant. Spat from within the same group of larvae also grew to a similar size in this time, irrespective of whether they were collected during the first 3 days of settlement, or after a further 5 days. In practical te rms, the results of these experiments show that it is cost-effective f or hatchery operators to maintain batches of larvae for as long as set tlement continues, to maximize the output of spat numbers without any danger of wasting resources by producing inferior quality or slower gr owing animals.