Abstract
The most valuable cultured pearls in the world are produced from the silver-lip pearl oyster, Pinctada maxima. Although widely distributed throughout the tropics, with the exception of the north west coast of Western Australia there are no wild fisheries capable of supporting pearling industries. Further development of pearling industries based on P. maxima must therefore be based on hatchery production.The culture of Pinctada maxima spat is still a new and developing technology. Hatchery production has been successful in producing large numbers of spat, but there are still many unexplained mass mortalities in hatcheries and nurseries which have resulted in a significant reduction in the numbers of hatchery oysters seeded compared to the potential number. This study was initiated in response to persistent unexplained spat mortalities in commercial hatcheries in northern Australia.
The general aims of this work are to investigate and quantify some of the factors which may influence the performance of P. maxima spat during the hatchery and nursery phases of production, and to identify the optimum for each parameter for commercial production.
For all of the laboratory based trials, spat were maintained in temperature controlled flowthrough systems. Spat survival in the laboratory was greatest between 23 and 32°C. At 20°C, survival was slightly lower, while 35°C was lethal. The calculated minimum tolerance limit was 17.7°C. The optimum temperature for spat growth was between 26 and 29°C.
Spat maintained at an algal concentration of 6 cells µL-1 of an equal mixture of Chaetoceros muelleri and T. Iso had the highest survival rate. Otherwise, spat survival was not affected by algal food concentrations between 12 and 110 cells µL-1 . Spat growth was not significantly affected by algal densities between 12 and 110 cells µL-1, but was reduced at 6 cells µL-1 . High algal concentrations resulted in the inefficient usage of algae. The ash free dry weight (AFDW) of spat increased proportionally with algal concentration from 11% at 6 cells µL-1, up to a maximum of ca. 13% at 54 cells µL-1 .This increase in spat tissue weight was primarily as lipid, particularly neutral lipid. Spat performance after transfer from the laboratory to a farm based nursery was not related to the feeding rate or organic content of fed spat. However, spat from unfed treatments, and those obtained directly from a hatchery both had a low organic content (<10%), and showed poor survival after transfer to the nursery.
During the first week following the transfer of spat, lipid reserves declined sharply, and spat with insufficient lipid reserves may be compromised.
Spat showed a well developed ability to select algal cells from an algal/particulate inorganic matter (PIM) suspension. This ability was not immediate, but required ca. five days before acclimation to the diet suspension was complete. Even after acclimation, spat still lost a proportion of the available algal cells as pseudofaeces, commensurate to the level of PIM in the diet. Absorption efficiency was reduced at high PIM concentrations, but was compensated for by an increase in filtration rate and ingestion, hence maintaining a relatively stable absorbed ration. This was reflected in the similar growth and survival of spat maintained at PIM levels from 0 to 80 mg L-1. There was no demonstrated advantage of including PIM in the diet.
A variety of water pressures and intervals for cleaning spat were trialed during farm nursery growout up to seedable size. Overall growth averaged ca. 1 mm week -1, and growth rate was not related to the initial size of the spat at the beginning of the trial. Spat survival was greatest at a combination of 1800 kPa pressure and a 28 day interval between cleans. Spat growth was greatest at a combination of 2400 kPa and 14 days. Oysters cleaned at low pressure (600 kPa) had the slowest growth. The optimum combination of pressure and interval for cleaning effort (cleaning time per month per panel of spat) was at 2400 kPa every 28 days.
It is recommended that spat are maintained at temperatures between 26 and 29°C and fed using a continuous feeding method which maintains the algal density at between 20 and 30 cells µL-1 . Two weeks prior to transfer to farm based nurseries, the algal concentration should be increased to 50 cells µL -1 to maximise spat organic content. Nursery PIM levels should be less than 40 mg L-1 in order to minimise the amount of algal food that is discarded as pseudofaeces.
The results of this work has been taken up by the Western Australian and Northern Territory pearling industries, and has led to significant improvements in the spat production of P. maxima.
Based on the results of this study, and utilising results from other authors, in particular J. J. Taylor and associates, a protocol for the culture of P. maxima spat from settlement to seeding is presented.
It is recommended that further work be conducted on the effects of food quality and quantity, and particulate inorganic matter on the growth, survival and feeding physiology of P. maxima spat and adults under field conditions.
| Date of Award | 2004 |
|---|---|
| Original language | English |
| Supervisor | Jim Luong-Van (Supervisor) |