Spatio-temporal factors affecting the growth of cultured silver-lip pearl oyster, Pinctada maxima (Jameson) (Mollusca: Pteriidae) in West Papua, Indonesia
Lee, Anne Michelle (2010) Spatio-temporal factors affecting the growth of cultured silver-lip pearl oyster, Pinctada maxima (Jameson) (Mollusca: Pteriidae) in West Papua, Indonesia. PhD thesis, James Cook University.
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Abstract
This thesis addressed various growth aspects of cultivating P. maxima in a commercial farm in Indonesia with special emphasis on the influence of the environment. P. maxima of three age-classes were grown at three sites (Ganan, Manselo and Batu Terio) and two depths over a period of 18 months and various aspects of somatic growth (linear and weight measurements), gonad growth (visual and histological observations) and factors which influence growth (environmental and biological) were monitored.
Environmental monitoring of the three sites and two depths showed that seawater parameters varied spatially between sites and depths as well as temporally throughout the sampling period. Some of the parameters measured were physico-chemical properties (water temperature, salinity, pH) and particulate matter (suspended particulate matter, particulate organic matter and chlorophyll a, b and c). These environmental descriptors provide the basis for comparison of growth rates of P. maxima held at the three sites in subsequent chapters.
Total growth (GT) and monthly instantaneous growth (G30) of both length and weight were computed as part of the study on growth of P. maxima. G30 is a better indicator of growth as it is a standardised measure and permits comparison to be made between various age-classes of oysters with different shell sizes as well as at a specific point in time. Growth studies showed age has an inverse relationship on P. maxima growth, with both GT and G30 decreasing with increasing age. Multivariate testing showed that growth of all age classes was affected by culture depth, but not culture site. Growth rate at a depth of 5 m was higher than at 15 m. When oysters were partitioned by size before analysis, there was a site effect on growth rate for medium and small oysters. Spatial differences in P. maxima growth was shown to be linked to the local culture environment, where variation in somatic grow was influenced by varying pH, salinity and pH levels, and gonad growth was influenced by water temperature, pH, SPM, POM.
Growth of P. maxima was fitted into five mathematical models i.e Special Von Bertalanffy Growth Function (VBGF), General VBGF, Gompertz, Richards and Logistic and various growth parameters computed. The criteria used for best fit was low mean residual sum of squares (MRSS), high coefficient of determination (r2) and low deviation of the asymptotic length (L!) from the maximum length (Lmax). Based on these criteria, the Special VBGF and the General VBGF equally provided the best fit to length-at-age data for all the pearl oysters grown at the area. However, when data was plotted, General VBGF tended to underestimate L!. The Special VBGF best described the growth of P. maxima cultured at the farm, with growth parameters estimates of L! = 168.38 mm, K = 0.930 y-1, t0 = 0.126.
Biofouling studies showed that six classes of macro-fouling which settled on the shells of P. maxima were invertebrates from the classes Maxillopoda, Polychaeta, Bivalvia, Demospongiae, Foraminifera and Ascidicea. The quantity and diversity of biofouling was found to affect growth in medium and small oysters. The spatial and temporal variation in quantity and diversity of the six classes of biofouling was in turn affected by various environmental parameters. Regression analysis provided information on environmental parameters acting in concert to affect biofouling while principal component analysis showed the interaction between different biofouling taxa and environmental parameter. Together, they allowed examination of the interaction between various parameters, apportionment of environmental factors towards taxa of fouling and the degree a particular environmental variable affects fouling. Chlorophyll levels, pH and salinity were found to have a greater affect on biofouling settlement than SPM, POM and seawater temperature.
Macroscopic investigation of gonads and comparison to histological data in this study support previous reports that gonad colour and appearance may be used to determine sex and stage of development in P. maxima. A fundamental difference in the colour and the area occupied by the developing gametes made it possible to distinguish between the gender and various stages of development of P. maxima oysters with relative ease. While most of the oysters observed appeared to be of indeterminate sex, enough male and female oysters were observed to show that gametogenesis in cultured P. maxima occurred between August to February, with spawning occurring twice during that period; once in October/November and again in February. Sex ratio in cultured P. maxima was overwhelmingly biased towards maleness, with no spatial difference in sex ratio between oysters cultured at various sites and depths. The expression of maleness was weakly correlated to water temperature, pH and rainfall, while there was no correlation between femaleness and environmental descriptors. Size, and not age, was more important in determining the sex of P. maxima.
In summary, this research presented new data on growth of different age classes of P. maxima cultured in a farm situation in Indonesia. It has added to our knowledge the importance of various environmental factors and biofouling on somatic and gonadal growth of P. maxima. This information can be utilised to improve farming management practices through judicious selection of future culture sites. It is hoped that this will form a basis for further study into grow-out of P. maxima in the pearling industry in Indonesia and South-East Asia and lead to further improvement and expansion in the industry for the future.
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