| Summary: | M.SC.BIOLOGY To investigate the growth, mortality and condition of European and Japanese oysters (Ostrea edulis and Crassostrea gigas) in Maltese inshore waters, floating cages were placed at Marsaxlokk bay, Mistra bay and Rinella creek at depths of 3m and 8m. 5 and 15mm C. gigas spat and 10mm O. edulis spat imported from Seed Oysters (U.K) Ltd., were used. Samples from each location and depth were monitored at two-month intervals for shell growth, condition and survival. The more relevant properties of the water in which these oysters were placed were monitored concurrently. These included temperature, salinity, oxygen content, chlorophyll a levels, as well as a qualitative and quantitative analysis of the natural phytoplankton populations. Batches of oysters laid on trestles at Marsaxlokk and Mistra were swept away by rough weather. This part of the experiment was discontinued. So also was an attempt to culture oysters in bags on the bottom. These latter were lost probably through pilferage. In an attempt to define the nutrients critically limiting the growth of phytoplankton, the nitrate and phosphate levels were also regularly measured. Moreover, the effects of added amounts of these nutrients on the natural phytoplankton community of Marsaxlokk bay were also tested in laboratory experiments. O. edulis suffered catastrophic summer mortalities at all locations when the ambient temperature 0 of the water reached about 25 C. This, besides making impossible the drawing of any conclusive parallels between the performance of the two species of oysters, virtually rules out any possibility of starting an oyster industry based on this species. This study shows that of the sites and depths tested only Marsaxlokk 3m appears to hold promising prospects for the culture of C. gigas. At this site, this species showed reasonable growth, relatively low mortality and normal shell growth. Although at Rinella growth of C. gigas was equally promising, these oysters exhibited abnormal and thickened shell growth which resulted in the oysters being of a much inferior condition. Growth at Mistra (both depths) and Marsaxlokk 8m was stunted. An interesting finding was that while C. gigas at Rinella showed abnormal and thickened shell growth O. edulis grew normal shells. Apart from this difference, there was a striking similarity in patterns of overall growth and shell growth shown by the two species of oysters (and by the two spat sizes in the case of C. gigas at each particular depth and location. O. edulis overcame the adaptive stresses associated with the initial relaying period much more successfully than C. gigas. In fact, while the former species experienced a initial mortality of about 10%, the latter species exhibited an initial mortality of about 30% in the case of the 15mm spat and 80% in the case of the 5mm spat. The almost total mortality experienced by O. edulis in summer seems to have been a consequence of thermal starvation. Later mortalities of C. gigas were, to a large degree, correlated with stunted growth. No clear correlation could be established between the performance of the oysters at the different sites and depths and the environmental parameters monitored. Several factors that may have been responsible for the observed differences in oyster performance are identified. The thickened shell growth exhibited by C. gigas at Rinella is tentatively attributed to some physical or chemical factor, present only or at least to a much larger degree in the waters of this creek, finding its way into the extrapallial cavity and inducing the mantle to secrete more shell. A number of possible factors are identified. The observation that the Rinella oysters had acquired a deep green colouration throughout their entire body prompted the monitoring of the copper levels in the oysters of Marsaxlokk and Rinella. The oysters at the latter site were found to carry a much higher level of this metal (about 300mg/kg wet wt.) than those at Marsaxlokk (about lOOmg/kg wet wt.). The results of the nutrient analysis and of the artificial enrichment experiments support those of other workers who contend that while both nitrates and phosphates are scarce in Mediterranean waters, phosphate is normally the primary limiting factor. This primary limiting role of phosphate contrasts with most seas of the world where nitrate is very often the primary limiting factor. Moreover, the present experiments failed to reveal any notable seasonal changes in the respective roles of phosphates and nitrates as factors limiting phytoplankton growth. The higher incubating temperatures of the June cultures resulted both in a quicker attainment of the peak values of chlorophyll a and in a higher peak concentration of this pigment being reached as compared with the November and February experiments. With few exceptions, there were no major changes in the percentage composition contributed by the dominant species within the experimental period, during all three enrichment experiments. Diatoms, mainly Skeletonema costatum and Chaetoceros spp., dominated all culture vessels. These results are discussed in the light of various theories of Margalef (1965, 1967, 1968) about the ratio of pigment absorption of 430nm to that at 665nm. In the final section, the main limitations of this study are outlined and several suggestions for future experimental work as follow-up to this study are made. N/A
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