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The role of phytoplankton and environmental variables in Pacific oyster (Crassostrea gigas) aquaculture in British Columbia Cassis, David


Pacific oyster aquaculture in British Columbia faces serious challenges, such as high cadmium (Cd) concentrations, low growth and high mortalities during summer, and inability to directly gauge stress levels. The goal of this dissertation was to address these challenges by investigating the role of biological, physical and chemical oceanographic parameters in controlling them in various oyster farms in the Strait of Georgia. Three studies were undertaken. The first, from August 2004 to July 2005, investigated the role of phytoplankton in controlling Cd levels in the oysters in a Deep Bay farm. Phytoplankton mediated the transfer of dissolved Cd to the particulate phase, accounting for 90% of the summer reduction in dissolved Cd. This suggests that phytoplankton act as a sink for dissolved Cd, reducing the main source to the oysters. Two descriptive models for annual oyster Cd concentrations were developed based on environmental variables. The second study, from June to October of 2008, investigated how environmental factors, culture depth and seed size controlled oyster mortality and growth in four farms. Farms with less stratified, colder waters rich in diatoms fared better than those with highly stratified, higher temperature waters and persistent blooms of flagellates. Larger oyster seed presented low mortalities, while smaller seed were more susceptible to adverse conditions due to their ineffective particle processing capabilities. The best yield was obtained at a culture depth of 3 m, despite higher mortalities. A depth manipulation technique was investigated as a means to reduce summer mortalities without success. The third study, during the summer and fall of 2007 in Deep Bay, investigated a novel proteomic technique to detect and quantify heat-shock proteins (HSP) 70 and 90 in oysters to assess their stress levels. Mortalities were relatively low during that year (8.5% accumulated). The abundance of HSP 70 sequences was positively directly with non-harmful diatom biomass and negatively with high temperature and reproductive state. In contrast, the levels of HSP 90 were correlated negatively to the biomass of non-harmful diatoms, and positively to that of potentially-harmful algae, indicating that HSP 70s and 90s may have different triggers.

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