Monthly variation of condition index, energy reserves and antibacterial activity in Pacific oysters, Crassostrea gigas, in Stansbury (South Australia)

This paper investigates the temporal responses of 2-year old Crassostrea gigas to environmental changes in Stansbury, South Australia from September 2005 to October 2006. A total of 360 oysters were grown in one-line baskets on the farm using six replicates that were sampled monthly. A range of environmental parameters were assessed and correlated against biological indicators for oyster condition, metabolism and antimicrobial activity. Food availability by chlorophyll a, was low throughout the study period (0.5-1.5\ua0μg L) and was significantly correlated to phosphorus concentrations. The condition index and shell weight of oysters significantly increased over the year, with the condition index dropping after spawning but then recovering within one month. Significant temporal variation in energy storage and utilization were observed in different tissues over the year. Glycogen in the mantle tissue was influenced by reproduction and correlated to chlorophyll a levels, but not in the gill or adductor muscle. The mantle glycogen and gill protein reached the lowest level in February when spawning occurred and presented evidence for seasonal variation in oyster metabolic activity. However, mantle and adductor muscle proteins did not drop after spawning indicating that these proteins contribute little to gametogenesis. Hemolymph protein was negatively correlated to water temperature and chlorophyll a, reaching the lowest level during summer. Hemolymph antibacterial activity significantly decreased after spawning, implying that the period of post-spawning is critical for oyster health. This study revealed trade-offs in the energy budget between immune resistance, growth, and reproduction. The results indicate that in a lean water environment, spawning events significantly regulate metabolic and immune capacities of oysters and a second year of rearing increased meat and shell weight but not the shell length. These findings are applicable to the management and development of oyster aquaculture within temperate southern hemisphere