Aquaculture Environment Interactions

The effects of environmental variables, culture depth, and phytoplankton abundance and composition on juvenile Pacific oyster Crassostrea gigas growth and mortality were studied be tween June and October of 2008 at 4 sites in the Strait of Georgia, British Columbia, Canada. In addition, the effects of temperature-triggered depth manipulation on growth and mortality of oysters were examined in order to assess potential control measures for mitigating high summer mortalities associated with high temperature, harmful algal blooms (HABs), and other environmental stressors. Control oysters were held at constant depths of 3, 10, and 15 m, while experimental oysters were kept at 3 m depth and lowered to 10 or 15 m when the surface water temperature reached 14, 16, or 18°C. Site and Depth significantly affected the growth and mortality of control oysters. At the site with the best growth, cumulative mortality was low (range: 6.4 to 19%) and negatively correlated with temperature and positively with transparency. At the high-mortality site (range: 64 to 98%), mortality was positively correlated with temperature, chlorophyll concentration, and the biomass of diatoms and potentially harmful algae. Cumulative mortality was generally higher at 3 m than at 15 m depth. Significantly larger oyster volume was obtained with the oyster controls at 3 m than with those held at 10 or 15 m at most sites. Temperature-trigger treatments did not significantly affect oyster volume or cumulative mortality, and oysters moved to 10 and 15 m had final volumes similar to the 10 and 15 m controls, independent of trigger temperature. Oyster growers could select their sites for maximal growth and minimal mortality based on temperature profile, freshwater input, and phytoplankton abundance and composition.