Can manipulation of culture depth reduce summer mortalities in Pacific oysters?



Oyster growers in British Columbia normally lose approximately 10% of their stock in raft-based operations during the summer. Temperature and harmful algae have been determined to be the environmental factors most closely correlated with these mortalities. Transient periods of very high temperature and the occurrence of harmful algal blooms can cause a massive increase in the mortalities, decimating the oyster stock by 50 to 100%. The seawater temperature in the Strait of Georgia reaches its maximum between the surface and 4-m depth during the summer, diminishing at a rate of about 1ºC (yearly average) every 3 m. Dense harmful algae blooms of various species are common throughout the summer in the Strait. These blooms normally present their highest abundance in the top 10 m of the water column. The management option that we propose to mitigate the impact of these two environmental factors involves lowering oyster culture trays to a depth where the temperature is lower and harmful algae are less concentrated. This should be done as soon as the temperature and seawater transparency reach certain critical values. The caveat of this method is that normal phytoplankton is also concentrated close to the water surface, and lowering the oysters would take them out of reach of their food source, thus reducing their growth. Field trials are required to assess the effect of such a procedure. In order to find the optimum balance between oyster mortality and growth, it is necessary to find the depth at which to lower the oysters, as well as the temperature that should trigger the change of depth. This proposal has been geared to study this problem with scientific methods applied to farm conditions throughout the Strait of Georgia. The results of this research will allow oyster growers to make management decisions to reduce the mortalities in their stock based on simple environmental monitoring.

Program Name

Aquaculture Collaborative Research and Development Program (ACRDP)


2008 - 2009


Pacific: Strait of Georgia

Principal Investigator(s)

Chris Pearce