Influences of temperature, salinity and available food upon suspended culture of the Pacific oyster, Crassostrea gigas: II. Condition index and survival

Abstract

Condition indices, dry meat weight:dry shell weight ratio and dry meat weight:internal volume ratio, and survival rate were determined for a common broodstock of oysters grown in 10 locations in British Columbia, Canada, over a 14-month period. Year 0 oysters (20–30 mm initial shell height) had lower overall survival rates (31.5–57.3%) than Year 1 oysters (40–50 mm shell height) (63.2–89.7%) although for a given age-class, differences in survival between most sites were not great. Most oyster mortalities occurred in the initial 2–3 months according to seasonal and annual instantaneous mortality rates. Monthly dry meat weight:internal volume condition index values did not appear to be related to site-related trends in growth rate. Dry meat weight:dry shell weight ratio was a more suitable index for assessing the condition of oysters provided available food and water temperature were the principal factors affecting growth. Slow growing oysters from areas with low salinity (< 20 ppt) had dry meat weight:shell weight ratios equivalent to those of rapidly growing oysters from areas with conditions of high salinity and abundant food supply. Intra-site correlations between the two condition indices were poor. Multiple regression analysis revealed that dry meat weight:internal volume ratio was primarily correlated to whole weight and secondarily to water temperature, salinity and chlorophyll a concentration ( r 2 = 0.39, P ≤ 0.001). Dry meat weight:dry shell weight ratio was correlated to the same environmental factors but not whole weight ( r 2 = 0.41, P ≤ 0.001). Ecophysiological studies of oysters should not rely extensively upon results of condition index analysis, particularly involving volume measurements, unless there are collaborating growth and environmental data.