Evaluation of live phytoplankton and live algae substitute diets for feeding juvenile oysters (crassostrea gigas) in nursery culture

The aim of this study was to mitigate summer mortality of juvenile Pacific oysters (Crassostrea gigas) by developing a low-cost, low-intensity system of supplemental feeding of juvenile oysters that could be used during times of low natural phytoplankton production. Three experiments tested the effectiveness of supplemental feeding of juvenile oysters in nursery culture using live algae and live algae substitute diets. Experiment 1 tested five dietary formulations consisting of two Schizochytrium-based live algae substitute diets (Algamac³⁰⁵⁰™ and Rotimac™) and Isochrysis galbana (clone T-Iso) in 1:1 (dw:dw) and 100% formulations. Oysters were fed for 60 days and the Algamac³⁰⁵⁰™/T-Iso diet produced the highest percentage increase in dry weight over 60 days (+106%) followed by the reference T-Iso diet (+65%). Experiment 2 tested the effectiveness of using protease absence/presence as an indication of dietary suitability. The oysters that were fed the diets that produced poor growth (Algamac³⁰⁵⁰™, Rotimac™ and the starved control) also failed to produce proteases. Experiment 3 tested three dietary formulations consisting of two mixed Schizochytrium-based live algae substitute (Algamac³⁰⁵⁰™ and Rotimac™) and live algae diets (using phytoplankton grown in outdoor bloom tanks dominated by Thalassiosira nordenskioeldii', 1:1 (dw:dw)) and a reference 100% T. nordenskioeldii diet. Both mixed diets produced a significant increase in mean oyster dry weight and overall energy storage levels compared to the T. nordenskioeldii diet following 30 days of feeding. The results of the three experiments suggest that juvenile oyster energy storage levels and overall health could be improved through a supplemental feeding regime