Evaluating survival after heat challenges and investigating their correlations with field summer survival for the juvenile Pacific oyster (Crassostrea gigas)

Mass mortalities of juvenile Pacific oysters (Crassostrea gigas) in aquaculture operations have been associated with sustained high temperatures during the summer months. Selective breeding programs to improve field survival in the C. gigas are expensive, labor-intensive, and often rely on prolonged field trials. Therefore, controlled challenge methods for improving selection response towards increased summer mortality resistance are required. The purpose of this study was to verify whether juvenile survival after heat challenges predicts field summer survival for full-sib families of C. gigas. We thermally challenged juveniles from 59 families using two separate methods (chronic heat stress and acute heat shock) and monitored their survival rates. Oysters were also deployed in two major oyster farms and then survival rates were compared between the two conditions. In this study, families from different genetic backgrounds differed in heat tolerance and field survival. Correlations between heat challenges and field trials were low to moderate and positive (0.321-0.346 for acute heat shock and 0.472-0.491 for chronic heat stress). Our results suggest that although current heat challenge trials cannot completely replace field trails in selective breeding programs, selection for heat tolerance is expected to indirectly improve field survival. An important outcome of this study is that heat challenges can discriminate some families that are resistant or susceptible to summer mortality at an early age, and that selection efficiency can be improved by identifying and eliminating poor-performing families prior to planting. In a separate experiment, we found that juvenile oysters had significantly higher summer survival when they were exposed to heat challenges than oysters that were not exposed earlier. We conclude that heat challenge of oysters is a useful method for assessing the survival capacity of hatchery-produced spat used by the oyster industry.