Warm seawater and infection with Vibrio aestuarianus, contributing factors for summer mortality of oysters

The Pacific oyster, Crassostrea gigas, is cultivated in many regions of the world and represents an important resource for local and regional economies. During cultivation, adult C. gigas are prone to mass mortality events during the summer months. A complex combination of environmental and biological parameters has been suggested as the cause of this disease. From late July 2018 to August 2018, up to 87 percent mortality was reported in adult Pacific oysters cultivated in Baynes Sound, British Columbia, Canada. Farmers in the area first observed mortalities of adult oysters on July 24, which coincided with a spike in sea-surface temperature in the Sound above the 90th percentile for 3 days (SST = 20.9C, 2.7C above climatology). Histological examination of C. gigas collected from the southern end of the Sound revealed 21.4 % of gaping oysters displayed early-stage tissue necrosis combined with a systemic infection comprising bacteria with uniform short rod-shape morphology. Microbiological characterization, combining 16S rRNA amplicon sequencing, bacterial culture and qPCR revealed bacterial compositions of these gaping oysters were dominated by bacteria belonging to Vibrio aestuarianus subspecies aestuarianus. The role of this bacterial species and elevated seawater temperature in causing mortality of adult C. gigas was confirmed in controlled laboratory conditions. Intramuscular injection of V. aestuarianus subsp. aestuarianus led to significantly higher mortality of C. gigas at 21C compared to 17C.Together, these observations suggest V. aestuarianus and seawater temperature anomalies likely contributed to the mass mortalities of adult C. gigas in Baynes Sound. Findings highlight the threat of ocean warming, especially acute warming events, to sustainable oyster aquaculture production in western Canada.