Role of the Vibrio community, reproductive effort, and environmental parameters in intertidal Pacific oyster summer mortality in British Columbia, Canada

Abstract

In recent years, mortality of cultured Pacific oysters (Crassostrea gigas) during July and August has regularly exceeded 50% in many oyster farms in Baynes Sound, British Columbia (BC), Canada. However, little is known about the bacterial diversity and environmental/biological conditions that may be contributing to such mortality events in the region. A study of Pacific oysters in intertidal culture trays was conducted during the summer of 2017 in Baynes Sound to monitor the occurrence of oyster mortality, while identifying potentially pathogenic bacteria present in the oysters, examining oyster reproductive development, and recording various environmental parameters (temperature, salinity, dissolved oxygen, chlorophyll a, carbon chemistry, phytoplankton species/abundance, zooplankton species/abundance) at three farm sites in Buckley Bay, Fanny Bay, and Ship's Point. Continuous mortality was observed throughout July and August, cumulative mortalities ranging from 16.5 to 35.1% at the three sites. Mortality over time was examined with a binomial generalized linear mixed-effects regression model, which demonstrated a correlation with date, tides, temperature, gonad somatic index, and gonad developmental phase (Rc2 = 0.598). Average cumulative mortality per module was highly correlated with average oyster gonad length and the proportion of female oysters in a multiple linear-regression model (p = 0.002, R2 = 0.824). A total of 158 bacterial isolates were cultured and identified based on sequencing of the recA gene. Among those isolates, Vibrio aestuarianus and V. harveyi, with 27 and three isolates, respectively, are well-documented pathogens of Pacific oysters in other regions. This study is the first to concurrently evaluate a broad array of factors potentially associated with Pacific oyster summer mortality in BC. We provide evidence that summer mortality of Pacific oysters in Baynes Sound is occurring with an etiology similar to summer mortalities of adult Pacific oysters reported in other countries.