In recent years, mortalities of unknown aetiology have occurred in Pacific oyster aquaculture in Baynes Sound, BC during the summer. Field studies were conducted to examine environmental, reproductive and microbial factors that could be contributing to these mortalities. In 2017, oysters were observed at three sites from July 5 to September 15. Each intertidal site had three modules containing seven stacked trays with 80 oysters per tray. Final mortalities ranged from 9.3 ± 1.9 to 38.8 ± 4.9% per module. The mortality per module correlated significantly with gonad length and the proportion of oysters that were female in a multiple linear regression model (R2=0.824, p=0.002). Vibrio aestuarianus, a well-documented pathogen of farmed Pacific oysters in France, was well represented in bacterial cultures from intertidal oysters in 2017 based on recA gene sequencing of 158 bacterial isolates. In 2018, juvenile Pacific oysters were monitored to characterize the onset of a summer mortality event in suspended culture. From May 11 to September 17, data on shell size, reproductive development, environmental conditions, and the microbial community of gill tissue was tracked at culture densities of 150, 300, 450, and 600 oysters tray-1. The onset of mortality was associated with a period of rapid growth, reproductive development, and elevated temperatures. Cumulative mortality per tray ranged from 34 to 75%, with the highest density trays having significantly lower mortality (p=0.023), smaller shell width (p=0.001), smaller shell length (p=0.002) and smaller gonad length (p=0.049) than the lowest density trays in a linear mixed-effects regression. Histology of oysters from August 12, during the mortality event, showed a mixed microbial infection in peripheral gill tissue. High-throughput sequencing of the 16S rRNA gene and qPCR of V. aestuarianus using species-specific recA primers suggest V. aestuarianus is temporally associated with summer mortality. Mortalities observed in 2017 and 2018 occurred in different age classes and with different oyster culture techniques, but all were associated with elevated water temperature, increased reproductive effort, and the presence of V. aestuarianus. / Graduate / 2021-08-06

In the northeast Pacific, salmon are an integral part of ecology, economics and culture. Nearshore areas, where juvenile salmon reside upon leaving their natal streams, are important habitat during a critical time where growth can determine overall survivorship. With the rise in human development in coastal areas, these valuable habitats are becoming increasingly modified, however, the ecological ramifications are not fully understood. This study focuses on two types of anthropogenic influence including shellfish aquaculture, which modifies intertidal areas by adding structures such as intertidal fencing and anti-predator nets, and plastic marine pollution in the form of microplastics. We beach seined at sites within an area extensively modified for shellfish aquaculture (Baynes Sound) to examine juvenile salmon abundance, condition, feeding intensity and prey at aquaculture and non-aquaculture areas. In addition, we also beach seined, and along the east coast of Vancouver Island to determine the incidence of microplastics in juvenile Chinook salmon and their nearshore environments. No significant differences were found between areas in the abundance, diets, condition or feeding intensity of juvenile Coho and Chinook. Chum had different prey and a higher condition and feeding intensity at aquaculture sites, suggesting that species such as Chum feeding on more benthic prey items have a higher probability of being impacted by shellfish aquaculture modifications and in this case we observed positive effects. Microplastic analysis showed juvenile Chinook salmon contained 1.15 1.41 (SD) microplastics per individual while water and sediment samples had 659.88 520.87 microplastics m-3 and 60.2 63.4 microplastics kg-1 dry weight, respectively. We found no differences in microplastic concentrations in juvenile Chinook and water samples among sites but observed significantly higher concentrations in sediment at our Deep Bay site compared to Nanaimo and Cowichan Bay. These differences may be due to site bathymetry and oceanographic differences facilitating settlement at the Deep Bay site and/or may be a result of differential plastic sources in the area including shellfish farming and a marina. Shellfish aquaculture had negligible or positive effects on juvenile salmon abundance, diet, condition and feeding intensity and Chinook microplastic concentrations were relatively low compared to literature values. Although fitness consequences and ecosystem-wide implications must be addressed in the future, it appears shellfish aquaculture and microplastics are not immediate threats to juvenile Pacific salmon along the east coast of Vancouver Island at this time. However, continued monitoring programs and larger-scale studies should be implemented as shoreline modification and plastic use continues to increase. / Graduate / 2017-08-14

Marine aquaculture is already vital to global food security and will continue to become more important in the coming years. Crassostrea gigas (Pacific oysters) is the primary oyster species cultivated worldwide. The FAO and IPCC predict that climate change will create uncertainty and challenges for marine aquaculture. Baynes Sound, British Columbia, is a productive region for aquaculture, producing >50% of British Columbia’s total annual bivalve production by live weight and value. Major summer mortality events have been documented in farmed Crassostrea gigas globally since the 1950’s. These events are believed to be caused by a multiplicity of factors including changes induced by anthropogenic climate change. One of the major contributors to summer mortality is the proliferation of Vibrio bacteria, specifically Vibrio aestuarianus, which has been shown to increase in abundance and virulence when seawater temperatures rise. Despite this connection and the economic importance of oyster farming in the region, little is known about the presence of V. aestuarianus in Baynes Sound. Our 17-month study sampled 7 sites in Baynes Sound on 33 occasions from May 2019 to September 2020. We found a positive correlation between seawater temperature and total Vibrio detected in water samples in Baynes Sound, an association that was stronger when the overall temperature regime was warmer. We found no significant correlation between any of the bacterial assays tested and salinity, pH, or Ωarag saturation. We also did not identify a geographic pattern to bacterial abundance or virulence amongst test C. gigas in the field. Understanding that flagellates are the predominant type of microalgae present in Baynes Sound when summer mortality events occur, in lab trials, we found that incorporating V. aestuarianus into marine aggregates with flagellate microalgae caused higher mortality than aggregates with diatoms or planktonic V. aestuarianus. These results were not statistically significant but led us to look at how exposure to husbandry stress pre and post inoculation with V. aestuarianus incorporated into marine aggregates affects mortality. We found that stress was a significant driver of mortality, particularly when administered 24h post inoculation, suggesting that farmers should avoid sorting or tumbling their oysters in the summer, and particularly immediately after a marine heatwave. Oysters lack adaptive immune systems and are grown in an open ocean environment where it is not possible to eliminate their exposure to pathogens. These factors make it impossible to use vaccines or antibacterial disinfectants to combat diseases. Therefore, breeding genetic resistance to V. aestuarianus may be the most effective way to fight summer mortality. Creating a repeatable and accurate protocol for inoculating oysters with marine bacteria is key to accurate heritability measurements and the estimation of breeding values of different families. Key factors include controlling for dose per animal, laboratory efficiency, and inoculation via a mechanism that mimics real-world infection and does not bypass the animal’s immune defenses. We designed a protocol which controls for these factors, separating each animal into individual containers and adding a controlled dose of planktonic bacteria to each. Previously used methods of injection or using an infected “donor” animal in a group tank do not control for dose or bypass parts of the oysters’ natural immune system, potentially creating inaccuracies in survival data generated with these methods. After designing this protocol, we tested 32 full-sib families and estimated the heritability of survival to V. aestuarianus on the observed and underlying liability scales to be 0.095 (SE = 0.043), and 0.15 (SE = 0.068) respectively. We also found a strong negative correlation between oyster size and survival, with a gram of additional weight creating a 73% increase in the risk of death. Fast growth and large size are two traits which have been explicitly bred into C. gigas stock. Our work suggests that breeding a slower-growing, smaller animal may reduce summer mortality. / Graduate / 2022-09-15