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Investigating the role of Vibrio aestuarianus in summer mortality of farmed Crassostrea gigas in Baynes Sound, British Columbia

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dc.contributor.author Khtikian, Natalie
dc.date.accessioned 2021-11-24T23:04:38Z
dc.date.copyright 2021 en_US
dc.date.issued 2021-11-24
dc.identifier.uri http://hdl.handle.net/1828/13534
dc.description.abstract 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. en_US
dc.language English eng
dc.language.iso en en_US
dc.rights Available to the World Wide Web en_US
dc.subject oyster en_US
dc.subject aquaculture en_US
dc.subject fish farm en_US
dc.subject vibrio en_US
dc.subject heritability en_US
dc.subject Crassostrea gigas en_US
dc.subject Crassostrea en_US
dc.subject gigas en_US
dc.subject Baynes Sound en_US
dc.subject marine heatwave en_US
dc.subject marine en_US
dc.subject shellfish en_US
dc.subject bivalve en_US
dc.title Investigating the role of Vibrio aestuarianus in summer mortality of farmed Crassostrea gigas in Baynes Sound, British Columbia en_US
dc.type Thesis en_US
dc.contributor.supervisor Flaherty, Mark
dc.degree.department Department of Geography en_US
dc.degree.level Master of Science M.Sc. en_US
dc.description.scholarlevel Graduate en_US
dc.description.embargo 2022-09-15


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