Chen Yin V. Walker’s research while affiliated with Fisheries and Oceans Canada and other places

In order to investigate how shellfish culture in the intertidal zone may serve as a means of conferring resilience prior to subtidal deployment for commercial grow-out, a common garden experiment was carried out with juvenile Pacific oysters. Individuals were cultured under intertidal or subtidal conditions for one year, following which intertidal animals were transplanted to the subtidal site and both treatment groups were cultured together under subtidal conditions for a period of five months. During that time, microbiome and gene expression were tracked in relation to a marine heatwave event. Multivariate analyses of samples indicated a significant effect of time on gene expression, but no significant effect of culture treatment prior to and during the heatwave. Significant differences in the expression of oxidative stress and immune response genes in post-heatwave samples in comparison to pre-heatwave and heatwave samples, and between culture treatment groups post-heatwave, were detected. Results also indicated significant differences in microbiome according to culture treatment and timepoint, with increased richness observed in intertidally-cultured oysters and over time. Similarly, while the overall composition of the oyster microbiome did not vary between culture treatments, changes were observed over time. At the end of the field trial, a laboratory-based temperature and Vibrio challenge experiment was conducted to compare genomic responses of both culture treatment groups following 24-h exposure to coinciding acute warming and pathogen stressor conditions. A significant interactive effect of temperature and culture treatment on gene expression was observed, further suggesting that stress response may be influenced by prior culture treatment.

In recent years, Pacific oyster growers in British Columbia (BC), Canada have experienced devastating losses due to summer mortality syndrome. While anecdotal evidence suggests that intertidally-grown oysters may fare better during mass mortality events than deep-water counterparts, there remains a lack of research examining how different culture conditions may influence severity. To address this, we compared growth, condition, histopathology, reproductive status, and survival between intertidally- and deep-water-cultured oysters over 2 years at three oyster farms in Baynes Sound (BC). A reciprocal transplant was carried out after 1 year to test the use of the intertidal as a mechanism for promotion of physiological resilience prior to deep-water deployment. Field trial results showed significantly higher final survival in oysters transferred from the intertidal to deep water (83.5%) compared to those maintained in deep water (63.6%), but only at one farm, likely as a consequence of varying physical and/or biological characteristics associated with particular farm locations. Histopathology showed little role of disease with regards to varying survival among treatments, though higher occurrence of Viral Gametocytic Hypertrophy was observed in Year 1 oysters under deep-water (62.2%) versus intertidal (37.8%) conditions. Additionally, after 2 years, there was no significant difference in oyster size nor condition index between oysters transplanted from the intertidal to deep water and those solely cultured in deep water. A laboratory-challenge experiment determined significantly different survival curves of Year 1 intertidally- and deep-water-cultured oysters under immersion/emersion and warming conditions, with final survival of 88% and 64%, respectively, under conditions of high temperature (25°C) and immersion. Likewise, Year 2 (i.e. post-transfer) intertidally- and deep-water-cultured oysters showed significantly different survival curves under laboratory-based Vibrio challenge conditions (16°C) with final survival of 63% and 34%, respectively. Results suggest that partial culture in the intertidal at some farms may be an effective method for conferring resilience to summer mortality in Pacific oysters.

Contamination of Pacific oysters, Crassostrea gigas, by human norovirus (HuNoV) is a major constraint to sustainable shellfish farming in coastal waters of the Northeast Pacific. HuNoV is not a marine virus and must originate from a human source. A barrier to effective management is a paucity of data regarding HuNoV dispersal in the marine environment. The main objective of this study was to identify the spatial distribution and persistence of HuNoV in an active shellfish farming region in the Northeast Pacific. Market-size C. gigas were sequentially deployed for two-week intervals at 12 sites during the 2020 winter risk period from January to April. Detection of HuNoV quantification was performed by reverse transcription real-time PCR (RTqPCR) according to method ISO 15216-1:2017, with modifications. RTqPCR did not detect GI HuNoV. The estimated prevalence of GII HuNoV in oyster digestive tissue was 0.8 ± 0.2%. Spatiotemporal analysis revealed that contamination of oysters with GII HuNoV changed through time and space during the surveillance period. A single cluster of oysters contaminated with GII.2 HuNoV was detected in a small craft harbor on 23 April. There was no significant increase in the proportion of positive pools in the next nearest sampling station, indicating that HuNoV is likely to disperse less than 7 km from this non-point source of contamination. Results from this study indicate that HuNoV contamination of coastal waters from non-point sources, such as small craft harbors and urban settings, can pose a significant localised risk to shellfish farming operations in the region.