Oyster Mortality Research Articles

Climate resilience and profitability thresholds in Chesapeake Bay oyster aquaculture

Shellfish aquaculture producers in coastal systems are facing uncertain future growing conditions as climate change alters weather patterns and raises sea level. We examined expected mid-century (2059–2068) changes in aquaculture profitability from recent conditions by integrating models of climate change, estuarine hydrodynamics and biogeochemistry, oyster growth, oyster mortality, and economics, using the Chesapeake Bay, USA as a case study. We developed an economic stochastic dynamic programming (SDP) approach that generates optimal grower behavior to maximize profits under uncertainty by dynamically choosing planting density, replanting and mitigation use, in response to changing oyster stock status and water quality conditions. Separate models were developed for bottom culture largely serving the cannery market, and container culture largely serving the half-shell market, to reflect different production costs, market prices, and oyster growth and survival. The coupled hydrodynamic-biogeochemical and oyster ecology models projected high spatial variability in oyster growth and mortality with the most favorable growing conditions in the lower north and upper mid bay, where mortality is lowest, and the upper south bay, where growth is highest. Climate change by late mid-century generated modest water quality changes and virtually no mortality rate changes. Nonetheless, our modeling revealed that even if growers made optimal management choices under uncertainty, the majority of modeled sites would see a decline in profitability under climate change, primarily due to potential reductions in food availability. Bottom culture was more resilient to the future climate at most sites, being less sensitive to small changes in growth than container culture. Information on how aquaculture conditions currently vary in space was more important for profitability than future climate forecasts. Our stochastic dynamic programming approach tailored grower behavior to each site and unfolding annual conditions, including highly targeted and cost-effective mitigation adjustments to boost oyster survival or growth.

Microbial education plays a crucial role in harnessing the beneficial properties of microbiota for infectious disease protection in Crassostrea gigas.

The increase in marine diseases, particularly in economically important mollusks, is a growing concern. Among them, the Pacific oyster (Crassostrea gigas) production faces challenges from several diseases, such as the Pacific Oyster Mortality Syndrome (POMS) or vibriosis. The microbial education, which consists of exposing the host immune system to beneficial microorganisms during early life stages is a promising approach against diseases. This study explores the concept of microbial education using controlled and pathogen-free bacterial communities and assesses its protective effects against POMS and Vibrio aestuarianus infections, highlighting potential applications in oyster production. We demonstrate that it is possible to educate the oyster immune system by adding microorganisms during the larval stage. Adding culture based bacterial mixes to larvae protects only against the POMS disease while adding whole microbial communities from oyster donors protects against both POMS and vibriosis. The efficiency of immune protection depends both on oyster origin and on the composition of the bacterial mixes used for exposure. No preferential protection was observed when the oysters were stimulated with their sympatric strains. Furthermore, the added bacteria were not maintained into the oyster microbiota, but this bacterial addition induced long term changes in the microbiota composition and oyster immune gene expression. Our study reveals successful immune system education of oysters by introducing beneficial microorganisms during the larval stage. We improved the long-term resistance of oysters against critical diseases (POMS disease and Vibrio aestuarianus infections) highlighting the potential of microbial education in aquaculture.

Scottish oyster mortality event and association with Vibrio aestuarianus

Pacific oysters, Crassostrea (Magallana) gigas, are the most commonly cultured invertebrate species globally. Recent years have seen outbreaks of summer mortality across the globe, mainly caused by viral and bacterial pathogens such as OsHV-1 µvar and Vibrio spp. Despite isolated outbreaks in southern England, the UK has remained largely free from these pathogens. A summer mortality event in Scotland was investigated after Vibrio spp. were implicated. We identified two key Vibrio species (V. aestuarianus and V. splendidus) but complete absence of OsHV-1. There was a high prevalence of V. aestuarianus among moribund and dead oysters, along with an increase in V. aestuarianus detected in the water column and sediment. In accordance with strains associated with oyster mortalities elsewhere in Scotland and mainland Europe, V. aestuarianus from this site belongs to Lineage A. Whilst V. splendidus, could be cultured from most infected tissue samples, V. aestuarianus could not be cultured from from frozen tissue , only from unfrozen infected tissue. In vivo infection experiments with V. aestuarianus resulted in significantly higher mortalities than with V. splendidus and were exacerbated by increased temperature. These findings suggest that mortalities are caused by V. aestuarianus, which may have been intensified by handling stress and rising temperatures.

Environmental Conditions Associated with Four Index Cases of Pacific Oyster Mortality Syndrome (POMS) in Crassostrea gigas in Australia Between 2010 and 2024: Emergence or Introduction of Ostreid herpesvirus-1?

Warm water temperature is a risk factor for recurrent mass mortality in farmed Pacific oysters Crassostrea gigas caused by Ostreid herpesvirus-1, but there is little information on environmental conditions when the disease first appears in a region-the index case. Environmental conditions between four index cases in Australia (2010, 2013, 2016 and 2024) were compared to provide insight into possible origins of the virus. Each index case was preceded by unusually low rainfall and higher rates of temperature change that could increase oyster susceptibility through thermal flux stress. Water temperature alone did not explain the index cases, there being no consistency in sea surface, estuary or air temperatures between them. Tidal cycles and chlorophyll-a levels were unremarkable, harmful algae were present in all index cases and anthropogenic environmental contamination was unlikely. The lack of an interpretable change in the estuarine environment suggests the recent introduction of OsHV-1; however, viral emergence from a local reservoir cannot be excluded. Future events will be difficult to predict. Temperature flux and rainfall are likely important, but they are proxies for a range of undetermined factors and to identify these, it will be necessary to develop comprehensive protocols for data acquisition during future index cases.

Oyster extractivism in the Amazon: characterization and perception of extractivists on the conservation of natural banks

The objectives of this study were to identify extractivists working in the region of Curuçá, Pará, Brazil, to evaluate the perception of these workers on the conservation and exploitation of natural oyster stocks. Applying snowball sampling, semistructured questionnaires were applied in the period from September 2019 to January 2020, resulting in 38 interviewed extractivists. Of the 38 respondents, 50% were older than 40 years, 78.9% were men, and oyster extractivism was the main source of income for 65.8%. In addition, 94.7% lived in the city of Curuçá, and 36.8% worked as extractivists for 7 to 11 years. The majority reported never having received any training on oyster collection or oyster bank conservation. The extractivists reported that their greatest challenge was collection sites. Lack of training was a risk factor for collecting during the rainy season (OR=1.52), while having received training was a protective factor for collecting seeds (OR=0.64). Additionally, 94.7% of the respondents reported an increase in oyster mortality in the rainy season. Encouraging extractivists to organize into associations or cooperatives with the support of public development policies and providing training focused on sustainable extraction can contribute to the conservation of natural oyster banks by preventing damage to the natural ecosystem.

Habitat-mediated direct and indirect interactions in a marine sedimentary system from Atlantic Canada

Trophic cascades focusing on direct (consumptive) effects have been well studied in marine ecosystems. However, less attention has been given to cascades involving indirect interactions embedded in distinct habitats. We focused on the interactions between a non-indigenous predator (the green crab Carcinus maenas), a consumer (the mud crab Panopeus herbstii), and a prey species (juvenile eastern oysters Crassostrea virginica). These interactions were studied in 3 small yet distinct habitats of increasing complexity: bare sediments, patches of blue mussels Mytilus edulis (BM), and a unique habitat consisting of giant Irish moss Chondrus crispus and mussels combined (IMBM). In the field, green crab predation rates on mud crabs were estimated in each of the abovementioned habitats using tethering experiments. The results showed that green crab foraging was most effective in bare sediments and least effective in IMBM, i.e. the least and most complex habitats, respectively. Trials conducted in the laboratory with mud crabs foraging over oysters showed a similar outcome: oyster mortality rates declined with increased habitat complexity. However, when trials were conducted in the presence of a green crab, this pattern reversed, and oyster mortality was lowest in bare sediments. Mud crab behavior was consistent with these results: in the presence of a green crab, mud crabs were less active and spent more time sheltering, whereas in its absence, the opposite pattern was observed, especially in bare sediments. These behaviorally driven indirect interactions are dependent on the presence of a non-indigenous predator and mediated by the type of habitat.

Characterization of water microbiota and their relationship with resident oysters during an oyster mortality event.

Microorganisms are vital for the health of marine invertebrates, and their assembly is driven by both deterministic and stochastic factors that regulate residents (innate to the host) and transients (from ambient water). However, the role of water microbiota and the significance of deterministic and stochastic processes in aquatic hosts facing mortality threats are largely unknown. This study examines the shifts in water microbiota during an oyster mortality event using amplicon sequencing and compared with those of resident oysters to disentangle the balance of the deterministic and stochastic factors involved. Water temperature and dissolved oxygen significantly shape the microbial community with a distinct monthly pattern, and Cyanobacteria blooms might exacerbate oyster mortality. The comparative analysis of microbial communities in oysters and water revealed that ≤ 21% of the genera were shared between oysters and water, implying that water microbiota cannot easily transfer into oysters. Furthermore, these shared genera had different functions, with oysters more involved in promoting host digestion and nutrient acquisition and water bacteria enriched more in functions promoting their own growth and survival. These findings illustrate that oysters may possess specific selection or barrier mechanisms that permit a small percentage of transients, controlled by stochastic factors and having a minimal effect on oyster mortality, to enter, whereas the majority of oyster microbiota are residents governed by deterministic factors. Consequently, oysters exhibit some plasticity in their symbiotic microbiota, enabling them to maintain microbial homeostasis and adapt to complex microbial surroundings. This may be a shared mechanism among marine invertebrates for survival in complex marine environments.IMPORTANCEPacific oysters are widely cultured and play vital ecological roles. However, the summer mortality hinders sustainable oyster farming. Untangling causative mechanisms of oyster mortality is a complex task due to the intricate "interactome" involving environmental factors, hosts, and pathogens. Interactions between hosts and microorganisms offer an ideal avenue for investigating the truth. We systematically investigated the microbial community in water and resident oysters during a summer mortality event and proposed that the assembly of oyster microbiota is primarily governed by deterministic processes independent of mortality. Pathogens mainly originate from resident members of the oyster microbiota, with a limited influence from the microbial community in the water. Additionally, environmental degraders, such as Cyanobacteria blooms, cannot be overlooked as a contributing factor of oyster mortality. This study evaluated the weight of deterministic and stochastic factors in microbial assembly during an oyster mortality event and greatly broadened our understanding of the "interactome" through the interaction between oysters and water in microbiota.

Strong genotype-by-environment interaction across contrasted sites for summer mortality syndrome in the Pacific oyster Crassostrea gigas

Genotype-by-environment (GxE) interaction in aquaculture is usually estimated for continuous traits and based on data from a limited number of 2–3 rearing sites. Here we report the results of a GxE study for resistance to Pacific oyster mortality syndrome (POMS), a multi-factorial disease that severely impacts Pacific oyster production worldwide. The syndrome is largely associated with Ostreid Herpes Virus 1 (OsHV-1). Resistance to OsHV-1 in Crassostrea gigas has been shown to be heritable, meaning that selective breeding is a suitable strategy for reducing mortalities. However, limited information was available about GxE interaction or the possible need to consider it in selective breeding. Survival of two cohorts (C1 and C2), consisting of a total of 104 full-sib families, was evaluated during the summer of 2013 in 7 sites along the French Atlantic, Channel and Mediterranean coasts. Mean survivals in autumn 2013 were 12.6% ± 10.9 and 4.6% ± 6.4 for C1 and C2, respectively. Genetic parameters were computed by MCMC, which is suitable for binary data like survival. Heritability estimates ranged from 0.16 to 0.42 depending on site and cohort, with a mean of 0.24 [0.20; 0.27] when including all data. GxE interactions were estimated by the genetic correlations between pairs of sites. Genetic correlations were high (ρ > 0.80) for C1 between most tidal Atlantic and Channel sites, and intermediate between tidal sites and a Mediterranean lagoon site, while they were lower and more variable for C2 (0.21–0.77). Expected genetic gains were maximal when production site was the same as selection site. They were closed to this expected maximum when the selection site was different from the production site along the Atlantic or Channel coast. Limited GxE interaction along the French Atlantic coast is favorable to wide dissemination of genetically improved oysters along this coast. Limited potential improvement was shown in the Mediterranean site if selection was carried out elsewhere, confirming the specificity of this environment. Consequently, a specific strategy such as dedicated breeding should be used to achieve genetic progress for this site.

Inhibitors of LAMP used to detect Tenacibaculum sp. strain Pbs-1 associated with black-spot shell disease in Akoya pearl oysters, and additives to reduce the effect of the inhibitors

Black-spot shell disease is an unresolved disease that decreases pearl quality and threatens pearl oyster survival. In previous studies, the bacterium Tenacibaculum sp. strain Pbs-1 was isolated from diseased Akoya pearl oysters Pinctada fucata, and a rapid, specific, and sensitive loop-mediated isothermal amplification (LAMP) assay for detecting this pathogen was established. This technology has considerable potential for routine diagnosis of strain Pbs-1 in oyster hatcheries and/or pearl farms; therefore, it is vital to identify substances in environmental samples that might inhibit LAMP and to find additives that can reduce the inhibition. In this study, we investigated the effects of six chemicals or proteins, otherwise known as conventional PCR inhibitors, on LAMP, using the DNA of strain Pbs-1 as template: humic acid, urea, iron (III) chloride hexahydrate, melanin, myoglobin, and Ethylenediamine-N,N,N′,N′-tetraacetic acid, disodium salt, dihydrate (EDTA; pH 6.5). Next, to reduce the effects of identified inhibitors, we tested the addition of bovine serum albumin (BSA) or T4 gene 32 protein (gp32) to the LAMP assay. When 50 ng of DNA template was used, 4 ng/μL of humic acid, 0.05% melanin, and 10 mM of EDTA (pH 6.5) inhibited the LAMP reaction, whereas myoglobin, urea, and FeCl3 had no effect. When 50 pg of DNA template was used, 4 ng/μL of humic acid, 0.05% melanin, 4 μg/μL of myoglobin, 10 μg/μL of urea, and 10 mM of EDTA inhibited the LAMP reaction. Thus, it was shown that the gene-amplification inhibitory effect of melanin, humic acid, and urea could be reduced by adding BSA or gp32 to the LAMP reaction mixture. This technique could be applied as part of a protocol to prevent mass mortalities of pearl oysters; moreover, the results enhance our knowledge about substances that inhibit LAMP and methods to reduce the inhibition, which have rarely been reported.

Investigations of the involvement of Vibrio species with Ostreid herpesvirus-1 in mass mortality events in the Pacific oyster Crassostrea gigas

Pacific oyster mortality syndrome (POMS) in Pacific oysters Crassostrea gigas is defined by mass mortality with Ostreid herpesvirus I (OsHV-1). In this study the association of Vibrio species with oysters before, during and after POMS events was investigated at population level to clarify their involvement with OsHV-1 in mass mortality events in the Georges and Hawkesbury River estuaries, Australia. In the POMS affected Georges River estuary, three patterns were observed concurrently at different sites: i) Vibrio counts and OsHV-1 DNA concentrations increased in adults and spat as mortalities started then decreased as mortalities stopped ii) bacterial counts increased in spat but not in adults associated with mortalities due to OsHV-1 iii) bacterial counts increased in the absence of mass mortality or high concentrations of OsHV-1 DNA. Although almost half of the 120 bacterial isolates were identified as an unknown Vibrio species belonging to the Splendidus clade with two predominant biotypes (A and H), there was no association between biotype and mortality. Concurrently in the Hawkesbury River estuary, neither mortality nor OsHV-1 were found, Vibrio counts fluctuated significantly and of 84 isolates identified, V. alginolyticus, V. parahaemolyticus and unidentified Vibrio sp. dominated; only five Splendidus clade isolates were identified. However, when POMS emerged in the Hawkesbury River estuary, the Splendidus-clade isolate became the dominant bacterial species. Mortality also occurred there without OsHV-1 and the dominant bacteria then were V. alginolyticus and V. harveyi, but the Splendidus-clade isolate was not detected. None of the bacterial species were specifically associated with POMS, although the Splendidus clade isolate increased in prevalence after POMS emerged in the Hawkesbury River estuary. V. aestuarianus was not detected in this study. While the findings suggest that bacterial proliferation follows infection with OsHV-1 in POMS, this was not a consistent feature. The number and diversity of bacteria within oyster tissues varied over time, between estuaries, between sites within an estuary and were associated with flooding, growing height, the age of oysters, mortality and OsHV-1.

Genome-wide association study and genomic prediction of resistance to summer mortality in Pacific oyster (Crassostrea gigas) using whole genome resequencing

The Pacific oyster Crassostrea gigas is one of the most important farmed marine shellfish worldwide. However, massive mortality during the summer months caused significant economic losses to the C. gigas farming industry. Understanding the genetic architecture of resistance traits has been an ongoing research issue, and the incorporation of genomic information into breeding programs is expected to accelerate the process of genetic improvement. Genomic selection (GS), as a new approach of genetic improvement, has great potential for breeding new lines with adaptive advantages. In this study, we conducted a genome-wide association study (GWAS) for summer mortality resistance and estimated the accuracy of genomic predictions using different methods. The heritability of the C. gigas summer mortality resistance was low, with heritability of 0.108 ± 0.036, 0.161 ± 0.102 and 0.134 ± 0.043 for PBLUP, GBLUP and ssGBLUP, respectively. In addition, we detected 9,783,674 SNPs and used a mixed linear model to identify 18 significant SNPs associated with summer mortality resistance. The phenotypic variance explained (PVE) for these SNPs ranged from 8.25% to 10.14%. Based on these significantly related SNPs, nine significant candidate genes were identified (TLR4, HEX, SLC22A8, PDE2A, HUWE1, SLMAP, RAD52, TBK1 and RAPH1). The prediction accuracy using PBLUP, GBLUP, ssGBLUP, and weight ssGBLUP (WssGBLUP) was 0.549 ± 0.026, 0.381 ± 0.074, 0.544 ± 0.026, and 0.869 ± 0.018, respectively. Therefore, WssGBLUP models are more suitable for genomic prediction of summer mortality resistance in C. gigas. Our results suggest a polygenic genetic architecture that provide new perspectives for studying candidate genes for resistance to summer mortality, which may facilitate the genetic improvement for resistance lines in C. gigas.

Effects of low salinity water inflow on eelgrass and cultured bivalves in subarctic lagoons: Analysis using hydrodynamic model

Brackish water areas are affected by river inflows and changes in tidal level, resulting in large fluctuations in salinity. Prolonged exposure to low salinity water can have adverse effects on the growth of marine plants and animals. Characteristics of lagoons such as catchment area, bottom topography, and seawater exchange rate with the open ocean vary widely and the horizontal and vertical gradients of salinity and spatio-temporal variability are also diverse. In this study, we evaluated the impact of low salinity water inflows due to heavy rainfall on seagrass and cultured bivalves in two lagoons (Akkeshi-ko Lagoon and Notoro-ko Lagoon), which are located in the subarctic region of Japan and have contrasting geomorphological characteristics. Continuous observations of the physical environment were conducted by installing measurement equipment near the seafloor, and we ran ten year replicate calculations using a three-dimensional high-resolution hydrodynamic model. Simulation results were used to assess the risk of low salinity associated with heavy rainfall to eelgrass and bivalves (oyster, scallop, and clam). The model analysis showed that the magnitude and frequency of exposure to low salinity water varied greatly between and within lagoons. Akkeshi-ko Lagoon, which has a relatively large catchment area and shallow basin, was sometimes exposed to extremely low salinities, which could lead to the mortality of oysters during heavy rains once every few years. In contrast, salinity decline in Notoro-ko Lagoon is negligible even in years with heavy rainfall due to the high volume of water within the lagoon and small catchment area. With projections that the intensity of heavy rainfall will increase due to climate change, this study was able to evaluate the impact of changes in river flow on coastal ecosystems and aquaculture, and we discussed adaptation measures that may be necessary in the future.

Efficacy and effects of three desiccation intervals on biofouling and Eastern oysters (Crassostrea virginica) at three commercial oyster farms in the Chesapeake Bay, MD

Oyster aquaculture is a growing industry both globally and nationally. Productive coastal waters support high-density containerized culture of oysters, but also sustain many other macrofaunal species which can settle on oysters and cage materials (biofouling), causing problems for oyster farms. Desiccation, or periodic air drying, of farm infrastructure and oysters has been used to control colonization and growth of biofouling organisms on oyster farms worldwide, but the minimum necessary interval of desiccation has yet to be thoroughly assessed. Given the known stresses to oysters associated with periods of emersion, an understanding of the effects of different weekly desiccation intervals is needed. This study investigated the use of desiccation in controlling biofouling and the associated impacts to oyster growth and mortality at three oyster farms within Maryland's portion of the Chesapeake Bay, including one low salinity site in an upper Chesapeake Bay tributary and two sites in the middle portion of the Chesapeake Bay. Three desiccation treatments were applied at each site: control (no desiccation), 8 consecutive hours of desiccation per week, and 24 consecutive hours of desiccation per week. Oysters were deployed between July–December 2018 and the percent valve coverage by individual biofouling species was monitored, along with worm abundance, oyster growth, and oyster mortality. Total biofouling was significantly greater on non-desiccated oysters but minimal differences in biofouling coverage were observed among oysters desiccated for 8 or 24 h weekly. Seven biofouling species were observed among the sites, and all except for a single macroalgal species (Ulva intestinalis) were controlled by the 8- and 24-h desiccation treatments. No clear treatment-derived differences in oyster growth or mortality were observed. Overall, results suggest desiccating weekly for either 8 or 24 consecutive hours may be a suitable biofouling management strategy for gross biofouling reduction on oyster farms.

Cross-talk and mutual shaping between the immune system and the microbiota during an oyster's life.

The Pacific oyster Crassostrea gigas lives in microbe-rich marine coastal systems subjected to rapid environmental changes. It harbours a diversified and fluctuating microbiota that cohabits with immune cells expressing a diversified immune gene repertoire. In the early stages of oyster development, just after fertilization, the microbiota plays a key role in educating the immune system. Exposure to a rich microbial environment at the larval stage leads to an increase in immune competence throughout the life of the oyster, conferring a better protection against pathogenic infections at later juvenile/adult stages. This beneficial effect, which is intergenerational, is associated with epigenetic remodelling. At juvenile stages, the educated immune system participates in the control of the homeostasis. In particular, the microbiota is fine-tuned by oyster antimicrobial peptides acting through specific and synergistic effects. However, this balance is fragile, as illustrated by the Pacific Oyster Mortality Syndrome, a disease causing mass mortalities in oysters worldwide. In this disease, the weakening of oyster immune defences by OsHV-1 µVar virus induces a dysbiosis leading to fatal sepsis. This review illustrates the continuous interaction between the highly diversified oyster immune system and its dynamic microbiota throughout its life, and the importance of this cross-talk for oyster health. This article is part of the theme issue 'Sculpting the microbiome: how host factors determine and respond to microbial colonization'.

Development of a nature-based solution for mitigation of Pacific oyster summer mortality: use of the intertidal zone to improve resilience to environmental stressors

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.

Attributions of cause of oyster mortality on the British Columbia coast: Oyster growers' and scientists’ perspectives

Unexplained oyster mass mortalities threaten to destabilize the oyster industry on Canada's west coast if left unexplored. A range of potentially causative factors have been identified by the industry and the scientific community but the effects and magnitudes of these factors on oyster mortality risks remain unknown. Through a structured expert elicitation questionnaire, this study presents industry and scientific experts' calibrated judgements of cause of this multifactorial problem. We document low agreement among scientists and oyster growers with regards to important causative groups of factors. There is some agreement that factors like water temperature, seasonality, bacterial pathogens, affect oysters suspended in water and reared in trays, particularly if they are large oysters. However, both scientists and growers harbored differing views about the age and size of oysters susceptible to mass mortalities and the role of food availability and gametogenesis in contributing to mass mortalities. We discuss potential sources of these differing opinions and misconceptions. Our findings also highlight the need for additional research to resolve some of the uncertainty in experts' perceptions of cause. Risk communication efforts in the future should focus on these differing perspectives to work toward mutually viable decisions and solutions.

Abundance of Vibrio aestuarianus, water temperature, and stocking density are associated with summer mortality of Pacific oysters in suspended culture

High mortality rates of cultured Pacific oysters (Crassostrea gigas) during the summer months have regularly occurred on oyster farms in British Columbia, Canada over the last 10 years, but little is known about the microbial and environmental conditions that contribute to such mortality events. The objective of the study was to determine correlative factors associated with the onset of a summer mortality event in oysters (mean ± SD shell height: 14.2 ± 0.5 mm) grown in suspended culture at four stocking densities (150, 300, 450, 600 oysters tray−1) from May 11 to September 17, 2018. Variables examined included both biotic (oyster growth, mortality, reproductive development, and microbiome (approximately every week); Vibrio and harmful algal species abundance) and abiotic (temperature, salinity, turbidity, dissolved oxygen, pCO2, pH, and aragonite saturation) ones. Both the absolute abundance of V. aestuarianus and the relative abundance of Vibrio spp. increased with observed oyster mortality and declining health. Mortality was highest on August 12 and associated with a prior period of elevated temperatures (i.e., increasing temperatures from early July to early August) and increased oyster growth/reproductive development. At that time, systemic mixed microbial infections and necrotic gill tissue in histological cross sections were observed in 19% of oysters that appeared healthy macroscopically. Cumulative percent mortalities per tray ranged from 34 to 75%, the highest-density trays having significantly less mortality and smaller shell width, shell length, and gonad length than lower-density trays. This study demonstrates the significant impact of summer mortality on Pacific oysters and highlights the biotic (host growth, reproductive development, and microbiome composition as well as Vibrio spp. abundance) and abiotic (water temperature) factors associated with the observed mortality in this region.

Increased abundance of potentially pathogenic Vibrio and a marine heatwave co-occur with a Pacific Oyster summer mortality event

The Pacific Oyster (Crassostrea gigas) is an important aquaculture species, that has been globally affected by “summer mortality” outbreaks. The causes of summer mortality are often unclear, but several contributing factors have been implicated. To better understand the factors contributing to C. gigas summer mortality, we performed a 24 week experiment in tanks filled by a continuous flow of unfiltered surface seawater during a summer mortality high-risk period. Throughout this period, we measured a suite of environmental parameters, with weekly sampling to characterise the bacterial community composition, the abundance and diversity of the Vibrio genus, and the expression of oyster immune response genes. We observed three clear phases of oyster mortality: Phase 1 (day 0 to 75) was characterised by sporadic and low levels (mean < 1% per week) of oyster mortality; Phase 2 (day 82 to 119) involved constant levels of ongoing mortality (mean < 10% per week); and Phase 3 (day 125 to 161) involved a sharp increase in mortality (mean > 10% per week) reaching up to weekly mortality of 51%. The shift between phase 2 and 3 coincided with heavy rainfall and a marine heatwave (MHW) event at our study site, which lasted 13 days, when the average water temperature was elevated by 0.8 °C when compared to the weeks prior and following the MHW, and a maximum water temperature of 27.7 °C was reached. Oyster mortality was positively correlated with the occurrence of the MHW as well as bacterial and Vibrio abundance, and negatively correlated with seawater salinity that decreased from a salinity of 34.6 to 31.4 psu following rainfall events. In addition, the relative abundance of several Vibrio species, including V. campbellii, V. rotiferianus and V. owensii displayed positive correlations to oyster mortality. Shifts in the oyster-associated bacterial community and oyster immunity gene expression profiles occurred in parallel to increased mortality levels, with genes involved in the regulation of antimicrobial peptides (IkB1 and NF-kB p105) significantly over-expressed during high mortality. Overall, we observed a C. gigas mortality event that coincided with a marine heatwave, rainfall event and a shift in bacterial community composition. Our analyses provides further evidence for the putative role of Vibrio in summer mortality events.

Relationships between high temperatures and Pacific Oyster disease and mortality in southeast Tasmania, Australia

Warm ocean temperature extremes, including marine heatwaves, have profound impacts on natural marine systems and aquaculture industries across the globe. In Tasmania, Australia, one aquaculture industry that has been significantly impacted by warm temperatures is Pacific Oyster (Magallana gigas, previously named Crassostrea gigas) farming, due to recurring outbreaks of the virus Ostreid herpesvirus 1. Such viral outbreaks are understood to be driven by high seawater temperatures, but the temperature threshold or duration for triggering disease and mortalities remain unclear. This study investigates the relationship between in-situ farm temperatures and oyster disease and mortality on the southeast coast of Tasmania, Australia using daily observations from three oyster growing areas (Pipe Clay Lagoon, Upper Pittwater, and Lower Pittwater) over three seasons. It is found that a 12-day averaged daily mean temperature is an excellent measure of the occurrence of high mortality. Specifically, a 21-day mean of 23.7 °C resulted in a 70% likelihood of high mortality, which is defined here as oyster losses of >15%. On the other hand, for lower levels of disease and mortality, a 12-day average of daily mean temperature gave the strongest relationship. A 12-day mean of 19.7 °C led to 70% probability of some disease and low mortality. The analysis also found in-situ farm temperature generally correlates well with remotely sourced temperature observations, indicating their potential usability for operational management. This study demonstrates a statistical risk analysis framework for the oyster farming industry, helping to improve the understanding of the detrimental impact of high temperatures on Pacific Oysters.

Susceptibility of shellfish aquaculture species in the Chesapeake Bay and Maryland coastal bays to ostreid herpesvirus 1 microvariants.

Ostreid herpesvirus 1 (OsHV-1) and its microvariants (µVars) cause economically devastating mass mortalities of oysters and pose a threat to the shellfish aquaculture industry globally. OsHV-1 outbreaks can cause up to 100% mortality in the Pacific oyster Crassostrea gigas. However, OsHV-1 and its variants have a broad host range and can infect at least 7 bivalve species, including bay scallops Argopecten irradians and eastern oysters C. virginica. Determining the susceptibility of economically and ecologically important bivalve species to OsHV-1 is critical for improving biosecurity and disease management to protect the aquaculture industry. Surveys of eastern oysters were conducted in June to August 2021 in the Maryland portion of the Chesapeake Bay to determine the prevalence and viral load of OsHV-1 at 5 aquaculture farms. Using quantitative PCR, OsHV-1 was not detected at any sites. Experiments examined the susceptibility of single stocks of eastern oysters and hard clams Mercenaria mercenaria to the virus and their ability to horizontally transmit it using OsHV-1 µVar SD (San Diego, California) and OsHV-1 µVar FRA (Marennes-Olreon, France). Results showed that OsHV-1 µVars did not cause mortality or symptomatic infection in the single stocks of eastern oysters and hard clams used in these experiments using natural infection pathways. However, the eastern oyster stock, when injected with OsHV-1, did transmit the virus to naïve Pacific oysters. Further experimentation using additional stocks and lines and establishment of surveillance programs along the east and Gulf coasts of the USA are necessary to prepare for the potential spread and impact of OsHV-1 related disease.