Oyster Production Research Articles

Adsorption of norovirus and ostreid herpesvirus type 1 to polymer membranes for the development of passive samplers.

This study was performed to develop a passive sampling methodology for the detection of two viruses in seawater in the area of shellfish production, the norovirus (NoV), a human pathogen implicated in gastroenteritis outbreaks linked to oyster consumption and the ostreid herpesvirus type 1 (OsHV-1), a virus associated with mass mortalities of Pacific oysters. Commercially, membranes were tested for their capacity to adsorb virus: zetapor, gauze, nylon, low-density polyethylene (LDPE) and polyvinylidene difluoride (PVDF). Laboratory exposures of membranes to contaminated water samples (stool, sewage, seawater) were performed. Our data show that the amount of NoV GII genome per membrane measured with qRT-PCR increased with the time of exposure up to 24h, for all types of membranes except gauze. After 15days of exposure, the amount of NoV GII per membrane continued to increase only for nylon and LDPE. The amount of OsHV-1 per zetapor membrane was significantly increased as soon as 4h ofexposure, and after 24h of exposure for all types of membranes. Exposure of membranes to serial dilutions of various samples revealed that the amount of NoV GII and OsHV-1 per membrane is significantly higher in diluted samples. The detection of NoV and OsHV-1, respectively, with zetapor and PVDF membranes was found to be more efficient than the direct analysis of sewage and seawater. All membranes immersed in contaminated samples adsorbed NoV GII and OsHV-1. The amount of both viruses increased with the time of exposure. Zetapor and PVDF membranes seem to be more adapted to NoV GII and OsHV-1 detection respectively. Membranes tested will be used as passive samplers to improve the detection of virus in oyster production areas. Also, passive samplers could be a valuable tool for microbiome analysis with new generation sequencing.

Genetic parameters of resistance to Vibrio aestuarianus, and OsHV-1 infections in the Pacific oyster, Crassostrea gigas, at three different life stages

BackgroundIn France, two main diseases threaten Pacific oyster production. Since 2008, Crassostrea gigas spat have suffered massive losses due to the ostreid herpesvirus OsHV-1, and since 2012, significant mortalities in commercial-size adults have been related to infection by the bacterium Vibrio aestuarianus. The genetic basis for resistance to V. aestuarianus and OsHV-1 and the nature of the genetic correlation between these two traits were investigated by using 20 half-sib sire families, each containing two full-sib families. For each disease, controlled infectious challenges were conducted using naïve oysters that were 3 to 26 months old. In addition, siblings were tested under field, pond and raceway conditions to determine whether laboratory trials reflected mortality events that occur in the oyster industry.ResultsFirst, we estimated the genetic basis of resistance to V. aestuarianus in C. gigas. Susceptibility to the infection was low for oysters in spat stage but increased with later life stages. Second, we confirmed a strong genetic basis of resistance to OsHV-1 infection at early stages and demonstrated that it was also strong at later stages. Most families had increased resistance to OsHV-1 infection from the spat to adult stages, while others consistently showed low or high mortality rates related to OsHV-1 infection, regardless of the life stage. Our third main finding was the absence of genetic correlations between resistance to OsHV-1 infection and resistance to V. aestuarianus infection.ConclusionsSelective breeding to enhance resistance to OsHV-1 infection could be achieved through selective breeding at early stages and would not affect resistance to V. aestuarianus infection. However, our results suggest that the potential to select for improved resistance to V. aestuarianus is lower. Selection for dual resistance to OsHV-1 and V. aestuarianus infection in C. gigas might reduce the impact of these two major diseases by selecting families that have the highest breeding values for resistance to both diseases.

COMMUNITY RADIO IN EMPOWERING FARM WOMEN THROUGH MUSHROOM PRODUCTION

Barh community radio station was established in 2011 at Krishi Vigyan Kendra, Barh, Patna to broadcast programmes on different issues of agriculture and allied sectors, health, hygiene and nutrition for three hours daily. Growing mushroom on agriculture byproducts is a handy enterprise, low initial investment, self employment creating, supplementary income generating, and nutritious food giving to the family. On listening to the talks on Radio, farm women visited KVK to enquire about mushroom production and after going through training, 55 women started oyster mushroom production at their hut for their own consumption and selling the surplus in their villages. Among them, 35 farm women started producing on medium scale to earn money by selling the produce in the local market. A small group of 10 farm women started producing mushroom at commercial scale and consulted vegetable shops and even hotels in nearby city for regular supply. This turn resulted in their financial empowerment. The large scale production of mushroom earned more than 1.25 lakh per woman in one season. These results clearly indicate that there is an impact of radio in empowering farm women.

The effects of shellfish harvesting by aboriginal inhabitants of Southwest Florida (USA) on productivity of the eastern oyster: Implications for estuarine management and restoration

Abstract Oyster reefs in Southwest Florida (USA) have been integral to estuarine ecology throughout the Holocene. Though Crassostrea virginica has never been commercially harvested, aboriginal people used the oyster substantially, accumulating middens between 5000 BC and AD 1700. A conservation paleobiological/historical ecological study of oysters from middens and modern reefs within Estero Bay (EB) and Pine Island Sound (PIS) determined if oyster productivity changed due to aboriginal overharvesting. Archaeological samples came from sites including the Late Archaic (LA, 2000–500 BC) of PIS and the Caloosahatchee (Cal, 500 BC–AD 1500) of EB and PIS. These samples were compared with natural oyster death assemblages from neighboring modern reefs. Methods comprised measuring oyster convex valve lengths and sectioning shells to count ligament pit growth lines that served as proxy for growth rate. The biologic taphonomic grade was also compared after scoring the interior valve surface; biologic grade is near pristine for oysters collected live for consumption. Archaeological samples exhibit significantly better taphonomic grades when compared to modern assemblages, confirming the hypothesis that oysters were harvested for food. Valve length decreased significantly from LA to Cal time, whereas modern assemblages were indistinguishable from LA collections. Because the Cal samples span 4 climatic intervals, the results suggest that climate change was not responsible for shifts in productivity. Results support the hypothesis of overharvesting during Cal times. They also suggest that modern oysters retain the capacity for growth, and indicate that aboriginal activity did not result in a permanent microevolutionary shift. The results are also relevant for the ongoing discussion surrounding the creation of an Anthropocene Epoch; the shell middens built throughout history greatly influenced both estuarine ecology and landscapes through fishing and engineering practices.

The effect of the Korea-US free trade agreement on governance of the aquaculture value chain

We present a value chain analysis of seafood exports from South Korea to major consumer markets in the United States, outlining the global value chain of oyster products from South Korea and its governance structure. A major issue faced by consumers is the lack of ability to determine where and how the seafood products are produced and processed. This can be addressed through standards set by the importing firm or country. In this article, we show that Korea–USA free trade agreement has led to development of high-capacity US-approved farms and processing facilities, and stricter standards have created a more efficient production and processing sector for exporting seafood products from Korea to the United States.

Ecological impacts of non-native Pacific oysters (Crassostrea gigas) and management measures for protected areas in Europe

Pacific oysters are now one of the most ‘globalised’ marine invertebrates. They dominate bivalve aquaculture production in many regions and wild populations are increasingly becoming established, with potential to displace native species and modify habitats and ecosystems. While some fishing communities may benefit from wild populations, there is now a tension between the continued production of Pacific oysters and risk to biodiversity, which is of particular concern within protected sites. The issue of the Pacific oyster therefore locates at the intersection between two policy areas: one concerning the conservation of protected habitats, the other relating to livelihoods and the socio-economics of coastal aquaculture and fishing communities. To help provide an informed basis for management decisions, we first summarise evidence for ecological impacts of wild Pacific oysters in representative coastal habitats. At local scales, it is clear that establishment of Pacific oysters can significantly alter diversity, community structure and ecosystem processes, with effects varying among habitats and locations and with the density of oysters. Less evidence is available to evaluate regional-scale impacts. A range of management measures have been applied to mitigate negative impacts of wild Pacific oysters and we develop recommendations which are consistent with the scientific evidence and believe compatible with multiple interests. We conclude that all stakeholders must engage in regional decision making to help minimise negative environmental impacts, and promote sustainable industry development.

Follow-Up of Norovirus Contamination in an Oyster Production Area Linked to Repeated Outbreaks.

A production area repeatedly implicated in oyster-related gastroenteritis in France was studied for several months over 2years. Outbreaks and field samples were analyzed by undertaking triplicate extractions, followed by norovirus (NoV) detection using triplicate wells for genomic amplification. This approach allowed us to demonstrate that some variabilities can be observed for samples with a low level of contamination, but most samples analyzed gave reproducible results. At the first outbreak, implicated oysters were collected at the beginning of the contamination event, which was reflected by the higher NoV levels during the first month of the study. During the second year, NoV concentrations in samples implicated in outbreaks and collected from the production area were similar, confirming the failure of the shellfish depuration process. Contamination was detected mainly during winter-spring months, and a high prevalence of NoV GI contamination was observed. A half-life of 18days was calculated from NoV concentrations detected in oysters during this study, showing a very slow decrease of the contamination in the production area. Preventing the contamination of coastal waters should be a priority.

Biomarkers of Winter Mortality resistance in selectively bred Sydney rock oysters (Saccostrea glomerata)

Sydney rock oysters (Saccostrea glomerata) are an important part of Australian cuisine and are considered to be a gourmet delicacy for their distinctive flavor. The species is native to Australian estuaries where it faces challenges from environmental stressors and human impacts. In addition, the Sydney rock oyster aquaculture industry has been periodically affected by outbreaks of diseases such as Queensland Unknown (QX) disease and Winter Mortality, a protozoan borne disease greatly impacting the production of the Sydney rock oyster industry. These diseases limit the production of Sydney rock oysters but do not affect other major oyster species farmed in New South Wales. Winter Mortality is a protozoan born disease affecting the production of the Sydney rock oyster industry greatly. Over the past few decades, breeding lines of Sydney rock oysters have been selected for resistance to Winter Mortality and Queensland Unknown disease. In this study, we report a proteomic comparison between the Winter Mortality selective breeding line and hatchery control oysters. We identified 22 protein features showing differential intensities between the two proteomes. The identities of 19 of these features were established using nanoflow liquid chromatography-tandem mass spectrometry. The differential proteome identified in the current study coincides well with previous analyses of stress responses in oysters, suggesting commonalities between intracellular reactions to disease and stress. For instance, cytoskeletal effects found in the current study to be associated with Winter Mortality resistance have been previously linked to intracellular stress responses. In addition, we identified novel proteins such as proteasome subunit alpha type-6 and calcium-dependent protein kinase 31, both of which are reported for the first time in disease resistant Sydney rock oysters. Results of this study will add to our understanding of stress responses and provide the first proteomic survey of Winter Mortality resistance in Sydney rock oysters.

Single or dual experimental infections with Vibrio aestuarianus and OsHV-1 in diploid and triploid Crassostrea gigas at the spat, juvenile and adult stages

French production of the Pacific cupped oyster, Crassostrea gigas, is currently threatened by two pathogens, OsHV-1 and V. aestuarianus. While oysters selected for their higher resistance to OsHV-1 are now available for the industry, the impact of V. aestuarianus on such oysters is unknown, especially for triploids. In addition, experimental infection has used the virus or the bacteria alone, but there have been no investigations of dual exposure to these pathogens. This study is the first report of single or dual exposure in spat (Spat1 and Spat2), juvenile and adult naïve oysters. For each of the two stocks evaluated, unselected oysters and oysters selected for their higher resistance to OsHV-1 infection were tested, as well as their triploid siblings of the selected oysters produced using cytochalasin B. We confirmed that resistance to OsHV-1 infection and susceptibility to V. aestuarianus increased with age and size, although selected oysters were not significantly impacted by OsHV-1 whatever their ploidy, size or age. We found different mortality patterns depending on the pathogen tested. The mortality pattern was similar for oysters exposed to OsHV-1 or to both pathogens in the Spat1 trial (4months old and 1.9g). The mortality pattern was similar for oysters exposed to V. aestuarianus or to both pathogens in the Adult trial (25months old and 63.1g). Surprisingly, mortality was much higher (ranging from 75.9% to 100%), in particular for the selected oysters, for the Spat2 (8months old/3.9g) and Juvenile trials (16months old/18.4g) given a dual exposure, regardless of the level of selection for OsHV-1 and the ploidy state. Our findings highlight an important threat for oyster farmers: oysters exposed to both pathogens could experience dramatic mortality rates, even in oysters selected for their higher resistance to OsHV-1. Finally, our study demonstrated for the first time that triploid oysters were more susceptible to experimental challenges with V. aestuarianus at the spat stage than their diploid siblings. However, the difference in mortality between the triploids and diploids remained limited and ranged from 22.9% to 6.6% for spat and adults, respectively with a relatively regularly decrease in the difference with increased age.

Quantification of reproductive effort of the triploid Pacific oyster, Crassostrea gigas raised in intertidal rack and bag oyster culture system off the west coast of Korea during spawning season

Oyster culture forms one of the major produce of aquaculture industry in Korea. Due to the demand for production, it is necessary to maintain a steady culture process. However, as a result of oil spill accident and other environmental problems in the west coast, there is a need to find a quick recovery in the production of oysters. While oyster culture in Korea is dominated by use of diploid oysters, an alternative to fast recovery for oyster industry is to introduce the use of triploid oysters. Triploid oysters have advantage in terms of faster growth and negligible quantity of reproductive effort during spawning, resulting in shorter grow-out period and improved meat quality compared to diploid oysters. In this study, we determined the reproductive effort in triploid females of Pacific oyster, Crassostrea gigas using an indirect enzyme-linked immunosorbent assay (ELISA). Two years old triploid and diploid female oysters raised in rack and bag oyster culture system on the west coast of Korea were collected during spawning and post-spawning season in 2013. Histology revealed that the diploid females spawned from June to September, with a peak in June. Unlike diploids, the triploid females exhibited no sign of spawning as the diploids were actively engaged in spawning. During post-spawning season, the triploid oysters reabsorbed the mature remnant eggs in their follicles. Gonad somatic index (GSI), determined using ELISA, of the diploid females reached its peak in June as 22.2%. In contrast, GSI of the triploids recorded during spawning peak was only 3.4%. Carbohydrate and glycogen contents in the triploid oyster tissues during spawning season were much higher than in diploids, suggesting that the triploids used minimum energy for reproduction in summer season, containing very small amount of gonad and high level of glycogen. Statement of relevanceReproductive efforts (amount and number of eggs) of the diploid and triploid Pacific oysters measured in this study clearly demonstrated sterility of the triploid oysters during spawning season in summer. The present study also report gametogenic pattern of the triploid, crucial in the management of oyster aquaculture.

Variability in larval period, post-setting growth and survival of the oyster Crassostrea belcheri produced by gamete stripping method

Gamete stripping is the most practical method in oyster production because the time of fertilization can be precisely controlled. However, it was found that there was much greater variability in the speed of egg development compared with natural spawning. In this study, three groups of larvae (aged 21, 27 and 29 d) were successively separated from fertilization to the pediveliger stage among all the larvae produced by the stripping method. No differences in the spat growth rate were found among the larval periods at 15 d of the experiment (p ≥ 0.05), but spat growth and survival rates of juvenile oysters in the larval periods of 21 and 27 d were significantly higher than those for 29 d at the end of the experiment (30 d). The positive correlation between larval growth rates and the spat daily yield at 30 d was significant. These results indicated that the gamete stripping method produced high variations in the larval period and post-setting growth and survival in the oyster, Crassostrea belcheri.

Plasma cholinesterase activity as a biomarker for quantifying exposure of green sturgeon to carbaryl following applications to control burrowing shrimp in Washington State.

Willapa Bay (Washington State, USA) has been 1 of the rare intertidal locations where large-scale pesticide applications occur. Until recently, carbaryl was applied to control burrowing shrimp that decrease commercial oyster productivity. The bay is a critical habitat for green sturgeon (Acipenser medirostris), an anadromous species listed as threatened under the US Endangered Species Act. However, the hazard that carbaryl poses is unknown. Surrogate seawater-acclimated white sturgeon (A. transmontanus) were exposed to 0 μg L(-1) , 30 μg L(-1) , 100 μg L(-1) , 300 μg L(-1) , 1000 μg L(-1) , and 3000 μg L(-1) carbaryl for 6 h, and brain acetylcholinesterase (AChE) and plasma butyrylcholinesterase (BChE) activities were measured. Enzyme recovery was measured in an additional cohort exposed to 1000 μg L(-1) carbaryl for 6 h. Activity of AChE was reduced (p ≤ 0.001) at concentrations ≥ 100 μg L(-1) with recovery in the 1000 μg L(-1) cohort by 72 h. Surprisingly, BChE activity was greater than controls at concentrations ≥ 300 μg L(-1) (p > 0.05), a finding confirmed in additional fish exposed to 3000 μg L(-1) for 6 h (+30%, p < 0.001) with apparent recovery by 48 h. Plasma samples were collected from free-living green sturgeon before and 4 d to 5 d after application of carbaryl in Willapa Bay. Activity of BChE after application was reduced 28% (p < 0.001), indicating exposure to the pesticide. However, the lack of congruence between BChE and AChE activity in captive white sturgeon exposed to carbaryl indicates that further studies are needed to better understand the risk carbaryl exposure poses to green sturgeon. Environ Toxicol Chem 2016;35:2003-2015. © 2015 SETAC.

Economic impact of ocean acidification on shellfish production in Europe

Ocean acidification (OA) is increasingly recognized as a major global problem. Despite the scientific evidence, economic assessments of its effects are few. This analysis is an attempt to perform a national and sub-national assessment of the economic impact of OA on mollusc production in Europe. We focus on mollusc production because the scientific evidence on the biological impact on calcifying organisms is ample relative to other types of marine organisms. In addition, Europe and its regions are significant producers of marine molluscs. By performing a partial-equilibrium analysis, we show that the highest levels of overall impact are found in the countries with the largest current production, such as France, Italy and Spain. For Europe as a whole, the annual impact will be over 1 billion USD in 2100. Due to the different production foci of the individual countries and their regions, the distribution of the impact is extremely uneven across countries and their respective regions, with the most affected sub-national regions being those on the Atlantic coast of France, which is an important region for oyster production.

Assessment of pollution of the Boca de Camichin Estuary in Nayarit (Mexico) and its influence on oxidative stress in Crassostrea corteziensis oysters

Boca de Camichin Estuary is one of the main producers of Crassostrea corteziensis oysters in Mexico, but the presence of pollutants can affect oyster production. Molluscs produce reactive oxygen species (ROS) in response to changes in the environment and pollution. These ROS induce oxidative damage in biomolecules. The main objective of this study was to evaluate pollution in the estuary and the subsequent oxidative stress in C. corteziensis oysters during the 2010 production cycle. For this aim, we performed monthly samplings in the oyster farms from January to May. We took water samples to quantify polycyclic aromatic hydrocarbon (PAH) and metal content; also, we evaluated oxidative damage (lipoperoxidation, lipidic hydroperoxides, protein oxidation) and enzyme activity (CAT, SOD, GPx, GST and AChE) in oyster gills. The results show the presence of Cu, Fe, Mn, naphthalene, benz[a]anthracene, pyrene, benz[a]pyrene and benzo[k]fluoranthene. On the other hand, AChE activity was not inhibited, which suggests that organophosphorus pollutants or carbamates were absent. Regarding oxidative stress, oysters from the estuary had oxidative damage in lipids, not proteins, and altered antioxidant enzyme activity, when compared to control organisms. Interestingly, we did not observe any correlation between the pollutants and the oxidative stress parameters evaluated in this study. Thus, we cannot rule out that a synergistic effect between the environmental variables and the pollutants is causing the oxidative stress in these oysters.

Improvements in desirable traits of the Pacific oyster, Crassostrea gigas, as a result of five generations of selection on the West Coast, USA

The Pacific oyster, Crassostrea gigas, is one of the most important global aquaculture species due to its potential high growth rates and tolerance of a wide range of environmental conditions; however, most farmers use seed from wild, non-domesticated stocks for production. In this study we estimated genetic parameters for performance traits of oysters from the Molluscan Broodstock Program (MBP), a family-based selective breeding program designed to improve yields of Pacific oysters on the West Coast, USA. Covariance components were obtained using AIREMLF90, and a multiple-trait animal model. A total of 15,236 records were analyzed from five generations of selectively bred oysters. Heritability estimates for field traits at harvest were all positive over the five analyzed generations, ranging from 0.58±0.03 (3rd generation) to 0.30±0.04 (5th generation) for yield, from 0.55±0.03 (1st generation) to 0.12±0.02 (4th generation) for survival, and from 0.51±0.03 (2nd generation) to 0.40±0.03 (4th generation) for mean individual weight at harvest (growth). Declines in heritabilities for survival and yield in the 4th and 5th generations were perhaps a result of changing ocean conditions due to increased upwelled hypoxic and acidified seawater occurring on the West Coast, USA, that affected the quality of seed from the MBP hatchery/nursery. Realized heritabilities were all positive and medium-to-high across generations, ranging to 0.11±0.42 (for survival in the 3rd generation) to 1.20±0.35 (for individual weight in the 5th generation); however, standard errors for realized heritabilities were high (ranging 0.15 to 1.47), especially in the 5th generation. There was a gradual improvement in genetic gains for survival and yield over the five generations of selection, resulting in accumulated gains of +15.7% and +19%, respectively, in the 5th generation; however, little improvement was achieved after the 2nd generation in genetic gain for individual weight, with an accumulated gain in the 5th generation of +11.3%. Realized gains for performance traits were less than predicted by genetic gains. Realized gain in survival consistently improved over the selection period, resulting in an accumulated gain of 11.7% in the 5th generation compared to that of wild, non-selected controls; however, gains in individual weight and yield at harvest were not consistent across generations and gains in the 5th generation were −9.8% and 0%, respectively, compared with those of controls, perhaps due to inconsistent genetic quality of control broodstock sampled from wild populations. Across generations, there were positive genetic correlations between yield and both survival (0.38±0.04) and individual weight (0.90±0.01) as well as between survival and individual weight (0.25±0.04). Positive medium-to-high genetic correlations among harvest traits suggest that indirect gains in yield can be achieved by selection for either higher growth or survival. Statement of relevanceThe Pacific oyster is an important aquaculture species both globally and on the West Coast, US. This study describes long-term (over 5 generations) genetic and phenotypic responses of Pacific oysters planted at commercial farms, in response to selection for yield, survival and mean individual weight at harvest. The breeding program spanned a period of increased ocean upwelling and acidification on the West Coast, US, that may have reduced heritabilities for survival due to impacts of the quality of hatchery seed. Results will be of interest to other breeding programs for Pacific oysters in France, Australia and New Zealand, as well as breeders of other extensively farmed aquaculture species. Genetically improved broodstock will help support future oyster production under more challenging conditions, due to the global effects of acidification and warming of coastal waters.

Restocking Natural Beds with Remote-Set Disease-Resistant Oysters in Connecticut: A Field Trial

ABSTRACT Oyster production in Connecticut historically has had large fluctuations due to two major factors: disease epizootics caused by a protozoan parasite multinucleate sphere X (Haplosporidium nelsoni), and long periods without successful natural sets. A field trial was performed to investigate whether the remote set of oysters can be used to restock natural beds during mortalities caused by disease epizootics and lack of recruitment due to periods without sets. Disease-resistant oysters were remote set in a hatchery and placed either in or without biodegradable nets on a natural bed. Aged, clean cultch was used as the control, and the growth, mortality, predation, fouling, and prevalence of disease were monitored for two growing seasons. The oysters grew to market size (3 inches, 76.2 mm) in 15 mo. Predation, due mainly to Atlantic oyster drills Urosalpinx cinerea, and overgrowth of oysters caused a mortality of 72% in the oyster seed during the first growing season. More abundant wild oysters set bu...

Epizootiology of Perkinsus sp. inCrassostrea gasar oysters in polyculture with shrimps in northeastern Brazil.

Bivalve culture is of considerable economic and social interest in northeastern (NE) Brazil. The polyculture is an alternative approach to traditional monoculture for reducing the environmental impact of shrimp farming and improving oyster culture. Perkinsus marinus and Perkinsus olseni were found infecting oysters in NE Brazil and can threaten oyster production. This study evaluated Perkinsus spp. occurrence in Crassostrea gasar during all production stages. Oyster spats were produced in a hatchery and grown in shrimp ponds in Rio Grande do Norte state. Perkinsus spp. were surveyed by Ray's fluid thioglycollate medium and confirmed by polymerase chain reaction. Prevalence and intensity of infection were determined in oysters until they reached 7 cm. Results showed that the broodstock was already infected by Perkinsus (60%), but the derived spats were Perkinsus-free. Oyster spats acquired Perkinsus infection when transferred to ponds. The prevalence gradually increased in the seven months following placement in ponds (73%), and then decreased to 17% by the tenth month. The infections were initially mild, but intensity increased at the final growth stage. In conclusion, it is possible to produce Perkinsus-free C. gasar oyster spats from infected broodstock, and their culture in shrimp ponds is feasible.

Exposure to the toxic dinoflagellate Alexandrium catenella modulates juvenile oyster Crassostrea gigas hemocyte variables subjected to different biotic conditions.

The Pacific oyster Crassostrea gigas is an important commercial species cultured throughout the world. Oyster production practices often include transfers of animals into new environments that can be stressful, especially at young ages. This study was undertaken to determine if a toxic Alexandrium bloom, occurring repeatedly in French oyster beds, could modulate juvenile oyster cellular immune responses (i.e. hemocyte variables). We simulated planting on commercial beds by conducting a cohabitation exposure of juvenile, "specific pathogen-free" (SPF) oysters (naïve from the environment) with previously field-exposed oysters to induce interactions with new microorganisms. Indeed, toxic Alexandrium spp. exposures have been reported to modulate bivalve interaction with specific pathogens, as well as physiological and immunological variables in bivalves. In summary, SPF oysters were subjected to an artificial bloom of Alexandrium catenella, simultaneously with a cohabitation challenge. Exposure to A.catenella, and thus to the paralytic shellfish toxins (PSTs) and extracellular bioactive compounds produced by this alga, induced higher concentration, size, complexity and reactive oxygen species (ROS) production of circulating hemocytes. Challenge by cohabitation with field-exposed oysters also activated these hemocyte responses, suggesting a defense response to new microorganism exposure. These hemocyte responses to cohabitation challenge, however, were partially inhibited by A.catenella exposure, which enhanced hemocyte mortality, suggesting either detrimental effects of the interaction of both stressors on immune capacity, or the implementation of an alternative immune strategy through apoptosis. Indeed, no infection with specific pathogens (herpesvirus OsHV-1 or Vibrio aesturianus) was detected. Additionally, lower PST accumulation in challenged oysters suggests a physiological impairment through alteration of feeding-related processes. Overall, results of this study show that a short-term exposure to A.catenella combined with an exposure to a modified microbial community inhibited some hemocyte responses, and likely compromised physiological condition of the juvenile oysters.

Hyperspectral remote sensing of wild oyster reefs

The invasion of the wild oyster Crassostrea gigas along the western European Atlantic coast has generated changes in the structure and functioning of intertidal ecosystems. Considered as an invasive species and a trophic competitor of the cultivated conspecific oyster, it is now seen as a resource by oyster farmers following recurrent mass summer mortalities of oyster spat since 2008. Spatial distribution maps of wild oyster reefs are required by local authorities to help define management strategies. In this work, visible-near infrared (VNIR) hyperspectral and multispectral remote sensing was investigated to map two contrasted intertidal reef structures: clusters of vertical oysters building three-dimensional dense reefs in muddy areas and oysters growing horizontally creating large flat reefs in rocky areas. A spectral library, collected in situ for various conditions with an ASD spectroradiometer, was used to run Spectral Angle Mapper classifications on airborne data obtained with an HySpex sensor (160 spectral bands) and SPOT satellite HRG multispectral data (3 spectral bands). With HySpex spectral/spatial resolution, horizontal oysters in the rocky area were correctly classified but the detection was less efficient for vertical oysters in muddy areas. Poor results were obtained with the multispectral image and from spatially or spectrally degraded HySpex data, it was clear that the spectral resolution was more important than the spatial resolution. In fact, there was a systematic mud deposition on shells of vertical oyster reefs explaining the misclassification of 30% of pixels recognized as mud or microphytobenthos. Spatial distribution maps of oyster reefs were coupled with in situ biomass measurements to illustrate the interest of a remote sensing product to provide stock estimations of wild oyster reefs to be exploited by oyster producers. This work highlights the interest of developing remote sensing techniques for aquaculture applications in coastal areas.

How network analysis of oyster movements can improve surveillance and control programs of infectious diseases?

Animal movements are one of the main ways to introduce and spread pathogens. In French oyster farming, many stakeholders and premises are heterogeneously divided over the country and a highly dynamic flow of oysters exists among them. In the context of animal disease surveillance and control, analysis of the animal movement’ network can provide useful information to build adapted surveillance strategies or to develop risk management. Movement network analysis has been widely used in terrestrial production, to evaluate the vulnerability of animal movement network to the spread of a specific disease. In France, since 2008, Pacific oyster spat (Crassostrea gigas) has been affected by massive mortality outbreaks associated with the detection of a newly reported variant of ostreid herpesvirus type 1 (OsHV-1). These mortality events have a direct economic impact causing considerable concern to oyster farmers. A previous epidemiological study has highlighted the potential role of oyster transfers in the spread of these outbreaks. However, neither mandatory database nor reliable data is publicly available concerning oyster transfers in France. In this context, a field study was carried out in the main oyster production area in France, Charente-Maritime bay, to map oyster movements and to characterize the corresponding network structure related to potential disease spread. Seventy-five oyster farmers were randomly selected in Charente-Maritime bay between July and September 2010, according to a stratified sampling design based on farm category regarding production type and location (i.e. spat producers, local farmers, beyond farmers, local farmer-senders, beyond farmer-senders). Data related to the farm characteristics and activities, routine rearing scheme and potential changes in husbandry practices were collected during a face-to-face interview of the oyster farmer, using a standardized questionnaire and a land register. Movement data were spatialized and analyzed using social network analysis. A 85.3% participation rate to the study was obtained. Results showed a seasonal variation of oyster transfers in all the sampling strata, with peaks of transfers in spring and autumn, and different geographical patterns of oyster transfers between strata. Growing sites were shared by farms belonging to different farm category which have different behaviour related to oyster movements in time. These results show heterogeneity of contacts between strata both in time and space. Network measures allowed identifying the growing sites having a high risk of becoming infected, of transmitting diseases to other sites or controlling the flow of oysters from one part of the network to another part. The growing sites at high risk of becoming infected could be targeted for surveillance activities (Figure 1). The growing sites at high risk of transmitting diseases to other sites could be the first ones where implementation of early or enhanced control measures (e.g. quarantine, depopulation) could be focused. The growing sites controlling the flow of oysters could be considered as key players in the initial spread of a disease. If these sites would be removed from the network by early movement restriction, this may contribute to contain the disease spread in a smaller part of the area, providing that the propagation by the marine currents and the effect of environmental factors are considered all together. As a conclusion, movement network analysis may provide valuable information to policy makers in preparedness (surveillance) and response (control) to an epidemic.