Oyster Growers Research Articles

Oyster grower perspectives on green crab (Carcinus maenas) interactions: using applied social science to inform research and engagement

Research that informs the New Hampshire Aquaculture industry, which has grown significantly over the past few years, and helps the overall associated watershed community to keep Great Bay Estuary healthy is vital. Specific study of green crabs, Carcinus maenas, and their potential effects on oysters, Crassostrea virginica, is important to gain a better understanding of the impact invasive species may have on these ecologically and economically important bivalves. The aim of this study was to document and understand New Hampshire (NH) oyster growers’ perspectives on the issue of green crab predation of oysters. Through a survey, this study documented a preliminary understanding of farmer perspectives and research needs regarding the population of green crabs in NH waters. More specifically, this survey provided a social science perspective and information on areas of potential research - abundance, distribution, and diet of green crabs - that could be beneficial to the oyster industry. This study fills knowledge gaps, helps improve future research (including how to better engage with end users), and facilitates creating and implementing solutions that benefit the NH oyster aquaculture industry.

Winter mortality syndrome in Hong Kong oyster (Crassostrea hongkongensis): Causes and impacts

Cultivation of Hong Kong oyster species (Crassostrea hongkongensis, also known as Magallana hongkongensis) in estuarine areas of South China including Hong Kong contributes to almost a quarter of global edible oyster production. Recently, many growers and scientists observed that mass mortality of this species in winter months, i.e., at the time of their harvest. However, the characteristics and patterns of aforementioned winter mortality syndrome (WMS), as well as the potential underlying causes and impacts are yet to be evaluated. In this study, WMS was first described and examined within Hong Kong's cultural areas by employing a combination of interviews, field observations, histological examinations, and molecular analyses to identify the potential pathogen responsible for WMS in this region. Significant mortality in field experiment confirmed the presence of WMS and matched with results from interviews with oyster growers. Eugymnanthea japonica is identified as one of the contributing factors to winter mortality in Hong Kong. Nevertheless, it cannot be definitively established as the primary and sole cause of WMS. Our results and observations suggest that multiple environmental stressors driven primarily by high salinity stress combined with infection by specific parasite, serves as the main cause of this WMS. This study highlighted the knowledge gaps warranting future investigation and proposed potential short-term mitigation strategies to tackle WMS in the region.

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.

Optimizing the post-harvest salting of cultured oysters (Crassostrea virginica) using a recirculating wet storage system by monitoring adductor hemolymph osmolality

The oyster aquaculture industry is steadily growing in the United States, principally driven by the rise of small farms. With an increasingly crowded marketplace, farmers are searching for methods to diversify their crops and remain competitive. Enhancing oyster liquor in wet storage while preserving the water's signature flavor, or ‘merroir,’ is one way to create a new product line and differentiate crops, especially with oysters grown in low-salinity environments. Previously, growers have attempted to increase the salinity of their product by ‘finishing’ their oysters in a saltier water body; however, mass mortality may accompany such a transfer. Therefore, implementing a wet storage system may be a useful intermediary step to reduce crop mortality prior to ‘finishing’ oysters in alternative waters. This project sought to understand how to effectively use recirculating aquaculture systems (RAS) as wet storage to expose harvested oysters to a chosen salinity and temperature to function as a modified wet storage system. Additionally, this experiment describes the effects of both short-term and long-term salinity and temperature stress on the survival, condition index, and shelf life of adult oysters. The shelf life and rate of osmoconformity of low-salinity adult oysters depend on ambient temperature and acute changes in salinity. While increasing ambient temperature improves the rate of hemolymph osmoconformity, we observed high mortality above 25 °C. Additionally, the shelf life of oysters was negatively affected by the length of exposure time to higher salinity and increasing temperature. These data can guide oyster growers in low-salinity regions who wish to diversify their product using recirculating tanks or better understand the potential risks associated with transferring cultured oysters across water bodies of varying salinity.

No trade-off between growth and OsHV-1 tolerance in Pacific oysters (Crassostrea gigas) farmed on the West Coast, USA.

An important pathogen of concern for Pacific oyster growers in the USA is ostreid herpesvirus 1 (OsHV-1). Currently, oyster stocks exist that are tolerant to OsHV-1; however, it is uncertain if a trade-off exists between their tolerance to OsHV-1 and their growth in different environments. To investigate any potential trade-offs, Pacific oyster families with varying levels of OsHV-1 tolerance were grown in a bay where OsHV-1 is endemic (Tomales Bay, CA) and in a bay where OsHV-1 is absent (Willapa Bay, WA). In Tomales Bay, we found that oysters from OsHV-1 tolerant families grew faster than oysters from OsHV-1 susceptible families, while in Willapa Bay, no statistically significant difference in growth was found between oyster families with different levels of OsHV-1 tolerance observed in Tomales Bay. These findings indicate that Pacific oysters bred to be tolerant to OsHV-1 would not be expected to have a longer time-to-market regardless of the presence of OsHV-1 in the growing environment.

Market opportunities for Florida farmed oysters

The oyster aquaculture industry has grown quickly in recent years. New farming techniques target specific attributes that increase oysters' value and allow them to be sold in the high-value, half-shell market. A lack of economic data makes it difficult for growers to determine which specific attributes increase oyster value, however. This publication summarizes findings from a recent study that examined restaurant menus to obtain economic data on raw, half-shell oysters. Major findings indicate that brand is an oyster attribute that provides value, and that marketing oysters with information on two or more attributes can also result in higher prices. Additionally, locally sourced oysters are often sold at cheaper prices, providing an opportunity for growers from outside regions to enter new markets. The findings will be useful to Florida oyster growers as attempts to expand Florida’s oyster aquaculture industry continue.

Growth Rates of Vibrio parahaemolyticus Sequence Type 36 Strains in Live Oysters and in Culture Medium.

The pathogenic marine bacterium Vibrio parahaemolyticus can cause seafood-related gastroenteritis via the consumption of raw or undercooked seafood. Infections originating from relatively cool waters in the northeast United States are typically rare, but recently, this region has shown an increase in infections attributed to the ecological introduction of pathogenic sequence type 36 (ST36) strains, which are endemic to the cool waters of the Pacific Northwest. A 2005 risk assessment performed by the Food and Drug Administration (FDA) modeled the postharvest growth of V. parahaemolyticus in oysters as a function of air temperature and the length of time the oysters remained unrefrigerated. This model, while useful, has raised questions about strain growth differences in oyster tissue and whether invasive pathogenic strains exhibit different growth rates than nonclinical strains, particularly at lower temperatures. To investigate this question, live eastern oysters were injected with ST36 clinical strains and non-ST36 nonclinical strains, and growth rates were measured using the most probable number (MPN) enumeration. The presence of V. parahaemolyticus was confirmed using PCR by targeting the thermolabile hemolysin gene (tlh), thermostable direct hemolysin (tdh), tdh-related hemolysin (trh), and a pathogenesis-related protein (prp). The growth rates of the ST36 strains were compared to the FDA model and several other data sets of V. parahaemolyticus growth in naturally inoculated oysters harvested from the Chesapeake Bay. Our data indicate that the growth rates from most studies fall within the mean of the FDA model, but with slightly higher growth at lower temperatures for ST36 strains injected into live oysters. These data suggest that further investigations of ST36 growth capability in oysters at temperatures previously thought unsuitably low for Vibrio growth are warranted. IMPORTANCE Vibrio parahaemolyticus is the leading cause of seafood-related gastroenteritis in the United States, with an estimated 45,000 cases per year. Most individuals who suffer from vibriosis consume raw or undercooked seafood, including oysters. While gastroenteritis vibriosis is usually self-limiting and treatable, V. parahaemolyticus infections are a stressor on the growing aquaculture industry. Much effort has been placed on modeling the growth of Vibrio cells in oysters in order to aid oyster growers in designing harvesting best practices and ultimately, to protect the consumer. However, ecological invasions of nonnative bacterial strains make modeling their growth complicated, as these strains are not accounted for in current models. The National Shellfish Sanitation Program (NSSP) considers 10°C (50°F) a temperature too low to enable Vibrio growth, where 15°C is considered a cutoff temperature for optimal Vibrio growth, with temperatures approaching 20°C supporting higher growth rates. However, invasive strains may be native to cooler waters. This research aimed to understand strain growth in live oysters by measuring growth rates when oysters containing ST36 strains, which may be endemic to the U.S. Pacific Northwest, were exposed to multiple temperatures postharvest. Our results will be used to aid future model development and harvesting best practices for the aquaculture industry.

Investigating the Relationship between Nitrate, Total Dissolved Nitrogen, and Phosphate with Abundance of Pathogenic Vibrios and Harmful Algal Blooms in Rehoboth Bay, Delaware.

Vibrio spp. and phytoplankton are naturally abundant in marine environments. Recent studies have suggested that the co-occurrence of phytoplankton and the pathogenic bacterium Vibrio parahaemolyticus is due to shared ecological factors, such as nutrient requirements. We compared these communities at two locations in the Delaware Inland Bays, representing a site with high anthropogenic inputs (Torquay Canal) and a less developed area (Sloan Cove). In 2017 to 2018, using light microscopy, we were able to identify the presence of many bloom-forming algal species, such as Karlodinium veneficum, Dinophysis acuminata, Heterosigma akashiwo, and Chattonella subsalsa. Dinoflagellate biomass was higher at Torquay Canal than that at Sloan Cove. D. acuminata and Chloromorum toxicum were found only at Torquay Canal and were not observed in Sloan Cove. Most probable number real-time PCR revealed V. parahaemolyticus and Vibrio vulnificus in environmental samples. The abundance of vibrios and their virulence genes varied between sites, with a significant association between total dissolved nitrogen (TDN), PO4-, total dissolved phosphorus (TDP), and pathogenic markers. A generalized linear model revealed that principal component 1 of environmental factors (temperature, dissolved oxygen, salinity, TDN, PO4-, TDP, NO3:NO2, NO2-, and NH4+) was the best at detecting total (tlh+) V. parahaemolyticus, suggesting that they are the prime drivers for the growth and distribution of pathogenic Vibrio spp. IMPORTANCE Vibrio-associated illnesses have been expanding globally over the past several decades (A. Newton, M. Kendall, D. J. Vugia, O. L. Henao, and B. E. Mahon, Clin Infect Dis 54:S391-S395, 2012, https://doi.org/10.1093/cid/cis243). Many studies have linked this expansion with an increase in global temperature (J. Martinez-Urtaza, B. C. John, J. Trinanes, and A. DePaola, Food Res Int 43:10, 2010, https://doi.org/10.1016/j.foodres.2010.04.001; L. Vezzulli, R. R. Colwell, and C. Pruzzo, Microb Ecol 65:817-825, 2013, https://doi.org/10.1007/s00248-012-0163-2; R. N. Paranjpye, W. B. Nilsson, M. Liermann, and E. D. Hilborn, FEMS Microbiol Ecol 91:fiv121, 2015, https://doi.org/10.1093/femsec/fiv121). Temperature and salinity are the two major factors affecting the distribution of Vibrio spp. (D. Ceccarelli and R. R. Colwell, Front Microbiol 5:256, 2014, https://doi.org/10.3389/fmicb.2014.00256). However, Vibrio sp. abundance can also be affected by nutrient load and marine plankton blooms (V. J. McKenzie and A. R. Townsend, EcoHealth 4:384-396, 2007; L. Vezzulli, C. Pruzzo, A. Huq, and R. R. Colwell, Environ Microbiol Rep 2:27-33, 2010, https://doi.org/10.1111/j.1758-2229.2009.00128.x; S. Liu, Z. Jiang, Y. Deng, Y. Wu, J. Zhang, et al. Microbiologyopen 7:e00600, 2018, https://doi.org/10.1002/mbo3.600). The expansion of Vibrio spp. in marine environments calls for a deeper understanding of the biotic and abiotic factors that play a role in their abundance. We observed that pathogenic Vibrio spp. were most abundant in areas that favor the proliferation of harmful algal bloom (HAB) species. These results can inform managers, researchers, and oyster growers on factors that can influence the growth and distribution of pathogenic Vibrio spp. in the Delaware Inland Bays.

"Economics of Oyster Mushroom (Pleurotus spp) Production in Bhagalpur District of Bihar"

"The mass population of India was vegetarian and attracted to mushroom consumption because they have contained more nutrients like vitamins, minerals, fibers, and medicinal value. The mushroom species (oyster, button, milky and paddy straw) was identified for cultivation in upper and hilly regions of the Indian states suitable for white button mushrooms and plain areas of lower altitude regions in favor of oyster ‘Dhingri’ (Plutorus sages Kaju) mushroom. Moreover, growers have doesn’t fully organize form of mushroom per unit sale in Bhagalpur & territories market of Bihar as well as other states of India. The strategic economic problem of the net sown area was limiting shrink factors day by day due to increase the infrastructure alternate option is open but human population is pressurized for fulfilling the demand become inevitable to best alternative option to supplementary mushroom growing opportunity. Mushroom production in locale area under Bhagalpur district of Bihar was purposively studies of the fourteen each block and village panchayat with the help of snowball referral to subsequent respondents collected 71 complete lists enumerated then obtained sample size (Taro Yamane) 60 oyster growers and cumulative cube root frequency distribution techniques were used. The major variable costs of oyster mushroom that spawn cost was higher expenses followed by the human labour, chopped dry straw, plant protection and polythene bags. The overall farms of fresh oyster mushroom production were accounted to be on an average 16.90 Kg (m2) per square meter, which were varied to be on an average 18.48 Kg (m2) per square meter higher for large farms followed by 16.99 Kg (m2) per square meter of medium farm and 15.16 Kg (m2) per square meter for the small farm. The overall farm benefit-cost ratio (BCR) was estimated to be on average 1.31 which varied to 1.40 higher for large farms followed by 1.31 for medium farms and 1.19 for small farms. Further, the cost of oyster mushroom production per unit was decreased when the increase in the size of farms and vice-versa."

Oyster Farming in Ilocos Sur, Philippines

The study looked into the management and culture methods used by oyster farmers in Ilocos Sur. The support given by numerous groups was also noted, along with a number of challenges and issues. Using an expert-validated survey questionnaire data were collected from 148 oyster producers from six municipalities namely, Candon, Sta. Maria, Narvacan, San Vicente, Caoayan and Sto. Domingo. According to the findings, the growers cultivate oysters using three different techniques: long line hanging, raft hanging, and stake (tulos) methods. Empty oyster shells, PVC, rubber strips, and bamboo poles are used for spat collection. The growing season is typically carried out in estuaries, rivers, and fishponds. According to the farmers, oyster prices are erratic but tend to rise around the holidays. Every production cycle, their oyster farming brings in an average of Php5,000 to 15,000 in revenue. Despite receiving technical and financial assistance from various authorities. oyster growers encounter a number of socioeconomic and environmental constraints that affect their production processes. In light of the aforementioned problems, it is recommended that institutions support growers by organizing and registering them and strengthening capacity building on sustainable oyster production technologies, including marketing and post-harvest management assistance. In order to create business opportunities in the neighborhood and increase household income, additional skill training should be offered. In addition, the province needs to develop marketing plans to control pricing.

Off‐farm employment in aquaculture: A case study of New England's oyster growers

AbstractGlobal aquaculture markets have experienced a great deal of recent success. However, growers are still highly susceptible to risks caused by seasonality, interannual variability in earnings, and environmental factors. As a result, we see growers turning to alternative sources of income, and participating in off‐farm labor. We utilize a series of zero‐inflated count models to examine the relationship between an aquaculture grower's off‐farm labor decision and relevant explanatory variables including demand‐side potency, which is often omitted in the literature. We also look to identify and analyze the importance of professional social networks among growers as a stabilizing mechanism and a way to mitigate risk to growers. This study expands upon an economic model that has traditionally been used for analyzing off‐farm labor choices within an agricultural context and applies it to an increasingly important industry, aquaculture.

Sustainable Oyster Aquaculture, Water Quality Improvement, and Ecosystem Service Value Potential in Maryland Chesapeake Bay

The United States has a $16 billion seafood deficit that the U.S. Department of Commerce and states are attempting to close by legislative policies, encouraging expansion of aquaculture in the United States. One of these policies, the 2011 National Shellfish Initiative, recognizes the benefits to water quality of cultivation of bivalve shellfish aquaculture in addition to the provision of seafood product. More recently, research addressing these policies has resulted in approval of the use of harvested oysters as a nutrient best management practice in the Chesapeake Bay region. Also discussed, but not yet fully implemented, is the inclusion of oyster growers in nutrient credit trading programs where economic compensation is provided to oyster growers for the nutrient removal ecosystem service that their oysters provide. This study used field sampling and a local-scale oyster production model to compare water quality, oyster production, and oyster associated nitrogen removal at two bottom and four water-column Maryland Chesapeake Bay oyster farms. Objectives were to highlight differences in water quality (i.e., oyster food), resultant differences in oyster production, and differences in estimated oyster-associated nutrient removal among farms. An avoided, or replacement, cost economic valuation analysis was performed to also compare the potential payment to the oyster growers for the nutrient removal service if they were included in a fully developed nutrient credit trading program. Production at the six sites varied from 1.78 to 25 metric tons of harvestable oysters acre–1 y–1. Oyster filtration–related N removal was estimated to be at a range of 28–457 kg N acre–1 y–1. The potential economic value of the total N removed by a farm was estimated to be at a range of $0.56 × 103–$12,446 × 103 y–1 among farm sites, depending on the alternative management measure used to assign the value.

The role of ecosystem services in the decision to grow oysters: A Maryland case study

Ecosystem services provided by oysters are regularly cited to gain support for the continued development of oyster aquaculture, but we have limited understanding of whether and how these benefits influence those who grow oysters. Participant observation and semi-structured interviews occurred with 57 oyster growers in Maryland (United States) to detail factors motivating entry into the oyster aquaculture industry. Results, framed under a lens of ecosystem services, indicate that cultural services are more likely to motivate aquaculture participation than provisioning, regulating or supporting services. This study emphasizes the significance of cultural ecosystem services and defines the need to better understand those provided by oysters and other farmed shellfish. A more complete description of the ecosystem services provided through shellfish aquaculture could serve to enhance support of farmed shellfisheries. In addition, this participatory approach highlights challenges within the ecosystem services framework regarding linked services and their complexity.

Testing the efficacy of bouncing-bucket nursery systems for enhancing shell strength and thickness in on-bottom cultured Eastern oysters (Crassostrea virginica)

Through physical manipulation, oyster growers can modify the morphological features of young oysters in order to improve their robustness and resiliency to predation. In this study, the efficacy of a novel passive culturing technique using floating buckets that “bounce” oysters prior to the benthic grow-out phase for enhancing shell thickness and strength, while not compromising oyster shape, was investigated. Using a field experiment, we quantified and compared the shell thickness (mm), compressive strength (Newtons), and shape (fan ratios and cup ratios) of juvenile oysters subjected to this bouncing technique with oysters grown in floating bags, the current industry standard. Results indicated that bouncing increased shell thickness and strength for oysters >45 mm shell height compared to control oysters, but also decreased shell thickness and strength for oysters <45 mm shell height compared to control oysters. There was no difference in shell shape between bounced and control oysters, with both groups having similar fan and cup ratios. Ultimately, our results suggest that the size at which bounced oysters are released to the benthos for grow out will dictate the usefulness of bouncing technique for reducing predation-related mortality.

Farmer monitoring reveals the effect of tidal height on mortality risk of oysters during a herpesvirus outbreak

Abstract The intertidal zone is characterized by a sharp vertical gradient of environmental stress, which structures species distribution and their interactions. Few studies, however, have examined the influence of tidal height on host–pathogen interactions. Here, we investigated how the tidal height influence outbreak of the Ostreid herpesvirus type 1 (OsHV-1) affecting the Pacific oyster. A volunteer network composed of 20 oyster growers monitored the survival of 28 batches of oysters during an epizootic event in Southern Brittany, France. Oysters were spat from wild collection or hatchery production. The sampling sites were spread over a 150-km2 area with a tidal height ranging from 0.98 to 2.90 m. Concomitantly, we followed survival of oyster spats in relation with OsHV-1 DNA detection at two sites and conducted risk analysis. We found that tidal height was associated with a lower risk of mortality. This effect was higher for hatchery than for wild oysters probably reflecting differences in health status. Our study opens perspectives for mitigation strategies based on tidal height and emphasizes the value of volunteer science in marine epidemiological studies.

Using geospatial analysis to identify potential sources of contamination for two norovirus outbreaks associated with commercial oyster harvesting in Tomales Bay, California

Using geospatial analysis to identify potential sources of contamination for two norovirus outbreaks associated with commercial oyster harvesting in Tomales Bay, California

Examination of the potential relationship between boring sponges and pea crabs and their effects on eastern oyster condition.

The eastern oyster Crassostrea virginica provides a number of ecosystem services and is an important commercial fishery species along the US East and Gulf Coasts. Oyster populations have declined dramatically due to overharvesting, habitat loss, and disease. As restoration efforts and aquaculture of oysters continue to increase throughout their range, it is important to consider the impacts of a number of potential oyster pests, including the boring sponge Cliona spp. and the pea crab Zaops (Pinnotheres) ostreum, on oyster populations. Both of these have been demonstrated to reduce oyster growth, condition, and in some instances, reproductive output. Boring sponges in particular are a major concern for oyster growers and managers. Our monitoring efforts have suggested that pea crabs might be more prevalent in sponge-infested oysters; we therefore conducted an observational study to determine if there was any relationship between pea crab prevalence and sponge presence, and to examine whether the presence of both pests had synergistic effects on oyster condition. At 2 very different sample sites, North Carolina and New Jersey, oysters with 1 pest (i.e. boring sponge) were significantly more likely to have the second pest (i.e. pea crab) than the background population. Furthermore, sponge presence negatively affected oyster condition in North Carolina only, while pea crabs significantly reduced condition at both locations. When sponges and pea crabs were present together, the effects on oyster condition were additive. This study provides further evidence that interactions between an individual and a fouling/pest organism can alter oyster susceptibility to other parasites.

Assessing the risk to green sturgeon from application of imidacloprid to control burrowing shrimp in Willapa Bay, Washington-Part I: exposure characterization.

Willapa Bay and Grays Harbor (WA, USA) comprise the largest region of commercial oyster cultivation on the Pacific Coast. The activities of 2 species of burrowing shrimp impair growth and survival of oysters reared on the intertidal mudflats. To maintain viable harvests, the oyster growers have proposed controlling the shrimp by applying the insecticide imidacloprid onto harvested beds. Green sturgeon (listed in the Endangered Species Act) forage on burrowing shrimp and could be exposed to imidacloprid in the sediment porewater and through consumed prey. Studies were conducted to evaluate the likelihood that green sturgeon would be exposed to imidacloprid and to characterize the subsequent environmental exposure. Comparisons between treated and untreated control beds following test application of the insecticide suggested that green sturgeon fed opportunistically on imidacloprid-impaired shrimp. The highest interpolated imidacloprid residue concentrations in field samples following chemical application were 27.8 µg kg(-1) and 31.4 µg kg(-1) in porewater and shrimp, respectively. Results from modeled branchial and dietary uptake, based on conservative assumptions, indicated that the porewater exposure route had the greatest contribution to systemic absorption of imidacloprid. The highest average daily uptake from porewater (177.9 µg kg(-1) body wt) was 9.5-fold greater than total dietary uptake (18.8 µg kg(-1) body wt). Concentrations and durations of exposure would be lower than the levels expected to elicit direct acute or chronic toxic effects.

Assessing the risk to green sturgeon from application of imidacloprid to control burrowing shrimp in Willapa Bay, Washington--Part II: controlled exposure studies.

The activities of 2 species of burrowing shrimp have a negative impact on the growth and survival of oysters reared on intertidal mudflats in Willapa Bay and Grays Harbor, Washington (USA). To maintain viable harvests, oyster growers proposed the application of the neonicotinoid insecticide imidacloprid onto harvested beds for the control of burrowing shrimp. In test applications, water column concentrations of imidacloprid were relatively low and dissipated rapidly. The foraging activities of the green sturgeon (listed in the US Endangered Species Act) could result in exposure to higher, more sustained imidacloprid concentrations within sediment porewater and from the consumption of contaminated shrimp. Controlled experiments were conducted using surrogate white sturgeon to determine acute and chronic effect concentrations, to examine overt effects at more environmentally realistic concentrations and durations of exposure, and to assess chemical depuration. The 96-h median lethal concentration was 124 mg L(-1) , and the predicted 35-d no-observed-adverse-effect concentration was 0.7 mg L(-1) . No overt effects were observed following environmentally relevant exposures. Imidacloprid half-life in plasma was greater than 32 h. Measured concentrations of imidacloprid in porewater were significantly lower than the derived acute and chronic effect concentrations for white sturgeon. Exposure risk quotients were calculated using the effect concentrations and estimated environmental exposure. The resulting values were considerably below the level of concern for direct effects from either acute or chronic exposure to an endangered species.