Improving Oyster Aquaculture in Rhode Island: Development and Testing of the "Rhodoyster"

Final Report for LNE07-256

Project Type: Research and Education
Funds awarded in 2007: $127,254.00
Projected End Date: 12/31/2010
Region: Northeast
State: Rhode Island
Project Leader:
Dr. Marta Gomez-Chiarri
University of Rhode Island
Expand All

Project Information


Oyster aquaculture is the largest segment of aquaculture in Southern New England. In Rhode Island, 33 farmers generated almost $1.8 million of production in 2009. The highest risk on oyster farms is disease. The goal of this project is to stimulate the growth of the oyster aquaculture industry in southern New England by: a) developing the Rhodoyster, a disease-resistant line well-adapted to local growing conditions, consisting of a hybrid of a local line and a disease-resistant line; and b) involving at least 25 of 200 oyster farmers in the Rhode Island region in evaluating the performance of a disease-resistant line. Direct participation of farmers in line testing, combined with several outreach and extension activities including on-site training and workshops on best management practices and health management will encourage at least 11 oyster farmers in Rhode Island to adopt at least one of the following practices: 1) using more than one line in their farms, therefore potentially avoiding catastrophic losses due to disease (increased diversification); and 2) using at least one line that is disease-resistant and well adapted to local conditions, leading to a 10% increase in yield. In 2008, 2009, and 2010, 10 to 14 farmers in Rhode Island received seed from 3 lines, a disease-resistant line (NEH), a local stock (GHP), and a hybrid line between the two (HYB), and were trained on how to evaluate performance and recognize losses due to disease or predators. As of December 2010, data on performance for each of the year classes was received from 8 farmers. The major findings of this study were that: a) oyster growth was driven by a combination of oyster genetics and the growing environment; b) oyster performance was driven by survival in the farms that experienced disease outbreaks, with the disease-resistant line NEH showing significantly higher survival (11 to 76% compared to 2 – 62%, depending on the farm) and performance (2 – 4 fold better) than a local stock (GHP) and a hybrid between the two (HYB); c) no significant differences in performance between the lines were seen in farms that did not experience disease pressure; and d) the hybrid line between the disease-resistant line NEH and the local stock GHP did not show improved performance compared to the parental lines. These results show that using disease-resistant lines can lead to significant decreases in losses and significantly higher performance and economic gains in case of disease outbreaks. They also show that seed diversification is a useful strategy, since lines show relative differences in growth performance depending on the environmental conditions of each site and year. Based on farmer input, we have made arrangements for maintaining survivors of the NEH and GHP lines after exposure to heavy disease pressure from Juvenile Oyster Disease and moderate disease pressure from MSX disease; these broodstock oysters are available to hatcheries upon request for the creation of new lines. Availability of research data on line performance in several farms representative of different growing conditions in Rhode Island will help farmers in New England area to make wise decisions on line choice.


Farming the American cupped oyster (Crassostrea virginica) is one of the oldest, most productive, and fastest growing segments of US aquaculture, with a total value of $45 million in 2008, up from $26 million in 2003 (FishStat Plus version 2.3, Food and Agriculture Organization. Oyster aquaculture in Rhode Island (RI) has a unique and interesting history. During the late 1800s and early 1900s, oyster leases covered one-third of Narragansett Bay, employing thousands of people and producing many millions of dollars in revenue (Rice et al., 2000). A headline from the Evening Tribune, a newspaper from Providence, in 1914 stated that “Rhode Island ships 7000 gallons of oysters daily”, and continues saying that the “State secures a revenue of $700,000 a year from lease of beds”. However, a combination of factors, including user conflicts, water degradation, and the Hurricane of 1938, followed by World War II, lead to the total collapse of the industry. All leases were revoked by the State of RI in 1952, and it was not until the 1970s that new leases were approved. Led by hard-headed pioneers that had to weather and change a regulatory and public environment strongly against aquaculture, the oyster aquaculture industry is now expanding in RI. Aquaculture production and the number of farms in RI have been steadily increasing in the last decade, growing from 8 active farms and a production valued in $160,000 in 1999 to 33 farmers generating a value of more than $1.7 million in 2009 (Alves, 2007; Beutel, 2009). The growth in the aquaculture sector is a prime example of the exciting rebirth of RI’s agriculture, with farmers adding value to small farms through innovative marketing, diversification of products, and the development of brand names (like Moonstone Oysters, Matunuck Oysters, and many more), associated with quality product. Furthermore, RI farmers have been instrumental in promoting the environmental and economic benefits of oyster farming nationwide, demonstrating that it is a sustainable practice with many added ecological benefits. These benefits include the improvement of water quality by filter-feeding oysters that remove nitrogen and phosphorous from eutrophic waters, as well as providing habitat for other fish species (Shumway et al. 2003). The combined efforts of farmers, extension and outreach agents, regulators, and other agencies have resulted in a more favorable climate and less user conflicts between the public, the fishing industry, and aquaculture, paving the way for a healthy and ecologically sustainable expansion of oyster aquaculture in the region (Byron et al., 2010).

Disease, however, is still a major constrain to the expansion of oyster aquaculture in the region. The major diseases threatening RI oysters are dermo, Multinucleated Sphere X (MSX), and Juvenile Oyster Disease (JOD) (Davis and Barber, 1999; Burreson and Ford, 2004; Villalba et al., 2004). The combined impact of dermo and MSX diseases has caused hundreds of millions of dollars in losses in the Chesapeake and Delaware Bay region since the 1950s (Ewart and Ford, 1993). Recently, the range of dermo disease has expanded northward, causing high mortality in Massachusetts in the late 1990’s (Ford and Smolowitz 2006). Although levels of dermo disease in RI cultured oysters have remained low, disease prevalence and intensity have increased significantly in a few oyster leases since 1998, indicating that dermo disease could potentially have a serious impact in Rhode Island oyster farms in the future. The disease MSX, caused by the protozoan parasite Haplosporidium nelsoni, routinely causes heavy mortality in areas in the Northeast (Buresson and Ford, 2004). Juvenile Oyster Disease (also known as Roseovarius Oyster Disease) has the heaviest impact on seed oysters in the Northeast; losses in populations of oysters less than 25 mm in shell height that can often exceed 90% of yearly production (Davis and Barber, 1999). Farmers in Rhode Island have sporadically reported significant losses in outbreaks showing the characteristic signs of JOD. Since no treatments are available or feasible in the bays and ponds in which oysters are grown, alternative management techniques should be developed to combat these diseases.

Several attempts have been made to selectively breed individual lines of oysters that are able to withstand exposure to these diseases (Allen et al. 1993, Barber et al. 1998, Guo et al. 2003). To date, there are about five lines of oysters from the Mid-Atlantic and Northeast regions that have been selected for resistance to dermo and MSX diseases, with a few also being selected to counter JOD (Table 1). In addition, the creation of hybrid lines, based on the concept of heterosis, is an alternative technique used for oyster breeding. Hybrid lines attempt to pass on desired genetic characteristics of the parental lines to their offspring, intentionally creating a genetically superior individual.

Although studies have been conducted in various locations in the Northeast to determine the resistance of available lines in a field setting, no published data existed on the performance of disease-resistant lines in Rhode Island. Rhode Island growing conditions and growth techniques differ from those in other locations along the east coast, because oysters are subjected to high summer water temperatures similar to more southern locations, along with winter conditions similar to northern locations. Furthermore, all three major shellfish diseases (dermo, MSX, and JOD) can be found in Rhode Island and have the potential to occur simultaneously throughout a given growing season. Rhode Island oysters are grown in relatively high salinity waters (~28-34 psu) which favor the development of these three diseases. Culture techniques for oysters in Rhode Island are site-specific; oysters are typically grown in the water column in cages at deeper sites or on the bottom in a rack and bag system or directly on the bottom in shallower sites.

This study aims to evaluate the performance (growth and survival) of two year classes of three oyster lines/stocks (disease-resistant line NEH, a local wild stock GHP, and a hybrid cross HYB of the hatchery line NEH and wild stock GHP), in farms representative of local growing conditions. The Rhode Island wild stock originated from Green Hill Pond (GHP), a coastal salt pond. Oysters from this pond have been historically exposed to moderate to high dermo and MSX disease pressure (Gomez-Leon, et al., 2008). By creating a hybrid line (HYB) between the disease-resistant NEH and the local GHP stock, we hoped to develop a line well adapted to local growing conditions in Rhode Island (high salinity, cold winters, and hot summers) and exhibiting resistance/tolerance to JOD and dermo and MSX diseases. The main objectives of this study were to determine the influence of disease pressure, local growing conditions (environment), and genetics on the performance of these lines in Rhode Island farms. This knowledge will inform hatchery managers and farmers in Rhode Island on the major factors affecting oyster performance in this geographic location.

Performance Target:

Performance target. By the end of this project, the development of a new line well-suited to the RI region will be initiated. Eleven of the 200 farmers in the Southern New England area will adopt at least one of the following practices: 1) using more than one line in their farms, therefore potentially avoiding catastrophic losses due to disease (increased diversification); and 2) using at least one line that is disease-resistant and well-adapted to local conditions, leading to a 10% increase in yield.

We have met our performance target. Through the participation of up to 12 farmers in the study, we have determined the relative performance of the 3 lines in Rhode Island farms, determined which diseases are a major risk to Rhode Island farmers, evaluated the effect of genetics and environment on line performance, and kept survivors from disease outbreaks as broodstock for future line development. At least 14 farmers in Rhode Island are aware of the benefits of maintaining seed from at least two different hatchery sources in their farms and use at least one line that is disease-resistant, such as the Flower’s line (FMF, see table 1 in the introduction). The Rhode Island oyster industry continues to grow at a healthy rate, up 11% in value in 2009 from 2007. Productivity per acre also continues to increase, up to $13,300 in 2009 from $12,900 in 2007 (Beutel, 2009). The East Coast Shellfish Growers Association understands that further benefits can be achieved by more targeted selective breeding efforts, and is lobbying for the creation of a Shellfish Breeding Center.


Click linked name(s) to expand
  • David Alves
  • Dale Leavitt
  • Kathryn Markey
  • Perry Raso


Materials and methods:

This study aimed to involve farmers in evaluating the benefits of using disease-resistant lines of oysters in their farms. This was done by: 1) informing farmers of results from previous research that showed improved performance of disease-resistant lines; 2) seeking farmers’ input on which are the most common problems they encounter in their production runs and which are the most desirable oyster traits; 3) selecting lines to be tested based on farmers’ input; 4) developing the oyster lines by collecting and spawning oyster broodstock; 5) providing seed from the selected lines to as many farmers as possible; 6) training those farmers on how to measure oyster performance, how to recognize disease problems, and how to provide oysters for disease diagnosis; 7) collecting and analyzing data on environmental conditions and oyster performance at each participating farm; 8) repeating this process for one more growing season to reach more farmers and evaluate if performance results are consistent from year to year; and 9) disseminating the results of the trials to farmers and other relevant stakeholders.

Farmer recruitment and evaluation of needs

Different venues were used to inform farmers and stakeholders from other interest groups (hatcheries, shellfish pathologists, researchers, environmental groups, and state managers) about the goals of the project, including contact in person or by phone through our extension specialist and the Rhode Island Aquaculture Coordinator and presentations at regional and national meetings (the Northeast Aquaculture Conference and Exposition in Mystic, Connecticut, December 2006, and Portland, Maine, December 2008; Aquaculture 2007, San Antonio, TX, March 2007; Milford Aquaculture Seminar, New Haven, Connecticut, February 2008; and National Shellfisheries Association, Providence, Rhode Island, April 2008), a class at Roger Williams University (Practical Shellfish Farming, Spring 2007, 10 students) for people that are planning to go into the shellfish aquaculture business in Rhode Island and Massachusetts, and at two meetings of the Ocean State Aquaculture Association. A document containing a summary of the presentation was distributed to the farmers and stakeholders, along with a short survey requesting input on farmers concerns and needs.

Broodstock Spawning and Seed Distribution

Eastern oyster (Crassostrea virginica) broodstock was obtained from Rutgers University (NEH) or collected from Green Hill Pond, RI (GHP, 41 degrees 22 min 28.94 sec N, 71 degrees 36 min 48.57 sec W). Broodstock oysters were conditioned and spawned at the Roger Williams University Shellfish Hatchery, Bristol, RI, in February – March of 2008, 2009, and 2010 following standard hatchery procedures (Castagna et al., 1996). Oysters were individually set on micro-cultch and deployed when they reached between 3-5 mm in shell height (late-May or June) to 4 upwellers in selected locations representative of each growing area in Rhode Island. As the oysters grew, they were sieved and any oyster less than 10mm in early July was culled. Approximately 25,000 and 10,000 oysters from each line/stock were deployed to participating farmers in 2008 and 2010 respectively. Due to the small number of oysters reaching the first cull size (more than 6mm) by late-June 2009 only 200-900 oysters per line/stock were deployed to each farm in 2009.

Oyster deployment

At the time of oyster deployment, farmers were provided with the supplies necessary to evaluate oyster line performance and trained on how to perform the measurements. Farmers were instructed to provide monthly data on performance. They were also provided with brochures containing basic information on oyster diseases and predators, and informed on how to submit samples for disease diagnosis when unusual mortalities were observed. Four control locations representative of different growing conditions in Rhode Island were selected for more detailed sampling of performance and collection of environmental parameters (Figs. 1 and 2). Two of these farms were located in Narragansett Bay and the other two in coastal salt ponds. Oyster performance at these farms (control farms) was evaluated by our personnel.

Oyster maintenance

Oysters were held in the gear unique to each farm and cultured following the methods characteristic for each farm. Oysters held at the Narragansett Bay farms were cultured in cages. NB2 used ADPI mesh bags in cages that rest on the bottom and NB1 used a tiered plastic cage suspended in the water column. Oysters held at the coastal pond farms were grown in a rack and bag system (Fig 2, bottom left) a few inches off the bottom (CP1) or in cages with baskets suspended in the water column from a raft (CP2). Oysters were initially placed in three replicate bags and divided into additional bags at each sampling time point as they grew. Gear was rotated and cleaned as it became fouled with marine organisms (i.e. mussels, tunicates, barnacles).

Line/Stock Performance

Performance at the 4 control farms (total volume in L, shell height in mm, and percent survival) was measured every two-three weeks from the time of deployment (June) through the fall (October) for two years for the 2008 and 2009 year classes. Oysters from each replicate bag were placed in a graduated cylinder or container, mixed thoroughly, and the total volume (L) displaced by the oysters was recorded (Fig. 3). The volume for each replicate was added to attain the total volume (L) for each line at each farm. A random sample of oysters (n=100) from each line was measured for shell height using a caliper (Fig. 4; Barber et al., 1996). Live and dead oysters (boxes, Fig. 4) in 2 random samples of 250mL (for oysters larger than 30 mm in shell height) or 100mL (for oysters less than 30 mm) were counted and the cumulative mortality (CM) was determined for each time point using the formula CM= (T-L) divided by T, where T is the number of individuals in the sample and L is the number of live individuals (Barber et al., 1996). If half boxes (one valve) were found in the piles they were counted as a half oyster, therefore, 2 half boxes counted as 1 oyster. Performance measures at all the other participating farms were done by farmers on a monthly basis following the same procedures, with the exception that only 25 random oysters were measured for size.

Disease Diagnosis

Before transfer of the oysters from the hatchery to the upweller, two groups of 5 oysters per line were pooled and tested for the presence of Roseovarius crassostreae (causative agent of Juvenile Oyster Disease – JOD) following the method of Maloy et al. (2005). In addition, two pools of oysters per line and farm were tested for the presence of R. crassostreae after clinical signs of the disease were observed at a particular farm. These clinical signs included uneven valve margins, conchiolin deposits (Fig. 5), and high mortality (>30%) in oysters less than 25mm in shell height (Barber et al. 1996). Oysters (n=25 per line and farm) from the 2008 year class were also evaluated for the prevalence and intensity of the protozoan parasites Perkinsus marinus (dermo disease) and Haplosporidium nelsoni (MSX disease) during the second summer of sampling (week 30, late-July 2009). Prevalence and intensity (0 – 5, Mackin scale) of P. marinus infections were determined using the tissue Ray’s Fluid Thioglycollate Media (RFTM) assay, following standard procedures (American Fisheries Society, 2007). The weighted prevalence was determined by adding scores for the intensity of the infections and dividing by the total number of oysters tested (n=25). Samples of gill, mantle and gut tissue were collected to diagnose the presence of H. nelsoni via polymerase chain reaction (PCR), following the methods of Stokes et al. (1995). A histological examination was conducted in selected samples to confirm infection by haplosporidian parasites (American Fisheries Society, 2007). Intensity of infection was quantified by microscopic examination using a semi-quantitative scale ranging from 0 (no infection) to 3 (high, heavy infection).

Environmental Parameters

In order to evaluate the influence of environmental parameters on line performance, temperature, salinity, dissolved oxygen, turbidity, and pH were measured at each control farm four times each sampling day (8am, 10am, 12pm, and 2pm) to obtain a daily average. Temperature and dissolved oxygen were measured with a YSI instrument (YSI Y55) either at the depth of the water (CP farms) or as far as the YSI probe would reach in the water column (approximately 1 meter, NB farms). Turbidity was measured using a Secchi disk (Preisendorfer 1986). A water sample was collected to measure salinity with a refractometer and pH with a portable field pH meter (Hanna 9024C). The pH measurements were only taken during the 2009 sampling season.

Statistical analyses

Statistical analyses were run on SigmaStat 3.1 (SYSTAT) and differences were considered significant at p less or equal to 0.05. Two-way ANOVA followed by the Holm-Sidak method for multiple comparisons was used to test for significant differences among oyster lines and farms for mortality and size data at the end of each sampling season. Data found to fail assumptions of equal variance and normality were analyzed using nonparametric tests. Mortality data was expressed in percent cumulative mortality for two sub-samples in each replicate bag and arcsine transformed to determine differences in cumulative mortality among oyster lines and farms. Data for dermo disease weighted prevalence (index from 0 for non infected to 5 for heavy infections) was analyzed using a Kruskal-Wallis ranks test followed by Dunn’s test for multiple comparisons to examine to examine differences among the oyster lines and farms. Multiple regressions were conducted to evaluate potential correlations between selected environmental conditions (temperature, average and minimum dissolved oxygen, salinity, pH, and turbidity) and performance (growth and mortality) of each oyster line. To attain an accurate measurement of the average temperature, dissolved oxygen, and salinity occurring at each farm between the sampling time points, all four tidal cycle measurements for a given time point were averaged along with the four measurements from the previous time point. All of these data points were combined in order to account for the potential range in a given parameter occurring between the sampling time points. These average temperature calculations were compared to average temperature logger data for two of the four farms and were accurate to the ranges detected for the loggers (plus minus 2 degrees C). A backward elimination multiple regression technique was used to eliminate independent variables (temperature, dissolved oxygen, minimum dissolved oxygen, salinity, pH, turbidity) as they became insignificant. Regressions were run until significant variables were left.

Research results and discussion:
Farmer recruitment and evaluation of needs (Milestones 1 and 2)

Our goal was to request input from about 200 farmers from the RI region, as well as other stakeholders, and receive input from at least 50 farmers. These milestones were partially met. We used different venues to let about 30 farmers and about 100 stakeholders from other interest groups (hatcheries, shellfish pathologists, researchers, environmental groups, and state managers) know of our project (appendix 1). We received input from farmers in the Ocean State Aquaculture Association (25 farmers) and a few farmers in Massachusetts (5) regarding concerns and needs. We did not receive significant input from farmers from New England states other than Rhode Island, probably due to the differences in growing techniques (Connecticut farmers use mostly wild seed and the lines we developed were not adequate for farmers in Maine), regulatory environment in states like Massachusetts (which discourages in some cases the use of seed from out-of-state non-approved hatcheries to prevent transfer of disease), and culture conditions. Farmers that provided input were mostly focused on maximizing growth, but they also expressed concern about losses due to disease and predation. Most farmers had experienced at least one major loss to disease (more than 50% mortality in a period of 2 to 3 weeks) in the last 10 years. Most farmers were not willing to compromise growth performance to disease resistance. Based on data from previous projects, it was decided that the Rhodoyster would consist of a hybrid between local oysters from a local stock from Green Hill Pond (RI) and the disease-resistant line NEH from Rutgers University.

Broodstock Spawning and Seed Distribution (Milestones 3 and 5)

Our goal was to develop the lines and provide oysters to 20 (year 1, milestone 3) and 15 (year 2, milestone 4) farmers from the RI. These milestones were delayed for one year, due to problems with hatchery production in 2007. Oysters from the 3 target lines (the local Green Hill Pond, the disease-resistant NEH, and a hybrid between the two) were produced in Spring 2008 and 2009 and we provided seed and training to 12 (2008) and 10 (2009) farmers in Rhode Island. An additional batch was produced in 2010 using survivors of the GHP and NEH lines from JOD and MSX outbreaks that occurred in Narragansett Bay in 2008. This batch was provided to 7 farmers. We were unable to provide seed to interested farmers in Massachusetts (MA), since MA regulators did not allow importation of seed from our hatchery in Rhode Island due to concerns on the transfer of disease.

Line/Stock Performance and disease diagnosis (Milestones 4, 6, and 7)

The goal was to receive data from 15 farmers at the end of year 1, 12 farmers at the end of year 2, and 11 farmers at the end of year 3. We received input from 5 farmers in 2008 and 2009, and 8 farmers in 2010. We present in subsections below the results on the detailed evaluation of performance from the 4 control farms. Data from other farms confirmed the overall conclusions of the study, and is only presented if it provided additional information.

Major finding of this study regarding oyster performance include (see subsections below):

Oyster performance was variable depending on the farm (influence of the growth environment)

Differences in total volume of oysters (an indicator of oyster performance) were found among farms at the end of the first sampling season (October 2008) with the best performing farm NB1 outperforming the other three farms (Fig. 6). Differences in volume were also found among farms at the end of the second sampling season (October 2009) for the oysters deployed in 2008, but with a change in the ranking order of the farms. In 2009, CP1 was the best performing farm, outperforming the other three farms. Increases in oyster volume at the end of study ranged from 112-178 times the original volume at CP1, 37-86 times at NB1, 42-51 times at CP2, and 10-55 times at NB2. One major factor contributing to the difference in performance of NB1 relative to the first season was the loss (disappearance) of one of the two large cages holding the oysters during the winter of 2008 (50% loss for all lines, Fig. 6A). Considering the performance of the remaining oysters, NB1 may have still outperformed CP1 at the end of year 2 if it wasn’t for the loss of the cages. A few bags of oysters were also lost during the winter at CP1, resulting in a loss of 23% of the HYB oysters, 20% of the GHP, and 14% of NEH (Fig. 6C).

Differences in oyster performance at each farm were driven by a combination of differences in growth and survival

In the case of NB1, the relatively higher performance of oysters at this farm during the 2008 growing season was due to environmental conditions that favored growth in Narragansett Bay, as suggested by the oyster size data. Significant differences in oyster size (shell height in mm) for the 2008 year class were found among farms at the end of the 2008 and 2009 sampling seasons. Oysters grown in the Narragansett Bay farms were larger in size than oysters at the coastal ponds farms at the end of the 2008 and 2009 sampling seasons (p less than 0.001, Fig. 7). Oysters from the Narragansett Bay farms exhibited higher growth rates than oysters from the coastal ponds throughout the 2008 sampling season (0.23 to 0.31 mm/day depending on the line versus 0.17 to 0.23 mm/day). Oysters from CP2 grew the slowest compared to the other three farms. Even if conditions for growth were more favorable in Narragansett Bay in 2008, the farm NB2 showed the worst performance of all farms in 2008. Low performance at this farm was driven by mortality. Significant differences in cumulative mortality were found among farms at the end of the 2008 sampling season, with oysters from NB2 exhibiting a higher mortality than oysters at all other farms (p less than 0.001) and NB1 experiencing higher mortality than the coastal pond farms (p less than 0.001, Fig. 8A and B). At the end of 2008, cumulative mortalities of less than 10% were seen in the coastal pond farms, compared to 18 to 66% at the Narragansett Bay farms. At the end of 2009, when oysters deployed in 2008 were 18 months, oysters at CP1 had the greatest total volume (60 to 67L depending on the line) and the lowest cumulative mortality (higher than 37%), while oysters at NB2 had the lowest volume (8 to 28L) and had experienced the greatest mortality (70 to 91%).

Oyster mortality in seed (oysters less than 25 mm) in 2008 was due to Juvenile Oyster Disease

Seed oysters (less than 25 mm in shell height) with clinical signs of JOD were seen in all farms in 2008, but a high prevalence of conchiolin deposits was observed only in the Narragansett Bay farms (Fig. 9). The presence of a high proportion of oysters with conchiolin deposits, mortalities affecting primarily oysters less than 25 mm in shell height, and isolation of the bacterial pathogen Roseovarius crassostreae from affected oysters, strongly suggest JOD as the cause for mortalities at the bay farms. Conchiolin deposits and uneven valve margins were detected first in oysters from NB2 in early August (week 32), after the oysters were deployed to the farms from the upwellers, coinciding with mortality ranging from 13-33% at this time period (Fig. 8). Although clinical signs of JOD were observed in oysters from NB1 in late August (week 35), mortality at this farm was low until mid September (week 38). At this time, cumulative mortality increased from less than 10% on week 38 to more than 18% on week 42. The timing of mortlity (late summer), the fact that it affected oysters larger than 25 mm (JOD mostly causes mortality in seed oysters less than 25 mm), and detection of the presence of Haplosporidium nelsoni in other oysters at this farm through a monitoring program suggest that MSX disease could have been a contributing factor to mortality at NB1. Prevalence of dermo disease was low to moderate at all farms, and probably did not significantly contribute to mortality (weighted prevalences less than 1, not shown). Since mortality related to dermo disease typically occurs in oysters with infection intensities of moderate-heavy (3-5) on the Mackin index (Villalba et al. 2004), we ruled out dermo disease as a cause of the mortalities observed in the Narragansett Bay farms.

Differences in line performance were observed at those farms that experienced disease outbreaks (influence of genetics)

Significant differences in the total volume of the oysters deployed in 2008 were observed among lines at the Narragansett Bay farms, but not the coastal ponds, at the end of the 2008 and 2009 sampling seasons. In both seasons, the NEH line showed higher total volumes than GHP and HYB, with the lines at the coastal pond farms showing similar total volumes (Fig. 6). These are the farms that experienced higher mortality during the 2008 growing season. The significantly higher performance of NEH at NB1 may be due to the demonstrated resistance to MSX of this line. Furthermore, differences in the prevalence of conchiolin, a clinical sign of Juvenile Oyster Disease, were observed among lines at the Narragansett Bay farms, with NEH oysters showing a conchiolin prevalence of less than 10% in comparison to 25-50% for GHP and HYB at week 35. These results suggest that disease resistance to Juvenile Oyster Disease in the NEH line was also responsible for increased performance of the NEH line at the Narragansett Bay farms in 2008. This is a novel finding, since the NEH line has not been purposely selected for resistance to JOD, which is not found in Delaware Bay (the site for development of the NEH line). It is possible that some of the mechanisms conferring resistance to dermo and MSX diseases in the NEH line can confer resistance to the bacteria-caused JOD.

Differences in line performance were driven by differences in both growth and survival

NEH oysters showed the lowest mortality in comparison to HYB and GHP (p<0.001, Fig. 8) at the Narragansett Bay farms, where disease pressure was high, leading to higher volumes. NEH oysters from the 2008 year class were also overall significantly larger at the end of 2008 and 2009 growing seasons (p less than 0.001, Fig. 7). NEH oysters exhibited a higher growth at the end of the 2008 growing season with average growth rates of 0.27 mm per day versus 0.23 to 0.25 mm per day for HYB and GHP. Consequently, NEH oysters reached market size (75mm) sooner then HYB and GHP, with 3% at NB2 and 5% at NB1 reaching market size in early June 2008, only a year after their initial deployment. At this time, no oysters larger than 75mm were observed for any of the other lines. A higher proportion of market size oysters were also observed for the NEH line at all farms by the end of the second sampling season (Table 2).

Differences in oyster growth were driven by interaction between the genetics of the oysters and the growth environment (G x E interactions)

A significant interaction between farm and line was detected at the end of the 2008 and 2009 sampling seasons (p<0.001) for the oysters deployed in 2008. The interaction among the lines within the four farms in 2008 showed that NEH oysters at all farms were largest in size except at NB1, where all lines were of similar size. The interaction among the lines in 2009 within the four farms showed that the lines grew differently at each farm. No significant interaction between farm and line was detected for mortality (p=0.768).

Low performance of the 2009 year class oysters compared to the 2008 year class was driven by high seed mortality and slow growth

Seed deployed in 2010 showed similar performance to the seed deployed in 2008 (not shown). Volume at each of the farms for the 2009 year class, however, was less than 3L for each line by the end of the 2009 growing season. This low performance overall was due to heavy mortality and slow growth during the nursery phase (early summer). A small number of oysters from each line reached the cull size (more than 10 mm in shell height) in July 2009, resulting in less than 500 mL of oysters being deployed to all farms. Growth rates for the 2009 year class season during the first growth season were significantly lower than for the 2008 year class, ranging from 0.03 to 0.14 mm per day versus 0.23 to 0.25 mm per day. Consistent with the results obtained with the 2008 year class, oysters from CP2 resulted in the lowest total volume compared to the other three farms (not shown). Also consistent with the results from the 2008 year class, a significant interaction between farm and line in oyster size was found, with NEH oysters showing the largest size (p less than 0.001), and no significant interaction between farm and line detected for mortality (p=0.877). Relative overall performance of each line was different at each farm, with no line consistently outperforming others (not shown). Oyster mortality in oysters planted in 2009 during their first growing season was high at all farms, with oysters at NB1 experiencing the highest mortality (ranging between 70-90% depending on the line, p<0.001). Mortality at the other three farms ranged between 30 and 60%. Consistent with the 2008 year class, the NEH showed lower mortality than the GHP line at two of the farms (57% versus 85% in CP2; 89% versus 99% in NB2). In contrast to results for the 2008 year class, oysters from the NEH line deployed in 2009 at NB1 experienced significantly higher mortality during the nursery phase than the other lines (60% mortality at week 33 after deployment versus 15 – 30% for GHP and HYB). The exact causes for these losses and the slow growth are unknown, but the spring and early summer of 2009 were cooler and rainier than the spring and summer of 2008 and the seed in the upwellers may have been exposed to low salinity and low dissolved oxygen. Oysters at all farms showed clinical signs of JOD, including uneven valve margin and conchiolin deposits, the first week after the oyster were brought out to the farms (week 33, early August). The timing of the appearance of clinical signs of JOD mirrored the timing and intensity of mortalities (not shown). Roseovarius crassotreae bacteria, however, was only positively identified at one of the farms (NB2) for 2 pools of oysters collected in mid August (week 36). Differences in the mortality patterns due to JOD between farms and years led us to hypothesize either a possible differential effect of environmental parameters or the presence of some other disease or condition triggering mortalities. Therefore, we explored the relationship between environmental factors on oyster growth and mortality.

Some differences in oyster growth between lines are driven by environmental parameters

The four control farms had been chosen to showcase differences in environmental conditions. In general, although environmental parameters were similar on average, the coastal ponds (and in particular CP2, which has the lowest water exchange of all farms) experimented more extreme fluctuations in environmental conditions that the two farms in Narragansett Bay, reaching warmer water temperatures in the summer (max of 29 degrees C versus 25 degrees C) and experiencing the lowest levels of dissolved oxygen (min 3 mg per L versus 5 mg per dL), lowest salinities (min of 5 versus 28 psu), and lowest pH (7.3 versus 7.7). Regression analyses for the growth rate of the 2008 year class revealed a significant negative relationship with pH (p equal to 0.004) for GHP oysters, with 28% of the variability in the growth rate of these oysters related to this parameter. In addition, a positive relationship was detected for HYB (p=0.003) and NEH (p equal to 0.006) oysters and dissolved oxygen (DO, mg per L). Dissolved oxygen explained 13% and 15% of the variability in the growth rate of the NEH and HYB oysters respectively. Backward regression for the mortality of the 2008 and 2009 year classes revealed no direct relationship with any of the seven independent variables for the three lines. Due to the failure in detecting a relationship between mortality and JOD/MSX/dermo prevalence in 2009 or between mortality and the environmental parameters we have measured, we hypothesized that some unknown conditions may have been responsible for the low performance of the 2009 year class oysters in the upwellers. Further research should investigate the potential causes for mortality and poor growth at these farms, including harmful algal blooms.

Summary of results

In conclusion, results from our field trials show that: a) oyster performance was driven by survival in the farms that experienced disease outbreaks, with the disease-resistant line NEH showing overall significantly higher growth and survival than a local stock (GHP) and a hybrid between the two (HYB); b) no significant differences in performance between the lines were seen in farms that did not experience disease pressure; c) oyster growth was driven by a combination of oyster genetics and the environment, confirming that site selection is a key aspect of oyster aquaculture in Rhode Island; and d) the hybrid line between the disease-resistant line NEH and the local stock GHP did not show hybrid vigor, showing overall performance similar to either one of the parental lines. Although the NEH line showed improved growth and survival under disease outbreaks, it may, however, show a higher susceptibility to low dissolved oxygen than the GHP line. We have also identified JOD as a widespread disease problem in Rhode Island farms. Dermo disease was present in manageable levels (low or moderate weighted prevalence). MSX was detected as part of another monitoring program in a few farms in the East portion of Narragansett Bay in 2007 and 2008. Farmers in Rhode Island should consider the NEH line as an additional source of seed, in view of the potential impact of JOD, MSX, and dermo diseases on oyster performance.

Participation Summary


Educational approach:
  • Our outreach efforts included:

    On-site training sessions during site visits to farms in Rhode Island. During the 3 years of the project, we visited 15 farms in Rhode Island at least once a year, and at least 6 more than twice a year. These visits were coordinated with another project funded by the Natural Resources Conservation Services (NRCS) Environmental Quality Incentives Program (EQIP), which involved 14 farmers in Rhode Island. The goals of these visits were to: a) train farmers on how to recognize and manage disease and predator problems; b) train farmers on how to collect oyster performance data; and c) collect samples for disease monitoring and monitoring of environmental parameters. We provided to the farmers brochures describing the major disease and pests of oysters in the region (see appendices 3 and 4), as well as detailed instructions for measuring oyster performance and datasheets (appendix 5).
    Annual presentations at meetings of the Ocean State Aquaculture Association (see appendix 1 for one example).
    Outreach articles on oyster disease management in Rhode Island that were published as part of the annual Aquaculture reports of the Rhode Island Coastal Resources Management Council (see publications list below for link).
    Our extension specialist, Dr. Dale Leavitt, offers an annual course on “Practical Shellfish Farming” that offers individuals the technical information needed to confidently undertake a small shellfish farming enterprise in Rhode Island and nearby areas of southern New England. Dr. Gomez-Chiarri participates in this course by explaining disease management methods specific to the Rhode Island region in one lecture.
    Presentations at several regional and national meetings and trade shows, including the Northeast Aquaculture Conference and Exposition (Portland, Maine, 2008 and Plymouth, MA, 2010), the Milford Aquaculture Seminar (New Haven, Connecticut, February 2008, 2009 and 2010), the annual meeting of the National Shellfisheries Association (Providence, RI, 2008 and Savannah, GA, 2009), Aquaculture 2010 (San Diego); and the WERA099 Shellfish Genetics and Broodstock Management meetings (2008, 2009, and 2010). These presentations were designed to divulgate the results from the project (see publication list below).
    This research also involved training of 4 undergraduate students (Elizabeth Hansel, Nevan Richard, Steven Kloeblen, and Paul Torbett) and one graduate student, Kathryn Markey, who completed her Masters Thesis in 2009. A manuscript is in preparation and will be submitted shortly to a peer reviewed journal such as Aquaculture or Journal of Shellfish Research. The undergraduate students participated in the project as Coastal Fellows. The Coastal Fellowship at the University of Rhode Island ( is a competitive fellowship program that allows undergraduates to participate in research projects under the mentorship of faculty and graduate students. The students receive a stipend during the summer to work in their research projects, and then continue this research during the Fall semester for credit. At the end of the Fall semester, the students present the results of their research in poster format at a symposium celebrated at the University. See appendices 6 and 7 for two examples of posters prepared by the undergraduate students.

    Gomez-Chiarri, M. 2008. Disease considerations for the Rhode Island Aquaculture Plan. In: CRMC Working Group on Aquaculture Regulations. Rhode Island Coastal Resources Management Council.
    Markey, K.R. and M. Gomez-Chiarri. 2007. Survey of major diseases affecting Rhode Island cultured bivalves In: Aquaculture in Rhode Island. 2007 yearly Status Report. Rhode Island Coastal Resources Management Council.
    Markey K., D. Leavitt, K. Tammi, M. Gomez-Chiarri. 2008. Evaluation of the performance of three oyster strains in Rhode Island Farms. Journal of Shellfish Research 27(4):956. (Proceedings of the Milford Aquaculture Seminar).
    Markey K., D. Proestou, J. Korun, D. Leavitt, M. Gomez-Chiarri. 2008. Roseovarius Oyster Disease outbreaks in Rhode Island coastal salt ponds. Abstracts of the Annual Meeting of the National Shellfisheries Association, Providence, Rhode Island, April 2008.
    Markey, K., D. Leavitt, K. Tammi, M. Gomez-Chiarri. 2009. Juvenile Oyster Disease, a (manageable) curse for oyster aquaculture in Rhode Island. Abstracts of the 101st Annual Meeting of the National Shellfisheries Association Savannah, Georgia, March 2009.
    Markey, K.R. 2009. Performance of three lines of the Eastern oyster, Crassostrea virginica, in Rhode Island shellfish farms. Masters Thesis, University of Rhode Island.
    Markey, K., D. Leavitt, K. Tammi, M. Gomez-Chiarri. 2010. Are disease-resistant lines superior performers in Rhode Island oyster farms? Journal of Shellfish Research 29(2):555-556. (Proceedings of the Milford Aquaculture Seminar, February 2010).
    Markey, K., D. Leavitt, M. Gomez-Chiarri .2010. Performance of three lines of the eastern oyster, Crassostrea virginica, at Rhode Island farms: The 2008 and 2009 seasons. Abstracts of Aquaculture 2010, San Diego, CA, March 2010.
    Markey KR and Gomez-Chiarri M .2010. Performance of three lines of the eastern oyster (Crassostrea virginicia) at Rhode Island shellfish farms. Abstracts of the Northeast Aquaculture Conference and Exposition, Plymouth, MA, December 2010. This conference also included a workshop on shellfish disease.
    Markey, K., D. Leavitt, K. Tammi, M. Gomez-Chiarri (in preparation). Performance of three lines of the Eastern oyster, Crassostrea virginica, in Rhode Island shellfish farms. To be submitted to Aquaculture.

Additional Project Outcomes

Project outcomes:

Impacts of Results/Outcomes

The verification process involved: a) interviews and on-site training sessions during site visits to farms participating in this project, as well as another project funded by the Natural Resources Conservation Services (NRCS) Environmental Quality Incentives Program (EQIP) involving 14 farmers in Rhode Island; and b) input received during presentations at regional and national meetings (see outreach efforts below) as well as annual meetings with the Ocean State Aquaculture Association. Although we reached the performance target, the verification process could have been improved by maintaining better written records of farmers’ input. The most useful tool for recruitment and gathering input from farmers was the on-site interviews and training sessions, followed by presentations at the meetings of the Ocean State Aquaculture Association. The on-site visits allowed us to establish a relationship of trust with the farmers, leading to increased farmer participation in the project. This approach, however, was time intensive, and was only feasible due to the small size of the State of Rhode Island. Presentations at the regional and national research conferences and aquaculture trade shows allowed us to reach us other stakeholders, including hatchery managers, state and federal regulators, and researchers working on selective breeding and aquatic pathology. As a result of increased exposure of our research, the University of Rhode Island and Roger Williams University are now part of an East Coast Shellfish Breeding Consortium (appendix 2, see more information in sections below).

As a result of our research, at least 14 farmers are now aware of the impact of diseases such as Juvenile Oyster Disease, MSX, and dermo diseases in Rhode Island farms and of key management techniques that prevent losses due to this disease. These include: using disease-resistant lines, planting early in the season so oysters reach sizes larger than 25 mm by the time water temperatures reach 20°C, and increasing flow rates in upwellers containing oyster seed. Adoption of these techniques was widespread in 2010 in Rhode Island and may have contributed to the absence of reports of significant oyster losses in 2010. Farmer interest in the use of disease-resistant lines increased after we presented a rough economical analysis based on the results from the 2008 – 2009 growing seasons (see below). In 2010, based on the input from the growers, we saved broodstock from the survivors of the disease outbreaks experienced in two of the farms participating in the project to create 2 lines well adapted to the environmental and disease pressure conditions experienced in this area. This will provide farmers in the area with an additional source of oysters to grow in their leases.

Farmer awareness of the importance of selective breeding as a tool to increase performance has also increased after this project. The East Coast Shellfish Growers Association (, which represents more than 1000 farmers from Maine to Florida, considers disease one of the major problems for the industry and is actively lobbying for the creation of an Agricultural Research Services Center in Shellfish Genetics and Breeding, with the support of a consortium of Universities in the East Coast of the US, including the two universities participating in this project (University of Rhode Island and Roger Williams University) (appendix 2). This breeding center will provide the means to establish a shellfish breeding program geared to selection of lines that are well adapted to the diverse growing conditions in the East Coast of the US.

Economic Analysis

The farm gate value of Rhode Island’s oyster aquaculture industry has been increasing steadily since 1999, from a production valued in $160,000 in 1999 to 33 farmers generating a value of more than $1.7 million in 2009 (Alves 2007; Beutel 2009). This increase in value is due to both an increase in the number of farms (from 14 farms and 27 acres in 1999 to 33 farms and 134 acres in 2009), but also to an increase in the productivity of the leases (from $5,925 per acre in 1999 to $13,272 in 2009, a more than 100% increase). The most significant increases in farm productivity occurred in 2005 and 2006, and are thought to be due to the use of more efficient methods of culture, as well as a decrease in unused acreage. The lease agreements with the State of Rhode Island stipulate that leases that are not actively farmed will be revoked.

We determined the potential differences in economic profit among lines and farms at the end of the 2009 sampling season, based on the number of oysters reaching market size and the total cumulative mortality. The highest profit was consistently obtained with the oysters from the NEH line, regardless of the farm (Table 2). At the Narragansett Bay farms, the high percentage of oysters reaching market size for NEH was enough to override the negative impact of high mortality due to disease. The improved performance of the NEH led to a 2 to 4 fold-increase in the total profit compared to the line that ranked second in performance (GHP or HYB, depending on the farm).

Farmer Adoption

We discovered in this project that most experienced farmers in Rhode Island were already aware of disease problems affecting their farms and have devised site-specific management strategies to prevent losses. For example, we were not aware of the impact of Juvenile Oyster Disease at NB2, one of the farms in Narragansett Bay, until the farmer pointed out that he never moves his seed from the upweller to the lease site until the seed is large enough to withstand the disease (more than 25 mm). In order to confirm the potential impact of JOD at this site, we moved small seed (less than 25 mm) to the lease in 2008, and determined that JOD was responsible for large losses experienced in the seed from the project. The input from this farmer allowed us to test the performance of the disease-resistant line NEH during a JOD outbreak. Most of the farmers learn by (painful) experience how to deal with losses due to disease and maximize productivity of their leases. Like one of the most experienced farmers said: “You are not an oyster farmer until you loose your first million (oysters)”. In general, farmers were much more receptive after they experienced disease losses or hear from other farmers about disease outbreaks, so we had much more interest in the project after the low performance year of 2009. We also learned that the “Practical Shellfish Farming” course offered by Dr. Leavitt and our presentations at the Ocean State Aquaculture Association are the best venues to educate new crops of farmers that have little or no experience. This has resulted in new farmers putting more care into considering site selection and seed source, and decreased the amount of time that is required for a lease to be productive.

Assessment of Project Approach and Areas of Further Study:

Areas needing additional study

This research emphasized the importance of selective breeding as a tool for improvement of oyster aquaculture. The USDA Agricultural Research Services has committed funds for starting an East Coast Shellfish Breeding program in collaboration with the East Coast Shellfish Breeding Consortium, and is planning to hire a quantitative geneticist and shellfish breeder that will be housed at the University of Rhode Island. The East Coast Shellfish Growers Association is actively lobbying for the funds required to expand this program. The goal of future work is to unify the fragmented efforts in shellfish breeding occurring through the East Coast of the US. Ongoing and future research will focus on family-based selective breeding efforts and performing extensive field trials evaluating the performance of these new families through the different growing environments of the East Coast of the US. Research will also focus on employing newly developed genetic tools to explore the genome of oysters to identify genetic markers related to higher survival and growths. This could greatly accelerate selective breeding efforts to develop strains that survive longer, grow faster and have improved production traits (for more information, see


We dedicate this report to the fishermen and farmer Louis Ricciarelli, who tragically died in 2009 while diving for hard clams. We thank all the farmers that participated in this project, as well as the undergraduate and graduate students at the University of Rhode Island that helped in the collection and analysis of data. Special thanks to Karin Tammi and the students at the Roger Williams University shellfish hatchery, who made this project a reality by spawning the broodstock oysters and getting the oyster seed ready for deployment. We also thank the Ocean State Aquaculture Association and the East Coast Shellfish Growers Association for their support.

  • Allen, S.K., Gaffney, P.M., Ewart, J.W. 1993.Genetic Improvement of the Eastern Oyster for Growth and Disease Resistance in the Northeast. Northeast Regional Aquaculture Center Fact Sheet No. 210.
    Alves, D. Annual Aquaculture Report: 2007. Coastal Resource Management Council: 2007 Annual Report on Aquaculture in Rhode Island.
    American Fisheries Society Fish Health Section. Bluebook. Suggested procedures for the detection and identification of certain finfish and shellfish pathogens. American Fisheries Society, Bethesda. US.
    Barber, B.J., Carnegie, R.B., Davis, C.V., Mook, W. 1996. Effect of timing of seed deployment on growth and mortality of oysters, Crassostrea virginica, affected by juvenile oyster disease (JOD). Journal of the World Aquaculture Society 27:443-448.
    Beutel, D. Annual Aquaculture Report: 2009. Coastal Resource Management Council: 2009 Annual Report on Aquaculture in Rhode Island.
    Burreson, E.M. and Ford, S. E. 2004. A review of recent information on the Haplosporidia, with special reference to Haplosporidium nelsoni (MSX). Aquatic Living Resources 17:499-517.
    Byron, C., Bengtson, D., Costa Pierce, B., Link, J., Rheault, R., Beutel, D., Alves, D. Working toward consensus: Application of shellfish carrying capacity in management of Rhode Island aquaculture. Abstracts of the Northeast Aquaculture Conference &amp; Exposition, Plymouth, MA, December 2010.
    Castagna, M., Gibbons, M.C., Kurkowski, K. 1996. Culture: application. In: Eble, A.F. (Ed.), The Eastern Oyster. Maryland Sea Grant College, College Park, MD, pp 675-687.
    Davis, C.V. &amp; Barber, B.J. 1994. Size-dependent mortalitiy in hatchery reared populations of oysters, Crassostrea virginia, Gmelin 1791, affected by juvenile oyster disease. Journal of Shellfish Research. 13:137-142.
    Davis, C.V., and Barber, B.J. 1999. Growth and survival of selected lines of eastern oysters, Crassostrea virginica (Gmelin 1791) affected by juvenile oyster disease. Aquaculture 178:253-271.
    Ewart, J.W. and Ford, S.E. 1993. History and Impact of MSX and Dermo Diseases. Oyster Stocks In the Northeast Region. Northeast Regional Aquaculture Center Fact Sheet No. 200.
    Gomez-Leon, J., Villamil, L., Salger, S.A., Sallum, R.H., Remacha-Trivino, A., Leavitt, D.A, and Gomez-Chiarri, M. 2008. Survival of the Eastern Oysters, Crassostrea virginica, from three lines following experimental challenges with bacterial pathogens. Diseases of Aquatic Organisms 79:95-105.
    Guo, X., Ford, S.E., DeBrosse, G., Smolowitz, R., and Sunila, I. 2003. Breeding and evaluation of eastern oyster strains selected for MSX, Dermo and JOD resistance. Journal of Shellfish Research 22: 353–354.
    Maloy, et al., 2005. A PCR-based diagnostic assay for the detection of Roseovarius crassostrea in Crassostrea virginica affected by juvenile oyster disease (JOD). Diseases of Aquatic Organisms 67:155-162.
    Preisendorfer, R.W. 1986. Secchi Disk Science: Visual Optics of Natural Waters. Limnology and Oceanography, 31:909-926.
    Rice, M.A., A. Valliere, A. Caporelli (2000) A review of shellfish restoration and management projects in Rhode Island. Journal of Shellfish Research 19:401-408
    Shumway, S.E., Davis, C., Downey, R., Karney, R., Kraeuter, J., Parsons, J., Rheault, R., Wikfors, G. (2003) Shellfish aquaculture. In praise of sustainable economies and environments. World Aquaculture 34:15-17
    Stokes, N. A., Siddall, M.E., Burreson, E.M. 1995. Detection of Haplosporidium nelsoni (Haplosporidia: Haplosporidiiae) in Oysters by PCR Amplification. Diseases of Aquatic Organisms 23:145-152.
    Villalba, A., Reece, K.S., Ordas, M.C., Casas, S.M., Figueras, A. 2004. Perkinsois in molluscs: A Review. Aquatic Living Resources 17:411-432
Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.