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Managing Diseases in Shellfish Aquaculture Farms in Rhode IslandBy Marta Gómez-Chiarri and Dale Leavitt July 30, 2007. The phone rings. It’s Dale Leavitt, the Rhode Island Aquaculture Initiative (RIAI) shellfish extension agent. “Marta, one of our farmers is losing a lot of seed oysters in his nursery. Probably around a million; they are about half an inch in size. Can we meet at the site this afternoon to take some samples for testing?” As a shellfish pathologist, this is the type of phone call we hope never comes. But this is an important part of our jobs. The aquaculture programs at the University of Rhode Island (URI) and Roger Williams University (RWU), through research, outreach, and extension projects, provide support to a rapidly growing state shellfish industry that continues to experience annual percent increases of farm gate value in the double-digits, as observed during the last decade. One of the major challenges facing the bivalve aquaculture industry in Rhode Island (mostly oysters with a few hard clams) is disease. In the last century, several diseases caused by bacterial or parasitic pathogens have devastated the oyster industry in the United States, resulting in millions of dollars in losses. The major disease threats to the American oyster in New England are dermo disease, caused by a microscopic, single-celled parasite named Perkinsus marinus; MSX, caused by another microscopic parasite named Haplosporidium nelsoni; and the culprit of the disease outbreak that killed millions of seed oysters in Rhode Island during the summer of 2007, juvenile oyster disease or JOD, caused by a bacterium recently named Roseovarius crassostreae. Although losses due to disease have been rare in quahogs, in recent years another microscopic parasite named QPX (quahog parasite unknown) has been associated with mortalities, primarily in farmed quahogs in Massachusetts, Rhode Island, New York, New Jersey, Virginia, and eastern Canada. It is difficult to accurately estimate losses of shellfish to disease, but in Rhode Island, levels of mortality of 10 to 20 percent are considered normal, losses of about 50 percent are not unusual, and there is at least one report of a farm losing 100 percent of the shellfish to disease. Our approach to helping Rhode Island shellfish farmers manage disease is multi-pronged and builds on our research program, studying the mechanisms used by oysters to defend themselves against infectious diseases. Few tools are available to treat and prevent disease in oyster farms. Unlike vertebrates, oysters cannot be vaccinated. Furthermore, oysters cannot be treated during disease outbreaks, since oysters are grown in open waters and there are environmental and efficacy concerns to the release of drugs into the aquatic environment. Diseases like dermo, JOD, and QPX are difficult to avoid, since the pathogens are abundant in local waters during the warm summer months. The most commonly used tools to manage infectious diseases in shellfish aquaculture include constant monitoring of wild and farmed populations to watch for unusual mortalities, the use of common-sense strategies and regulations that prevent the introduction and transport of diseased shellfish to minimize the risk of spreading diseases, and good knowledge of the environmental conditions that trigger disease outbreaks. In order to be effective, these tools should be based on up-to-date knowledge on the distribution of diseases in both wild and farmed bivalve populations. With funding from the R.I. Department of Environmental Management, RIAI, and the U.S. Department of Agriculture (USDA) Natural Resources Conservation Service Environmental Quality Incentives Program, we have been monitoring diseases in bivalve populations in Rhode Island since 1998. These monitoring programs have resulted in a wealth of information that has been used in management decisions, such as regulating the transfer of seed and adult bivalves within state waters to avoid the spread of disease from “hot-spots” to “clean” areas. This information is available in the map gallery at the www.narrbay.org website, under static maps. However, monitoring, regulation, and good husbandry practices are not enough to prevent diseases from occurring at shellfish farms. One of the most environmentally and economically sustainable health management tools to counter the impact of disease in shellfish is the use of disease-resistant strains. Strains of oysters resistant to dermo, MSX, and/or JOD are being developed through selective breeding programs at universities, such as Rutgers and the Virginia Institute of Marine Sciences, and at a few commercial hatcheries. Few of these strains are of New England origin, and given the more southern source of most of the commercially available disease-resistant strains, the question arises as to whether these strains will perform adequately in more northern waters, such as Rhode Island. If disease resistance in oysters is gained at the expense of overall growth rate, winter tolerance, or some other production characteristic necessary for growing oysters in New England, then the gains achieved through selection for disease resistance may be lost in poor production. Moreover, none of these strains had been tested in local conditions, so Rhode Island farmers didn’t know how they would perform in their farms. We sought funding from the RIAI to test the performance of two disease-resistant oyster strains (the MSX and dermo-resistant Rutgers’ NEHY and the JOD-resistant strain from Frank M. Flowers and Sons hatchery in Long Island) plus a native strain from Green Hill Pond (GHP) at three shellfish farms in Rhode Island. Green Hill Pond in Charlestown, R.I., was chosen as the source for a representative of a local strain that could have developed resistance to dermo and MSX, since severe outbreaks of these two diseases have been observed in the pond in the past. We also used controlled experimental infections of oysters with the parasite that causes dermo and the bacterium that causes JOD in the Gómez-Chiarri laboratory at URI. Overall, no significant differences in performance were observed among the strains in the field-testing, and the “best performer” strain was different at each farm. These results indicate that strain performance is influenced by the environmental conditions at each farm and the growing techniques used by the farmer. However, some common patterns regarding survival were observed between farms: In all farms, the Flowers strain experienced more mortality than either the NEHY or the local GHP strain. This observation was also consistent with the results from the experimental laboratory infections, which showed that the Flowers strain was more susceptible to dermo than the NEHY or the GHP strains. Interestingly, the NEHY strain, a strain that supposedly has never been exposed to JOD, showed the most resistance to experimental infection with the bacterium that causes JOD and to infection with vibrios, a group of bacterial pathogens that commonly cause mortality in shellfish hatcheries. The next logical step in the process is to develop a local oyster strain that has disease resistance and is well adapted to local growing conditions, one that we plan to name “Rhodoyster” in honor of the late Luther Blount, a well-known lover of oysters and supporter of oyster research in Rhode Island. We are planning to exploit the phenomenon of “hybrid vigor,” which has been observed in oysters and other agriculturally im-portant species (corn is the best known example). We are hoping that by crossing the disease-resistant strain NEHY with the local GHP strain, we will produce a hybrid that will perform better in Rhode Island than either of the parental strains. With funding from the USDA Northeastern Sustain-able Agriculture Research and Education Program, we are in the process of producing the NEHY x GHP hybrid at the RWU shellfish hatchery and will involve as many local and regional farmers as possible in the testing of the performance of the strain. If this hybrid shows better performance and higher disease resistance than existing strains, farmers will benefit economically by increasing production and minimizing the probability of having catastrophic losses to disease. For more information: See the R.I. Coastal Resources Management Council Working Group on Aquaculture Regulations: Subcommittee on Biology, “Report on Biological Impacts of Aquaculture” at www.crmc.state.ri.us/projects/aquaculture.html for more information on regulations regarding disease in shellfish aquaculture farming in Rhode Island. A map showing the results from surveys on prevalence and intensity of diseases in Rhode Island wild and farmed oyster populations is available at www.edc.uri.edu/fish/imagemaps.html. A summary of the results from the surveys can be found in “Aquaculture in Rhode Island: 2006 Yearly Status Report,” found at www.crmc.state.ri.us/pubs/pdfs/aquareport06.pdf. Ewart, J.W. and S.E. Ford. 1993. History and impact of MSX and dermo diseases on oyster stocks in the Northeast region. Northeastern Regional Aquaculture Center Fact Sheet No. 200, found at www.nrac.umd.edu/files/Factsheets/fact200.pdf. Allen, S.K., P.M. Gaffney, and J.W. Ewart. 1993. Genetic improvement of the Eastern oyster for growth and disease resistance in the Northeast. Northeastern Regional Aquaculture Center Fact Sheet No. 210, found at aquanic.org/publicat/usda_rac/efs/nrac/nrac210.pdf. Ford, S.E. and M.R. Tripp. 1996. Diseases and defense mechanisms. Pp.581–660. In: V.S. Kennedy, R.I. Newell, and A.E. Eble (eds.), The Eastern Oyster, Crassostrea virginica. Maryland Sea Grant, College Park, Md. Harvell, C.D., K. Kim, J.M. Burkholder, R.R. Colwell, P.R. Epstein, D.J. Grimes, E.E. Hofmann, E.K. Lipp, A.D. Osterhaus, R.M. Overstreet, J.W. Porter, G.W. Smith, and G.R. Vasta. 1999. Emerging marine diseases—climate links and anthropogenic factors. Science 285:1505–1510. —Marta Gómez-Chiarri is a URI Fisheries, Animal and Veterinary Science Associate Professor; Dale Leavitt is an RWU Marine Biology Assistant Professor and Aquaculture Extension Specialist.
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