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Abstracts
Session: The State of Oyster Disease
Summer Mortality of the Pacific Oyster, Crassostrea gigas, along the West Coast of the U.S.: Is oyster herpes virus associated with losses of seed?
Presented By:
Carolyn Friedman and Chris Burge, University of Washington, carolynf@u.washington.edu
Cheney, D., Elston, R.A., Suhrbier, A., Pacific Shellfish Institute
Cherr, G.N., Griffin, F.J., Hamdoun, A., Braid, B.A., Bodega Marine Lab, UC Davis
Langdon, C., Hatfield Marine Science Center, OSU
Judah, L.R., Wilkerson, F.P., Romberg Tiburon Center, SFSU
Barber, B., University of Maine
Burreson, E.M., Stokes, N.A., Reece, K.S. Virginia Institute of Marine Science
Cheney, D., Elston, R.A., Suhrbier, A., Pacific Shellfish Institute
Cherr, G.N., Griffin, F.J., Hamdoun, A., Braid, B.A., Bodega Marine Lab, UC Davis
Langdon, C., Hatfield Marine Science Center, OSU
Judah, L.R., Wilkerson, F.P., Romberg Tiburon Center, SFSU
Barber, B., University of Maine
Burreson, E.M., Stokes, N.A., Reece, K.S. Virginia Institute of Marine Science
Mortality of the Pacific oyster, Crassostrea gigas, has occurred in the Pacific Northwest and Japan since the mid 1950's and has only recently affected oysters in California beginning in 1993. Multiple stressors have been associated with these mortality events. In an attempt to alleviate the >50 annual oyster mortality observed in California and variable losses in Washington state, we examined the interaction between survivorship, growth and stress response of family lines from the Molluscan Broodstock Program (MBP) of Oregon State University, and planting time and height, and selected environmental parameters. Three oyster families were each planted during Fall 1999-2001 and Spring 2000-2002 at 2-3 sites in California, 3 sites in Washington (Spring only), and 1 site in Oregon (Spring 2002 only). Fall plants survived significantly more than did oysters planted in the spring (p<0.05) in California. In addition, two families (one commercial strain and MBP family 10-115) outperformed MBP family 10-116 (p<0.001) at all locations. During the study period inter-annual variation in phytoplankton community structure was more pronounced than spatial variation. Distinct seasonal patterns of community structure also emerged and were typical of temperate estuarine communities. While suspected harmful algal species were present throughout the study period, phytoplankton did not appear to be directly involved in oyster mortalities. However, in 2000 California mortality coincided with a Gymnodinium sanguineum bloom, while 2001 and 2002 mortalities were not associated with any phytoplankton bloom. Extreme temperature and dissolved oxygen fluctuations were repeatedly associated with oyster mortalities at the Washington and California study sites.
At the Tomales Bay, California site during 2002 oyster herpes virus was found infecting C. gigas seed with prevalence ranging between 3.3 and 43. Oyster herpes virus has been reported from various species of oyster larvae and juveniles in French hatcheries in 1992 and subsequently claimed to be associated with high mortality rates, and "...abnormal mortality and morbidity." At the Tomales Bay site there was some correlation between mortality and herpes virus prevalence in the families, although this potential link will clearly require further investigation and surveillance. We are currently working on an east coast/west coast collaborative project examining Eastern (Crassostrea virginica), Pacific (C. gigas) and Suminoe (C. ariakensis) oysters for presence of the herpes virus using molecular and histological methods. During 2002 we screened spat and larvae from Maine to Louisiana along the US east and Gulf coasts, and along the US west coast in Washington and California. The purpose of this project is to determine if the virus is present in oysters in the United States and our findings to date have indicated that most U.S. stock of oyster larvae and spat are free of oyster herpes virus. Therefore, it is essential to further examine the significance of this positive finding in Tomales Bay. There has been some international resistance to listing the oyster herpes virus disease with the Organization Internationale Epizooties in 1995 (OIE, an international advisory organization) as a Notifiable disease (disease of highest significance that should be geographically contained) with the argument that the virus is widespread in oysters throughout the world. This project was initiated to further evaluate whether or not OIE listing or other management steps are needed for either exclusion or containment of the disease in North America.
We are employing molecular and histological assays for detection of the herpes virus. Sequence information and positive control material were provided to us by collaborators from IFREMER (T. Renault). The molecular methods were validated and initial U.S. collected field samples were examined during 2002. We are using both standard and nested PCR assays to ensure that we maximize the chance of detecting any strain variants of the virus. Selected samples are also evaluated by histology and in situ hybridization with DNA probes to localize and verify infection of oyster tissue. The positive PCR result from California indicats that it is of critical time importance to establish the nature and magnitude of this threat to U.S. producers of oysters by answering the following questions: (1) Does the virus exist in and infect oysters in various regions of the United States and, (2) in locations in which it does exist, does the herpes-like virus constitute a significant risk to U.S. fishery resources? Whether a difference in prevalence reflects differential susceptibility of different oyster stocks needs to be assessed.. This information is needed to make appropriate recommendations regarding management of the virus and to take steps to prevent its spread, if oyster herpes viruses are found to be highly localized in occurrence in the U.S., and of significance to the oysters' health.
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