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Abstracts
Workgroup: Frontiers in Disease Research
Mortality of the oyster, Crassostrea gigas: health screening, environmental links and management options
Principal Investigator(s):
Dr. Dan Cheney, Pacific Shellfish Institute, cheneydp@compuserve.com
Co-Investigator(s):
Dr. Ralph Elston, Pacific Shellfish Institute
Dr. Gary Cherr, University of California at Davis, Bodega Marine Laboratory
Dr. Gary Cherr, University of California at Davis, Bodega Marine Laboratory
Funding Period: 12-1-97 to 11-30-99
Research was begun in Washington and California to characterize the Pacific oyster, Crassostrea gigas, summer mortality disease in a variety of culture conditions and locations, and to describe the relationship to infectious diseases, and identify water quality and seasonal patterns. Also, as a continuing study was a field component to investigate the oyster thermal stress response and an assessment of induced thermal tolerance to reduce mortalities. There were obvious differences in the disease rates, with triploid oysters having consistently higher mortality rates than diploid oysters planted in comparable plots. Mortalities trended toward higher rates at or immediately after neap tides when DO was lowest and during periods of elevated air and water temperatures. A long period of neap tides with low and slack water during the evening was observed to result in daily and successive reductions in DO to levels ranging from 0.5 and 2 mg/L. The DO reductions were sometimes coupled with heavy macroalgae blooms and high phytoplankton densities. Low daytime tides coupled with intense insolation resulted in very high ambient air temperatures and elevated water temperatures on the incoming tide. During the summer 1998, peak temperatures neared 53 C (127 F) and frequently exceeded 40 C during low-tide exposure at several of the Washington sites. Relative densities of the phytoplankton Akashiso sanguinea, Ceratium spp., and Psuedo-nitzschia spp. were higher during the same late summer period of elevated mortalities; however, no mortalities of larval or juvenile oysters occurred when challenged with cultured Akashiso sanguinea. Systemic bacterial infections in moribund oysters were observed to uniformly result from gram negative rod-shaped bacteria. These infections were believed to occur in oysters weakened by the damage to the digestive gland. Finally, field data suggested that oysters which experienced chronic summer stress, responded by acquiring some degree of thermo-tolerance, but were unable to mount a complete stress response as observed in non-stressed animals. This inability to respond to additional acute stress appeared to be correlated to increased summer mortality.
IMPACTS and/or BENEFITS:
It is likely that Pacific oysters at the west coast study sites experienced varying degrees of chronic stress due to multiple environmental factors. This work indicated oyster summer mortality rates were strongly influenced by elevated air and water temperatures, lowered dissolved oxygen levels, and oyster ploidy. An inability to respond to additional acute stress appears to be correlated to increased summer mortality. As such, we proposed that the stress response can be used as an "early warning" indicator of summer mortality in Pacific oysters. Our evaluation of the data from two seasons of relatively high mortalities supported earlier reports on the rate and timing of mortality events. In addition, we identified and presented management practices for commercial cultivation as possible measures to reduce the frequency and extent of oyster losses.
PROJECT PUBLICATIONS:
Clegg, J.S., Uhlinger, K.R., Jackson, S.A., Cherr, G.N., Rifkin, E., and Friedman, C.S. 1998. Induced thermotolerance and the heat shock protein-70 family in the Pacific oyster, Crassostrea gigas. Molecular Marine Biology and Biotechnology 17(1):79-83.
Friedman, C.S.,Cherr, G.N., Clegg, J.S., Hamdoun, A.H., Jacobsen, J.L., Jackson, S.A., and Uhlinger, K.R. 1999. Investigation of the stress response, summer mortality and disease resistance of oysters, Crassostrea spp. Journal of Shellfish Research 18(1):297.
Friedman, Carolyn S.; Shamseldin, Ally; Pillai, Murali; Olin, Paul G.; Cherr, Gary N.; Jackson, Susan A.; Rifkin, Erik; Uhlinger, K. R.; and Clegg, James S. 1997. Summer mortality and the stress response of the Pacific oyster, Crassostrea gigas Thunberg. Journal of Shellfish Research. 16(1): 335. (related research, not directly supported by this project)
Shamseldin, Ally A.; Clegg, James S.; Friedman, Carolyn S.; Cherr, Gary N.; and Pillai, Murali C. 1997. Induced thermotolerance in the Pacific oyster, Crassostrea gigas. Journal of Shellfish Research. 16(2):487-491. (related research, not directly supported by this project)
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