NOAA/SeaGrant Oyster Disease Program (ODRP): Improvement of Disease Resistance and Understanding of Summer Mortality in Pacific Oysters

Purpose or Scope:

Over the last ten years, in response to input from the shellfish farming community and tribes, the Pacific Shellfish Institute and several west coast research institutions have collaborated on critical disease and health issues affecting the Pacific oyster. The research was initiated with local and national SeaGrant funded investigations of the Pacific oyster summer mortality syndrome, a condition which continues to cause significant and recurring economic harm to the industry. On the U.S. west coast mortalities typically begin to occur in diploid and triploid oysters during the 2nd year of growout, often when the oysters are nearing market size. These mortalities have a significant monetary impact on individual aquaculture producers. For example during a one month period in 2004 and 2005, three major growers in Willapa Bay and south Puget Sound, Washington reported average losses of market-ready diploid and triploid oysters of 35% to 55% on beds ranging in size from 5 to 10 ha, with a combined production loss of $455,000 per year. In California, annual losses of seed oysters have ranged from 13 to 90% per mortality episode since 1993. Cumulative losses during the summer ranged between 52 to 63%, depending on the year. The coast-wide impacts include not only well-documented mortalities and farm closures, but also the lost production due to changing environmental conditions that lead to poor growth and sub-optimal culture conditions.

Approach:

Since 1998, the Pacific Shellfish Institute, in collaboration with other west coast research organizations, has been conducting Oyster Disease Research Program (ODRP) funded research on the oyster summer mortality disease. This research has fundamentally improved our understanding the disease and the causative factors. These studies reveal the complexity of the summer mortality disease, and demonstrate the influences of environmental stressors, genetic selection, ploidy, infectious disease, and farm management on oyster survival, growth and other production related traits. We have: 1) described environmental factors triggering a mortality-inducing stress response; 2) begun research to assess survival and yield in selectively bred and hybrid families; 3) identified the relationships of culture practices to oyster survival, including the influence of elevation above the substrate; 4) assessed responses of oysters to bacterial pathogens and potentially harmful phytoplankton; and 6) worked with shellfish growers to better examine the extent and nature of mortality events on commercial oyster beds. Paralleling the research on U.S. west coast mortalities, a heightened incidence of summer mortality in the oyster producing regions of western France prompted the Institut français de recherche pour l'exploitation de la mer (French Research Institute for Exploitation of the Sea or IFREMER) to conduct a multidisciplinary research program termed MOREST (Mortalités Estivales). Employing specialists from a wide variety of disciplines, the MOREST team identified a number of key summer mortality risk factors, and tested selected strains of summer mortality resistant and sensitive animals.

Progress and Results:

Past field and laboratory observations and findings illustrate the influence of complex interactions between the host, one or more pathogens and numerous environmental factors in the summer mortality disease. It is clear from our observations and those made by MOREST that variations in the timing and intensity of peak water and air temperatures can alter the onset and perhaps the severity of summer mortality episodes. A primary consideration related to increased mortality at several of our Washington sites was heavy siltation and reduced dissolved oxygen. In general, the highest mortalities occurred between from late July/early August to late October sample periods. The DO levels were lowest and siltation rates were highest during this same period. Low DO and elevated temperatures may also be correlated with heavy losses of on-bottom cultured oysters.

Triploid and diploid oysters appear to have different optimal environments and definition of their respective requirements and responses is essential for maximizing their survival and productivity. Field experiments and grower observations have demonstrated sporadic high triploid mortalities and wide variation in survival compared with diploid oysters planted at the same locations. Overall, triploid oysters grown at Puget Sound test sites in 2000-2006 experienced a cumulative mortality of 8 to 28 percent above the mortality rates of diploid oysters planted in comparable plots.

An important finding of past research is that the ability of oysters to survive episodes of summer mortality is in part genetically determined and perhaps can be enhanced through selective breeding of families showing high survival traits. Currently, we are producing Pacific oysters for the dual purpose of producing lines of oysters resistant to summer mortality and to quantitatively assess reproductive allocation in oysters growing at different locations with varying susceptibility to summer mortality. In early summer 2006, PSI and Taylor Shellfish Farms placed 30 lines consisting of replicated arrays of diploid and triploid Pacific oysters contained in shellfish cages and reared on-bottom at project sites in south Puget Sound, Willapa Bay and Hood Canal, Washington for a test period of up to twenty-four months. These were produced in 2005 from diploid founder lines that survived an intensive mortality event in the summer of 2004 using protocols developed in the course of a Western Regional Aquaculture Center crossbreeding project. Variable mortality rates within these lines were reported within one month from at least one site, and all families will be reared through summer of 2007, by which time we expect summer mortality to have impacted the South Puget Sound and Willapa Bay sites. Interim measurements made on size at age and mortality include:

In addition the project team: 1) identified the relationships of culture practices to oyster survival, including the influence of elevation above the substrate; 2) assessed responses of oysters to bacterial pathogens and potentially harmful phytoplankton; and 3) worked with shellfish growers to better examine the extent and nature of mortality events on commercial oyster beds.

Outputs from this project will be to: 1) increase understanding of the etiology and pathogenesis of the summer mortality syndrome in Pacific oysters; 2) identify predictors of mortality and environmental stressors which may be utilized by growers to manage shellfish crops for reduced summer mortalities; 3) characterize hatchery production and broodstock selection methods in relation to observed summertime mortalities on growing areas; and 4) reduce the present economic losses now occurring on West Coast shellfish farms and increase their operational and economic efficiencies.

Publications and presentations of project results are available. The materials listed below may be viewed in PDF format (file sizes are indicated).

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