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Updated: Jan 25

Monitoring data shows that our waters are trending warmer. How might this effect our local oysters ?

As winter gives way to spring, oyster growers get excited for the start of the fast growing season. Seed purchased the previous season that grew to pre-harvest size in the fall will quickly reach market size and be ready for harvest and sale the following spring. Warmer waters in the spring translate into more algae in the water for oysters to eat. If warmer spring waters benefit oyster growth, might warmer waters year-round be a positive trend? Indeed, warmer trends will certainly enhance the growing season allowing operators to turn more product for profit.


Vibrio Forecasting Tool Vibrio parahaemolyticus (Vp) forecast models produced by NOAA are being improved and expanded to help both consumers and industry stay informed of (Vp) trends.

Yet, the same warm water that promotes algae growth also sets the stage for the undesirable growth of certain bacteria. Vibrio species are natural to our coastal waters. During periods of elevated concentrations (typically mid-late summer), specific strains are a public health concern. V parahaemolyticus (Vp) is the most common strain of Vibrio associated with infections from consuming raw seafood.

The Virginia Department of Health Division of Shellfish Sanitation (VDH DSS) is responsible for regulating the shellfish industry to reduce consumer risk. VDH officials visit operators every 6 months to inspect equipment, review records, and grant operational certificates for compliance. For example, oyster tags are required for all harvests. Tags follow oysters from harvest to the point of consumption and are the means of tracking their origin and facilitating product recalls if an outbreak of infection occurs. 

While this might keep others from getting sick from the same harvest, what about prevention? News of infection is devastating to the industry and keeping people well in the first place is obviously preferred. For this purpose, VDH also samples oysters for water quality and determines where and when one can harvest. For example, specific areas and times are off-limits for harvesting in order to control high-risk Vibrio exposure. Adhering to these rules and regulations is key to ensuring public health. But growers can also use best practices to lower Vibrio concentrations and your Lynnhaven Oyster Club operation is aligned with the latest research. 

Researchers with the Alabama Agricultural Experiment Station at Auburn University’s Shellfish Research Lab investigate how handling practices, gear type, and geographic region can affect the levels of Vibrio in farmed oysters, as well as determining the re-submersion period required to return elevated levels of Vibrio in harvested oysters to normal.

Study results show that all Vibrio types return to ambient levels after 7-14 days of re-submersion, regardless of handling type, gear type or region. This means that following routine handling treatment, farmers should allow the oysters to remain in the water for 14 days before harvesting for human consumption.

These research findings demonstrate that unhealthy Vibrio counts are influenced more by non-aquatic environmental factors and processing time incurred after harvest than the naturally occurring in-water concentrations.

LOC adheres to this best practice by separating all handling and processing steps from harvesting activity. Moreover, because of LOC’s size, we are able to work oysters in-water and in numbers that allow oysters to go from water to ice immediately, thereby suspending any Vibrio growth. Maintaining refrigeration temperature from point of harvest to end consumption is the next step in keeping Vibrio levels safe. A typical large-scale operator either brings large quantities of oysters on deck to sort, clean and package, or transport to a land-based processor to do the same. These activities increase risk by adding to the total time between the oyster’s in-water status to when the product can be put on to ice or into mechanical refrigeration.  

Bottom Line:

Vibrio’s ability to multiply in warm environments is in opposition to the longer growing season that might result from warming trends. In other words, what might extend the oyster growing season might also close it to harvest. With climatic warming trends, positive and negative changes will result. While the industry can celebrate positive outcomes such as faster return on investment, operators can also responsibly respond ahead of government oversight and build best practices into their businesses. This way, the industry can demonstrate self-regulation and the cost of operations from necessary regulation and oversight is minimized.

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