Hazards of Shellfish Consumption and Risk Management

Bivalve shellfish such as oysters and mussels are filter feeders, extracting microscopic particles from the surrounding waters. This means that they may absorb chemicals or microorganisms like bacteria and viruses as well as toxins produced by phytoplankton that could pose health risks to people from the water that they grow in. It is well documented that the condition of the land and water surrounding where shellfish grow and other environmental variables can greatly influence the risks associated with the consumption of raw or partially cooked shellfish. 

To minimise and manage health risks, the Shellfish Market Access Program (ShellMAP) establishes, reviews and monitors environmental management triggers or thresholds such as salinity, 24hr, 72hr or 7 day cumulative rainfall and river flow that can be statistically linked to microbiological levels in the water or the shellfish (Figure 1). The thresholds are monitored daily and reviewed annually through a structured process using data collected over a set period of time.

When these thresholds are exceeded, the risk of contamination to the shellfish is deemed too high and the area is temporarily closed to harvesting. Growers must also refrain from harvesting shellfish if they are aware of other possible food safety risks that may be present. Shellfish harvest areas are often managed on more than one trigger, as each trigger represents differing levels of risk that may be independent of the others. ​

Figure 1: ShellMAP environmental managment monitors a 
variety of influences in shellfish growing catchments

Potential sources of shellfish health risks that commonly impact Tasmanian shellfish growing areas and lead to temporary closures include:

  • Rainfall: Rain produces stormwater runoff which may transport pollution into shellfish harvest waters.
  • River Flow: Rivers can transport run-off from further up the catchment and affect a shellfish harvest area even if there has been no localised rainfall in the growing area. 
  • Agricultural/Urban run-off: Run-off from agricultural and urban sources can contain different types of pollution, from livestock waste to chemical pollutants. Ephemeral creeks may also serve as channels for localised run-off. This is usually associated with rainfall events.
  • Sewage Spills: Wastewater treatment facilities and human waste inputs from septic systems or public facilities can transport viruses into shellfish harvest waters. ShellMAP has developed a framework for managing the risk from sewage spills.
  • Increased Air and Water Temperatures: Increases in air and water temperature promote the growth of potentially harmful pathogenic organisms such as Vibrio parahaemolyticus, or Vibrio vulnificus. 
  • Wildlife:  Roosting birds may be a cause of potential pollution in the absence of weather events
  • Recreation/Gatherings i.e. boating, galas: Large gatherings of people and vehicles can cause unexpected pollution sources to shellfish harvest areas.  

Following a closure, growers usually provide water and or shellfish meat samples for laboratory testing as soon as possible. Shellfish harvest areas can be reopened when sampling/testing demonstrates that the water/meat quality parameters meet the requirements outlined by the national standards.

Standard tests include:

  • E. coli/faecal coliforms: These organisms are used as faecal bacteria indicators.
  • Vibrio sp:  Vibrio control plans focus around temperature management, but it can be tested for in the case of confirmed illness outbreaks.
  • Biotoxins: Harmful algal blooms (HAB) can cause temporary area closures as some algae species produce biotoxins and may cause the shellfish to become toxic.
  • Salinity: Salinity can be used to demonstrate that the residual risk of freshwater impact from runoff and/or river flow to harvest areas is low.

Adverse Pollut​ion Condition sampling

Identifying and implementing adequate food safety control measures helps to protect consumers and businesses by reducing or eliminating​ potential hazards when harvesting shellfish. The Australian Shellfish Quality Assurance Program (ASQAP)  provides the “National guidelines for managing the risks in the harvesting, relaying, depuration and wet storage of shellfish".

The monitoring program used to establish and review the Environmental Management Triggers on an annual and triennial basis is known as an Adverse Pollution Condition (APC) sampling strategy. APC sampling is described in the ASQAP manual​and is used throughout Australia (Figure 2). The approach has been developed with scientific methods accepted and applied throughout the world, that can establish relationships between environmental conditions and contamination thresholds.

Figure 2: 
Environmental Management Triggers for shellfish management

For any questions please contact the ShellMAP Regulatory Services Team.