SECTION F: Microbiological Guidelines for Shellfish-Gathering Waters
The microbiological water-quality guidelines for recreational shellfish gathering are as defined in the Shellfish Quality Assurance Circular (Ministry of Agriculture and Forestry 1995) for areas of approved shellfish-growing waters. These guidelines are used by the shellfish export sector and are internationally accepted as indicating that shellfish grown in such classified waters under given conditions of sanitary safety are expected to have suitable microbiological quality for public consumption.
Note: These recreational shellfish-gathering water quality guidelines only cover microbiological contamination. They do not cover marine biotoxins, which in certain places and locations can pose a significant risk to recreational shellfish gatherers.
F.1 The preferred indicator for waters used for shellfish gathering
The guidelines use faecal coliform indicator organism values to denote the potential presence of pathogenic bacteria, viruses and protozoa.
F.2 Recreational shellfish gathering guideline values
Compliance with these guidelines alone does not guarantee that shellfish grown in waters of this microbiological quality will be safe. The guidelines apply to waters in a catchment where a prior sanitary survey has shown that there are no point sources of pollution of public health concern. The guidelines are solely a management tool to measure any change from the conditions prevailing at the time of assessment.
The guidelines are also useful for assessing the impact of pollution from surface run-off after rainfall, and of tidal movement under storm conditions. Such factors are used to decide when gathering should be curtailed in commercial shellfish-growing areas when weather conditions cause pollution. They are equally applicable for recreational shellfish-growing waters.
The guidelines are set out in Box 3.
Box 3: Recreational shellfish-gathering bacteriological guideline values
The median faecal coliform content of samples taken over a shellfish-gathering season shall not exceed a Most Probable Number (MPN) of 14/100 mL, and not more than 10% of samples should exceed an MPN of 43/100 mL (using a five-tube decimal dilution test).
These guidelines should be applied in conjunction with a sanitary survey. There may be situations where bacteriological levels suggest that waters are safe, but a sanitary survey may indicate that there is an unacceptable level of risk.
Notes:
- The MPN method as described in Standard Methods for the Examination of Water and Wastewater; American Public Health Association (current edition), must be used to enumerate faecal coliforms unless an alternative method is validated to give equivalent results for the waters being tested.
- Sampling to test compliance shall be over the whole shellfish-gathering season.
- A sufficient number of samples should be gathered throughout the gathering season to provide reasonable statistical power in testing for compliance for both the median limit and the 90% samples limit.
