It is impossible to make direct comparisons between states or to assess trends over time based on this closure data. Standards, monitoring, and closure practices vary from state to state, making it difficult to know, for example, whether a state with many closings has vigilant health officials or more coastal pollution. Classification is based on surveys of water quality and pollution sources. Individual growing areas are classified either as approved for harvest or as one of several restricted categories. All harvest must be prohibited unless sanitary surveys indicate that water quality meets specific NSSP standards for the other categories.
Harvesting is permissible in approved areas year-round.
Some potentially productive growing areas remain prohibited for harvest because of inadequate state resources to conduct the requisite sanitary surveys. The primary basis for harvest restrictions is the concentration of fecal coliform bacteria associated with human sewage and with organic wastes from livestock and wildlife. In some cases, monitoring also includes heavy metals and toxins associated with so-called red tides. Some scientists and fishermen question the use of fecal coliform concentrations for indexing shellfish growing areas, particularly in waters that receive chlorine-disinfected effluents from wastewater treatment plants. The NOAA National Shellfish Register provides a record of the acreage of all classified shellfish-growing waters in the contiguous United States.