May 09, 2003 

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Shellfish and Salmonella (video)
July 12, 2001

Also on ScienCentral News

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Elsewhere on the web

The Environmental Protection Agency’s shellfish site, Office of Water

The National Oceanic and Atmospheric Association publishes the results of the 1995 NSSP surveys

Recreational clamming tips

Centers for Disease Control and Prevention information on food safety

Epicurious’ recipes for clams, mussels, and oysters


For many people, oysters are viewed as a delicacy and are even rumored to be an aphrodisiac.

But as this ScienCentral News video reports, a romantic mood isn’t worth anything if the waters that the oysters come from end up putting you in the hospital.

All Clammed Up

When shellfish beds are closed, it means the state Fish and Wildlife department has sampled the water and found it contaminated, making the clams, oysters, and mussels unfit for use as food. The states adhere to guidelines set by the National Shellfish Sanitation Program. The NSSP is a collaboration between the U.S. Food and Drug Administration, individual coastal states, and commercial shellfisheries.

Closures could be temporary, such as when heavy rain causes increased runoff from sewers and into fishing waters. A more permanent closure might be the result of new building developments that increase local waste.

Brunswick County North Carolina, for example, used to be a big shellfishing spot, says Jim Bahen, fisheries specialist for the North Carolina Sea Grant Marine Extension Program. "But because of development in Myrtle Beach and south in that area, a lot of it is closed now," he says.

If an area does not receive full clearance as an approved harvest area, a state classifies it under one of the four harvest-restriction categories: 1) conditionally approved, 2) restricted, 3) conditionally restricted, or 4) prohibited for harvest. Waters must be classified as prohibited unless proven to meet NSSP clean-water standards; potential harvest areas are therefore "guilty until proven innocent" of contamination. This also means that if a state does not have the resources to sample all its areas, some clean areas may remain prohibited for lack of proof of suitability.

When considering water safety for shellfish, contamination mainly consists of human sewage and livestock waste. This contamination cannot be measured directly; instead, state officials rely on the presence of bacteria that live in the waste, called fecal coliform bacteria. The lower the concentration of the bacteria in water, the less contaminated the water.

Shellfish beds may also be closed to harvest due to other contaminants or potential contamination events. Industrial toxins, periodic algal blooms known as "red tides," and weather events such as heavy rainfall that lead to excessive runoff can prompt a state to close a bed for a length of time.

Harvesting may take place year-round in approved areas. In conditionally approved and conditionally restricted areas, harvesting can occur under strict supervision of handling, transport, and storage. Restricted areas require that harvested shellfish spend a period, usually about two weeks, in clean water, either at an approved harvest area or a specific facility. This allows for depuration, in which the shellfish mostly rid themselves of contaminants, as clean water circulates through them.

Of course, depuration’s success may be limited, Dr. Joens explains. His research indicates Salmonella remains in shellfish after depuration, probably invading the intestinal tissues of the oysters and mussels. Shellfish are unique animals in that humans eat them whole, including the intestinal tract.

Thanks to the NSSP, the percentage of harvest-prohibited areas in 1995 (the most recent national survey) reached its lowest levels ever: Only 13% of the 33,000 square miles of U.S. coastal waters identified as potential growing areas fell into the prohibited category. The commercial harvest that year totaled about $200 million for 77 million pounds of shellfish. This doesn’t consider the recreational harvest by individual Americans.

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