Because fish are basically swimming filters, they soak up pesticides, industrial waste, and chemicals that have been dumped into waterways for decades. With more polluted waters than ever, the risk is rising that you'll find a contaminated fish on your dinner plate. Salmonella and scombroid poisoning, which is caused by a histamine produced by bacteria on fish, can also make certain fish unsavory.

Part of the reason that inedible fish are ending up in the grocery store is that there's no government-run process to inspect fish. In fact, fish and shellfish are the only major sources of protein that do not receive comprehensive government inspections for potential contaminations. The $9 billion industry is overseen by a piecemeal system led by the Food and Drug Administration, whose inspectors visit processing plants an average of once every four years.

In a 1991 report issued by the National Academy of Sciences, current safety regulations were deemed "insufficient" and "too limited in frequency and direction to ensure enhanced safety of seafoods." While some efforts to improve seafood handling have been made since then, substantial problems remain.

Freshwater fish are more likely to contain toxic chemicals than fish that spend their lives in the ocean. Because of industrial dumping and air pollution, the Great Lakes have been found to be swimming with polychlorinated biphenyls (PCBs), which are suspected of raising the risk of cancer and birth defects. Residues of DDT, the cancer-causing pesticide banned two decades ago, have been found in Great Lakes whitefish being sold in southern California.

Fish that live in deep, offshore waters, such as cod and haddock, are considered low in chemicals because the areas they live in aren't easily contaminated. But chemicals can be found in saltwater fish that live close to polluted urban shores, or in fish that commute to freshwater. Puget Sound, the Chesapeake Bay, and Santa Monica Bay are all considered risky sources, according to the National Academy of Sciences.

Among ocean-swimming fish, fin fish from tropical or subtropical areas have the most problems. Barracuda, grouper, amerjack, and certain tropical snappers have been linked to scombroid as well as ciguatera - a sometimes-severe disease that affects the nervous system and can cause vomiting and nausea. Neither bacteria is destroyed in the cooking process.

Methyl mercury poisoning from swordfish is a great enough risk that the National Academy of Sciences recommended that couples who intend to have children in the near future should avoid the fish. Tuna and mahi mahi can also contain methyl mercury in small amounts, but the danger in swordfish is much greater.

The riskiest seafood of all is molluskan shellfish such as oysters, clams, and mussels. The intestines and internal organs of these mollusks are fertile ground for contaminants. The problem is worsened by fishermen who illegally harvest shellfish in polluted waters. Many mollusks get contaminated because they live where rivers and seas meet and, because of nearby cities, these waters often are contaminated.

Oysters, clams, and mussels are also vulnerable to Norwalk viruses, which can cause severe diarrhea unless the shellfish is fully cooked. Another source of contamination is the algae called "Red Tides." The FDA and the coastal states all test for these blooms, and when they appear the waters are closed to all fishing.

Even oysters taken from clean waters occasionally harbor a naturally occurring, unfriendly bacteria known as Vibrio vulnificus, according to a recent report from the National Centers for Disease Control and Prevention. If contaminated mollusks are eaten raw and the bacteria remains alive, it can make a person very sick. Healthy people will probably just get a bout of indigestion. But for those with liver ailments and depressed immune systems, Vibrio vulnificus can be deadly. Symptoms can include sudden chills, indigestion, fever, nausea, vomiting, and stomach pain.

So what is safe? According to the National Academy of Sciences report, catfish, trout, salmon, and other farmed species are generally reliable as long as they are cooked immediately before serving. Processed fish such as fish sticks and fish nuggets are also safe bets because they are made from white-fleshed fish such as cod, haddock, and pollack. And canned tuna is not only the safest of all seafoods, but it is also the most popular.

No matter what the fish, there are some simple precautions you can take when shopping for and storing seafood, though. Here are some tips from Get Hooked on Seafood Safety, which is published by the U.S. Food and Drug Administration:

• Select seafood with a fresh, mild odor. It should not smell unpleasantly fishy. Fish fillets should be moist, without any traces of browning or drying around the edges.
• Check the gills, scales, and eyes. The gills should be bright pink or red. And the eyes should be bright and clear, not cloudy or sunken. Scales should cling tightly to the skin, and they shouldn't be slimy.
• Mollusks in the shell should be alive when you buy them. When a clam, oyster, scallop, or mussel is alive, its shell is tightly closed or closes when it is lightly tapped.
• Test shellfish for freshness. Hold the shell between your thumb and forefinger and depress it, as though sliding two parts of the shell across one another. If the shells move, the shellfish is not fresh. Throw away any mollusks whose shells aren't closed tightly.
• Unless you freeze it, cook fish within two days of purchase. Smoked fish, pickled fish, and vacuum-packed fish should always be refrigerated. Whatever the fish, keep it in its original wrapper and store in the coldest part of your refrigerator, which is usually under the freezer or in the meat drawer, until it is ready to be cooked.
• If you did freeze it, allow one day to thaw. Thaw frozen seafood in its own container in the refrigerator. Do not thaw seafood at room temperature or under warm running water.
• Store mollusks live. Keep mollusks in your refrigerator in a container covered loosely with a clean, damp cloth. Do not store live shellfish in an airtight container or in water.

We've started a library of "Living Footnotes" for this section, and welcome your suggestions and your URLs to make it more useful. Use our customized Lycos search to linkthink your way around the web, or feel free to consult the experts about the above topic.

Living Footnotes

• FDA: Seafood Information and Resources
• FDA: Seafood Hotline
• FDA: The Regulatory Fish Encyclopedia
• Another Seafood Information Source on the Web
• Seafood Recipes