The "Madison Declaration" last week included a worldwide warning to the public - especially children and women of childbearing age - to be careful about the amount and types of fish eaten. It's a message that seems to conflict with the advice given by many doctors - to eat fish as part of a healthy diet. For most people, the best way to reduce their risk is to limit the kinds of fish they eat to those that tend to contain the least mercury. Those that should be avoided include shark, swordfish, tilefish or king mackerel. And, besides heeding consumption advisories about fish and fowl taken from local waterways, it's a good idea to limit the amount of albacore tuna eaten to 6 ounces a week. These advisories are especially important for children, women who are pregnant, nursing or who may become pregnant and heart patients, particularly middle-aged men. All of these groups have vulnerabilities to mercury, with babies and children considered most sensitive to the effect of methylmercury, which can have irreparable neurological effects. That still leaves many types of fish on the menu for healthy diets. Several recent studies, including one published last fall by the Journal of the American Medical Association, suggest the benefits of eating fish outweigh the risks from mercury. -Judy Fahys