Can you enjoy a good salmon steak? Not if you are one of 7 million Americans, or 2.3 percent of the population, who are allergic to fish and shellfish. Fish is a lifelong allergy, and almost half of people with fish allergies first experience symptoms as an adult, which can include swelling, itching, cramps, wheezing, heartburn, dizziness and eczema. Determining a fish allergy can prove complicated, as people may be allergic to one kind of fish but not another. Fish can lurk in the strangest places, including salad dressing, Worcestershire sauce, meatloaf and barbecue sauce.