Fishing has long been a major provider of food and livelihood to people throughout the world. In the United States, 50 million people enjoy fishing as an outdoor recreation—38 million in fresh water and 12 million in salt water. Combined, they spend over $315 million annually on this sport. It is no surprise, then, that public attitude towards factors that influence fishing is strong.

The Environmental Protection Agency (EPA) is charged with overseeing the quality of the nation's waterways. In 1977 they received information on 503 separate incidents in which 16.5 million fish were killed. In 1974, a record 47 million fish were killed in the Black River near Essex, Maryland, by a discharge from a sewage plant.

Fish kills can result from natural as well as human causes. Natural causes include sudden changes in temperature, oxygen depletion, toxic gases, epidemics of viruses and bacteria, infestations of parasites, toxic algal blooms, lightning, fungi, and...