In the USA alone, between 1961 and 1976, 482 million fish were reported killed as direct result of pollution. These reports, based on voluntary notification, probably account for only a fraction of the fish that were killed. Many small kills are not noticed or are not reported, and large kills are often not included because of insufficient information to determine whether the kills were caused by pollution or natural by factors. Kills of 1 million or more were responsible for 77% of all reported fish killed in the 15 years. Low dissolved oxygen levels resulting from excessive sewage (primarily municipal) were the leading cause. The second most common cause was pesticides.