Fish diseases observed in the past three decades and which have been attributed to pollution include: haemorrhages; tumours; fin rot; deformed fins; and missing scales and tails. In industrialized countries, increasing numbers of fish are deemed inedible. 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 by natural factors. Low dissolved oxygen levels resulting from excessive sewage is one of the leading causes. The second most common cause is pesticides.