Soil erosion with the silting of rivers has, without doubt, caused the extinction of many fishes throughout the world. Human waste has polluted many rivers. Land clearing with move some of the topsoil into the rivers. The burning of trees with increase greenhouse gases. Aquaculture activities can have strong effects on the health and quality of down stream waters. Changes in oxygen, temperature, pH and the addition of metals, drugs, suspended solids, ammonia, organic nitrogen, phosphorus and hydrogen sulfide as acid rain are often measurable downstream from hatcheries.