Water temperature is an important factor for survival of aquatic life. Very high and very low water temperature can kill many aquatic plants and animals. The temperature can affect how an organism’s internal mechanisms (metabolism) work. For example salmon and trout prefer temperatures between 4.5° C (40°F) and 20° C (65° F). Different stages of the growth of fish are susceptible ranges of fish. In the summer when temperatures are high, juvenile fish actively look for the cooler pools of water. A fish must pump water across its gills to meet its need for oxygen. If the dissolved oxygen in water is low, the gills must move faster to get enough oxygen for the fish to survive.

Thermal pollution can occur when heated water is discharged into cooler streams or rivers. This heated water generally is from power plants or industrial processes. If the water is not cooled down before entering into a stream or pond, it can cause organisms to die of increased temperature.

Gills beat faster in warm water

Industrial plants can cause 
thermal pollution