There are indicators created by nature, living and very sensitive. They are organisms that react to environmental pollution by changing visible signs, which makes it possible to predict the pollution situation by measuring these changes. Bioindicator fish are used to monitor water quality.
People have learned not only to observe changes in the behaviour of various fish species, but also to use their characteristics to predict the weather, environmental conditions and impending natural disasters. Bioindicator fish are used in experimental and industrial activities. Some fish species are sensitive to the presence of foreign substances in the water. If they sense something is wrong, they will swim closer to the surface or die immediately. It is economically advantageous to use these live indicators. Filters installed to control the composition of the water are expensive and difficult to handle. Fish are not only cheap, they work much faster - a change in the water's composition will affect their well-being after two hours, whereas automatic equipment will give an alarm after fifteen.
Example: mirror carp and goldfish become restless in the presence of oil and chemical industry effluents in the water. The pike's high sensitivity to pollution makes it a reliable indicator of the state of drinking water. Siberian stonecutter and trout can serve as indicators of the purity of the reservoir.