The aquatic environment can be difficult to monitor because it is often not possible to directly observe whether a stream or a lake is affected by human activity. However, it can be important to investigate the wellbeing of fish in a stream or lake. Several fish species are of economic importance, and many species play an important role in the aquatic ecosystem because of their position high in the food chain. Two main methods are used to investigate how much, and which type of fish can be found beneath the surface: electrofishing and net fishing. Electrofishing is appropriate in running water, where fish within a particular stretch can be caught using an electrical field that stuns the fish. Net fishing is used in lakes and coastal waters to investigate fish numbers and species composition.