Fish
Below the surface of the water is a whole world of aquatic life. This ranges from algae and aquatic plants, to small creatures such as beetles, snails, dragonfly larvae and ... fish.
Fish are the most visible and conspicuous animals of that underwater ecosystem. They capture the imagination. The presence or absence of certain species can say something about the ecosystem. For example, species such as tench or rudd like to live in clear water with aquatic plants. A bream or a carp is somewhat less picky. Some species live in both fresh and salt water, depending on their life stage.
The Water Board manages water and thus has a lot to do with fish. There are several aspects to that: