Internal tags are defined as tags inserted or injected into the fish (body cavity, muscle or cartilage) and carried internally. They can be used to identify individual fish or groups of fish. Most of them, including Coded Wire Tags (CWTs), have to be removed from the fish to be identified, but the more advanced ones, such as Passive Integrated Transponder Tags (PITs) and visual Inplants, can be read without removing the tag, thus providing a very non-destructive way of identification.