Since fish and lampreys do not have individual-identifying external characteristics, they cannot be distinguished from one another as some land animals can be. Moreover, these animals move quickly over great distances in their native habitat, which is why they usually aren’t observable. Hence, the results of studies using individually marked fish, have contributed to knowledge gain regarding:

• travel distances and migration routes

• day and night (diurnal) activity of various species

• development of populations

• monitoring of actions to manage fish stocks and to reintroduce threatened species

• discovery and passage efficiency of fish ways

• acceptance and efficacy of fish protection systems and fish passage facilities for downstream migration

The tagging of individual fish is the only method available to gain insight into the hidden life of this group of animals, leading to information about where they originated and ranged throughout their lives.