Abstract
The aim of the investigation is to show how the Ayu (Plecoglossus altivelis), a fish living in clean streams, can return to an urban river that believed to be polluted in the past. It was found that the ammonia nitrogen concentration had decreased remarkably, by monitoring of the water quality at the river. Due to the qualitative improvement of the sewage treatment water that is the river's main discharge source, the ammonia nitrogen concentration was decreased. Ammonia is an important substance for the survival of the creature; and a safe level, which makes the Ayu's survival possible, was estimated.
