The Tama River runs through Tokyo. During the years of high economic growth, the river was called "river of death" due to extreme pollution. But as water quality improved, fish returned. One species symbolizing the restoration is the Ayu, also known as "queen of freshwater streams". Their population has rapidly increased, in some years recorded at 10 million. Meanwhile, some locals have been spotting more and more invasive species and are concerned about the effects on the ecosystem.