Aurelia in Tokyo's Sumida River

In the early afternoon while traveling by water taxi on Tokyo's Sumida river from Ryogoku to the Tsukiji fish market (3.5miles) I saw hundreds and hundreds of Aurelia sp. at and near the surface waters of the river in the calmer areas near the edges and docks of the river. Size range was from 3cm to 20cm. They all appeared healthy and were actively swimming. This was my first view of the river so I do not know if this is a common occurrence here. The river water was dark and dirty with abundant plastic materials floating at the surface. Barnacles and oysters were flourishing on the rocks so salinity must be raised by the influence of Tokyo Bay. A few Chrysaora sp were also seen.