Chrysaora sp in Tokyo's Sumida River

In the early afternoon while traveling by water taxi on Tokyo's Sumida river I saw a few Chrysaora sp. at the Hama Palace Garden dock next to the Tsukiji fish market. I saw two specimens at the surface in the calmer areas near the river boat dock. Size was 20cm. They both 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. Hundreds of Aurelia sp. were also seen.