Scientists in the far-eastern city of Khabarovsk caught fish with bleeding ulcers in the Amur and were still trying to determine what poisoned the fish.
A concentration of heavy metals exceeding the norm for human consumption has also been found in fish.
Fishing in the Amur has been banned since November after an explosion at a factory in northeastern China poured tons of dangerous chemicals into a tributary of the Amur.
Traces of pollution were found in the Amur in December, but tests determined they presented no danger to people. Russian scientists say the danger may rise with the spring thaw as large concentrations of chemicals are believed to be frozen in the river's ice.
(ITAR-TASS, AFP)