The Amur River flows for more than 2,800 km, much of it along the Russian-Chinese border, and is the largest and most commercially important spawning ground of chum salmon in Asia.
"A scientific expedition has studied spawning grounds in the Komsomolsk district and found that the die-out takes place in tributaries, not in the Amur bed as it was previously thought," said German Novomodny, the head of the Khabarovsk branch of an official fisheries research center.
Scientists said the die-out was caused by record low water levels in Amur tributaries and record high water temperatures. The majority of Amur tributaries used by the salmon as spawning grounds either shrunk or completely dried out during the hot summer, and many fish died of stress, unable to find a suitable place to lay their eggs.
"The abnormally high water temperature and a lack of oxygen in water could also be factors," said Boris Voronov, chief of the Institute of Water and Ecological Problems in Khabarovsk. "Salmon are under stress in warm water; they can becomes stranded and die."
"I do not rule out global warming as the reason behind the mass die-out," he added.