A wildlife official said on Monday that nearly half of the sockeye salmon migrating up the Columbia River through Oregon and Washington state have succumbed to hot water. Survival became very difficult for more than 507,000 sockeye salmon that were swimming between two dams along a stretch of the lower Columbia River.

Only 272,000 of them have survived the journey, said Oregon Department of Fish and Wildlife fisheries manager John North. This is the first time when moralities have happened at such a scale, said North.

US West Coast states are already troubled by drought conditions and the Columbia is recording the third-highest count of sockeye returning from the oceans to spawn since 1960.

Researchers have attributed the water temperature rise to hot air combined with abnormally low mountain snow melt. This has resulted in fishing restrictions and efforts to save beleaguered fish, including trucking salmon to cooler waters.

In mid-June, temperature of the Columbia River touched 70 degrees Fahrenheit. It happened a month earlier than usual and the fish were not prepared to acclimatize.

Counting for the sockeye was done between the lower Columbia's Bonneville Dam and McNary Dam, about 150 miles upstream.

Rain could play a role in bringing down water temperature along the Columbia River. Officials are also trying to reduce water temperature by releasing cold water from surround reservoirs.

If climate change persists, salmon in this area could face more threats to their survival and safe migration could become a big problem in years to come.