A large number of around 830,000 salmon fry released into Northern California’s Klamath River are believed to have died after suffering gas bubble disease, state wildlife officials said Monday.

The condition is caused by a severe change in pressure. It happened as the fish went through the Iron Gate Dam tunnel, the California Department of Fish and Wildlife said.

That tunnel and the dam of the same name will be removed later this year.

The around 830,000 Chinook salmon fry were the first release from the Fall Creek Fish Hatchery, which was a $35 million project designed to support salmon populations in the Klamath River once it is fully undammed, the department, known as the CDFW, said in a statement.

The fish were released on Feb. 26.

How many of the around 830,000 fry died was not clear, a spokesperson for the department said, but it’s being called a “high mortality rate.”

There’s no indication of water quality problems in the river, which is almost 270 miles long and travels from Oregon and through Northern California, and there are other healthy yearling coho and Chinook salmon that came from downstream from the dam, the CDFW said.

From now on until that dam and tunnel is removed, any other released will be done downstream of it, the department said.

The Klamath River was once the third largest salmon-producing river on the West Coast, and dams contributed to the decline since then, according to the National Oceanic and Atmospheric Administration.

The deaths of the fish are “yet another sad reminder of how the Klamath River dams have harmed salmon runs for generations,” the CDFW said.

This article was originally published on NBCNews.com