Last week’s record heat wave has had lethal aftereffects for steelhead in two Southwest Washington fish hatcheries. The Fallert Creek Hatchery on the Kalama River lost 135,000 summer and winter steelhead last weekend, said John Kerwin, fish health manager for the state Department of Fish and Wildlife.

About 99 percent of the hatchery’s fish — most of which would have been planted in the Kalama — were wiped out by a parasite that spread rapidly due to overheated water temperatures, he said.

The state hatchery on the upper Kalama River had no trouble. However, the same problem wiped out about 138,000 steelhead at the Washougal hatchery in Clark County, or about 12 percent of the hatchery’s steelhead population at the time, Kerwin said.

He said the fish losses were due to environmental conditions that “in my 20 years with the department is unprecedented,” he said in a phone interview Thursday.

Last week’s record temperatures heated rivers 15 to 20 degrees above seasonal norms, reaching 80 degrees in Washougal and 72 degrees at the Faller Creek hatchery, he said. The hot weather was largely to blame, but another factor was the drought that started in mid May in Southwest Washington, Kerwin said.

Warm water allows a naturally occurring parasite — commonly known as “Ich” because it has a long and cumbersome formal name — to reproduce rapidly, he added.

Kerwin said he examined samples of Fallert Creek steelhead Friday after staff noticed unusual levels of die off. He diagnosed the problem, and treatment with a chemical started the next morning when water temperatures dropped. But it was too late, he said.

“We were not able to bring the fish around,” he said.

Starting treatment immediately Friday afternoon may have been lethal itself, he explained. That’s because the chemical used to treat the parasite draws oxygen out of the water, and dissolved oxygen levels were already low because of the extreme water temperatures, he said.

There is little else hatchery managers can do when rivers that supply hatcheries heat to such high levels, he said.

“In surface water facilities, where you’re drawing water off rivers, your hands are tied. We were dealt with a tough set of circumstances.”