The Idaho Department of Fish and Game has received a number of reports about dead fish in Spirit and Blanchard lakes during the past week.
Jim Fredericks, regional fishery manager for the department, said biologists with the agency and the Idaho Department of Environmental Quality investigated the recent fish kills and determined they likely were water-temperature related.
The unusually cool May and June temperatures this year, with a few warm days mixed in, added to the kills of warmwater fish already stressed from spawning.
Bullhead catfish, perch, bass, crappie and sunfish expend a lot of energy spawning, which usually occurs in May and June. The biologists during their investigation observed several dozen dead fish, from a variety of species and sizes.
"In terms of the total number of dead fish, it's certainly not what I'd call a major die-off," Fredericks said.
He said the scientists observed many healthy fish, too, rising and swimming near the shoreline.
"Based on the condition of the dead fish, the indications are that it was temperature related, and not the result of any toxicant or disease," Fredericks said.
Erik and Ann Bird, both visiting from Huntington Beach, Calif., were fishing Thursday at Spirit Lake and observed some of the dead fish along the shoreline.
"I was thinking: 'Why are these fish dead?'" said Ann Bird, 45. "'Is there something wrong with the lake?'"
Fish, being cold-blooded, are sensitive to temperature changes. Dramatic swings, particularly during spawning, can take a toll.
Fredericks said it's not uncommon for people to see a few dozen fish dead or dying around the edges of North Idaho lakes this time of year.
"It's very common to see a die-off of bullhead catfish and sunfish in some of the regional lakes," he said.
In most cases, early summer die-offs are the result of a "thermal shock" on post-spawn fish.
There's generally no risk of swimming in the water or eating healthy fish caught from the lakes, he said.
Fish scientists here will investigate further to confirm that water temperature was the cause of the fish kills.
Fish and Game and a pathologist from the U.S. Fish and Wildlife Service will be collecting some fish to conduct tissue samples next week.
State fish and wildlife officials will continue monitoring Spirit and other lakes around the region where dead fish have been reported.
Fredericks said, "My guess is that we've seen the worst, but we'll definitely keep an eye on it."