LIBBY – At least 10,000 kokanee salmon were killed this week after a strong and fast-moving storm front Sunday churned up the waters of Lake Koocanusa.
The die-off appears to have been triggered by a period of hot, calm weather that allowed the reservoir to stratify, and algae to bloom, according to Montana Fish, Wildlife and Parks.
When the storm front and winds moved through, accompanied by a drop in atmospheric pressure, the algae mixed with deeper layers of water.
FWP fisheries biologist Mike Hensler says the kokanee ingest the algae, which includes some blue-green algae, as they feed.
The blue-green algae is toxic to fish.
“They become disoriented, come to the surface, their air bladders expand and they are unable to dive back to depth so the warm surface water kills them,” FWP spokesman John Fraley explained.
Hensler estimated at least 10,000 dead juvenile kokanee, measuring 8 to 10 inches, were scattered from Big Creek to the Canadian border after the Monday and Tuesday die-off following Sunday’s storm.
There was a similar die-off in 2005, and several others in the 1990s and earlier.
Fraley said the algae was not abundant enough to affect humans, and has since been dispersed by continuing winds. While 10,000 salmon is a lot of fish, the die-off only affected a small portion of what will be next year’s adult salmon.
There are no health concerns, Fraley said, with eating a fish from Koocanusa healthy enough to hit a lure.