Storm-driven algae blamed for Lake Koocanusa fish kill

At least 10,000 juvenile kokanee were killed in Lake Koocanusa after a strong and fast-moving storm hit the reservoir on Aug. 25.

Montana Fish, Wildlife and Parks fisheries biologist Mike Hensler estimates at least 10,000 dead juvenile kokanee measuring 8 to 10 inches long were found scattered along the shore from Big Creek to the Canadian border.

The fish were believed to have died on Aug. 26 and 27 after the storm’s strong winds mixed algae blooms on the surface with deeper layers in the reservoir. A period of calm, hot weather typically allows the reservoir water to stratify and for algae to bloom.

The kokanee then ingested the algae as they were feeding, including some blue-green algae that is toxic to the fish. The fish became disoriented, rose to the surface, their air bladders expanded, and they were unable to dive back to deeper water. The warm surface water then killed the fish.

The die-off lasted only a few days and affected only a small portion of next year’s adult salmon. Similar fish kills have occurred in 2005 and in the 1990s.

FWP officials say the algae is not abundant enough to affect humans, and the continuing winds have now dispersed the algae. FWP officials also said there are no health concerns with eating a fish healthy enough to hit a lure.