Weather events cause fish die-off in Koocanusa

A strong and fast-moving late August storm front appears to have triggered a kokanee salmon die-off in Lake Koocanusa, a reservoir that spans from Montana into British Columbia.

Fisheries biologist Mike Hensler estimates there were at least 10,000 dead juvenile kokanee measuring 8 to 10 inches long scattered over 21 miles of the reservoir from Big Creek to the Canadian border.

This is similar to a die-off in 2005 and several others in the 1990s and earlier, according to a news release from Montana Fish, Wildlife and Parks.

The fish deaths appears to be triggered by a period of calm, hot weather that lets the reservoir stratify and allows algae to bloom.

The storm front, accompanying winds and drop in atmospheric pressure allow the algae to mix with deeper water layers. The kokanee ingest the algae, which include some blue-green algae that are toxic to the fish, as they are feeding.

The kokanee 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.

The algae is not abundant enough to affect people and has now been dispersed by continuing winds. The die-off only lasted a few days and only affected a small portion of what would be next year's adult salmon.