Fish kill at Lake Koocanusa
by Montana Fish, Wildlife & Parks
September 4, 2013
A strong and fast moving storm front last Sunday night appears to have triggered a kokanee salmon die-off in Lake Koocanusa August 26 and 27. Fisheries biologist Mike Hensler estimates there were at least 10,000 dead juvenile kokanee measuring 8"-10" long scattered from Big Creek to the Canadian border.
This kill is similar to a die-off in 2005 and several others in the 1990s and earlier. The mortality 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 allows the algae to mix with deeper layers. The kokanee ingest the algae, which includes some blue-green algae which is toxic to the fish, as they are feeding. 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.
The algae is not abundant enough to affect humans and has now been dispersed by continuing winds. Likewise, the die-off only lasted a few days and only affected a small portion of what would be next year’s adult salmon. There are no health concerns with eating a fish healthy enough to hit a lure.
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