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Winter fish kill at McWenneger Slough in Kalispell due to lack of oxygen

Winter Fish Kill.jpg
Posted at 9:15 AM, Jan 30, 2023

KALISPELL - Long cold winters create ideal conditions for anglers, but it can cause a big problem for the fish.

Winter fish kills — like a recent incident at McWenneger Slough in Kalispell — occur on an almost annual basis.

These incidents will kill some fish in the water body, but luckily, not all of them.

“Winter fish kills are typically naturally occurring events that happen in these kind of long winters where we have a lot of thick, dark snow-covered ice,” said Montana Fish Wildlife and Parks (FWP) spokesman Dillon Tabish.

The snow and ice block sunlight from reaching the vegetation in the water, halting photosynthesis and the creation of oxygen.

Once the plants die and begin to decay, they absorb oxygen and deplete the dissolved oxygen levels in the water.

“Unfortunately, literally, fish don't have enough oxygen in the water to breathe. So we have these, these events in these pockets of these bodies of water where we do see fish kill,” said Tabish.

These winter fish kills typically occur in shallow, highly vegetated and nutrient-rich water bodies.

And the situation must be just right for these kills to happen.

“In deeper bodies of water, there's just more dissolved oxygen available and or in streams where there's oxygen moving around in the water, and there's usually less cover from ice or snow. There's plenty of oxygen,” said Tabish.

McWenneger Slough is the only known fish kill in the region so far this winter, but other waterbodies could be affected as the season continues.

While this is a tragic occurrence, fish are resilient creatures.

“It doesn't create any long-term worry about population size or worry in terms of these instances it's a momentary blip and then in spring and summer, those fish populations will hopefully bounce back,” said Tabish.

If you see signs of a fish kill, please contact FWP so they can investigate.