Thoughts on Oxygen Level

[Hillbiehle]

When it’s recommended not fishing in the summer time deeper than “x” depth due to oxygen levels and water quality; how does this carry over in the winter? Would you start fishing shallower than “x” or is it less applicable in the winter…or perhaps even more applicable?

[fishnowworknever]

You can find fish in 45' during the day, then those same fish in 15' at night. They move up and down through the water column themselves, slowly, with no adverse effects. It's when we bring them up quickly over 1 atmosphere when it causes issues.

Same goes for ice fishing...however oxygen is depleted in shallower water during the winter from the decaying aquatic plants...much more scientific explanations are out there

One atmosphere is right around 32-33'

If you're out and not planning on keeping any fish, try to stay shallower than 30' and you shouldn't have many issues.

[Hillbiehle]

Thanks for the reply. I think we're on the same track. I was investigating some different lakes on the DNR Lakefinder. A few lakes mention the absence of life beyond certain depths. For example, they say in the summer time to avoid fishing in 13' in the summer time on Lake Nokomis because of poor water quality and absense of fish. My questions is, does this 13' suggestion hold true in the winter time as well? Many lakes I fish are similar that have lots of algea or farm run off and I'm wondering if I've been fishing too deep in the winter.

[mwal]

In late fall when a lake turns over Oxygen levels should by fairly uniform from top to bottom. As winter progress's it slowly starts to stratify again due to aforementioned decaying plants using oxygen and lack of sunlight penetration due to snow or cloudy ice which can cause a lake to freeze out which just means fish die due to lack of oxygen not actually freezing to bottom. Just usually happens to shallow lakes due to the amount of weeds dying and using up oxygen in the decay process instead of creating oxygen if plenty of light gets through and some keep growing.

MWal

[Scott M]

Most Minnesota lakes are dimictic, meaning they stratify twice. The biggest contrast in stratification is summer, when the layering is very obvious in a vertical profile for both temperature and oxygen. In the winter, the stratification isn't much for contrast - the lake doesn't mix as its capped with ice and snow and the range of water temperatures are ~32 to 39 F, the 39 degree water sinking to the bottom. Only the topmost water is chilled by being next to the ice and cooling. But as far as oxygen - cold water holds oxygen quite well. I wouldn't worry about not having sufficient oxygen anywhere in a lake in the winter. Decomposition uses oxygen but its metabolically regulated by temperature...it can't use up enough oxygen because its too cold. Oxygen availability won't dictate where fish are in the winter - forage, habitat preference, predator avoidance, and other factors will.

[Nick Kuhn]

Fish need about 3mg/L to survive. The only place this really ever is below that is under the thermocline in the summer, or throughout a lake that summer/winterkills.

7 to 8 mg/L is pretty typical near the surface throughout the summer. This time of year I would wager it is still pretty close to that number all throughout a body of water.