I borrowed the chart below from an aquarium site because it has a Fahrenheit temperature scale. Whoever made it mistakenly labeled the oxygen scale as %, it should be parts per million (ppm). Otherwise accurate.
As previously mentioned, DO levels are directly related to water temperature. Saturation is the most DO that water can contain under natural conditions at atmospheric pressure and the indicated temperature. I posted this to illustrate what a narrow range there can be between DO saturation and lethal limits. For instance, at 86 degrees, the maximum DO is ~7.6 ppm, just 3.6 parts per million above lethality for most fish. At 50-59 degrees (common thermocline temps), the spread doubles to ~6-7 ppm.
Considering the dynamic, ever-changing nature of DO, it’s pretty amazing that we have as few DO-related fish kills as we do. Surface aeration replenishes DO, Biological Oxygen Demand depletes it. Then you have the daily phytoplankton cycle that replenishes DO in the day and depletes it at night. Inflows can be beneficial or detrimental simply depending on the specific conditions at that particular time and place.
I’ll make no claims as to what I think a fish knows or thinks, or whether it’s smart enough to sense potentially lethal conditions and relocate accordingly. In the case of walleye that are conditioned to summer thermocline temperatures, I know it wouldn’t do them much good to leave that zone because DO was dropping and move the mere ~30’ to the surface where it’s better, because the sudden change from ~55 degrees to ~85 would croak them in a fairly short time. Maybe that’s what happened.