The level at which the various fish species are affected by low oxygen levels in the water varies, according to the Minnesota DNR.
Trout are the first to feel the effects at oxygen levels below 5.0 parts per million. Fish next affected are bluegills and bass, which are moderately sensitive to declining levels of oxygen.
Walleye, yellow perch, northern pike, carp and crappies have been able to survive levels down to about 1.0 ppm for extended periods. The last to go, not surprisingly, are bullheads and fathead minnows.
Mackenthun said there is reason to be concerned once levels fall below 2.0 ppm. He also said that dissolved oxygen levels rarely are uniform across an entire lake and that frequently, refuge areas containing higher levels provide refuge for fish.
To view the most recent oxygen levels gathered by DNR Fisheries on some area lakes to to: http://www.dnr.state.mn.us/areas/fisheries/waterville/winterkill.html