Hi folks. I have one more �tangent� to toss into this discussion. My thought was stimulated by Bruce C.�s earlier suggestion that coolwater or coldwater fish might be more able to tolerate extended, colder water periods during winter (his example was 34 F vs. 39 F) than are warmwater fishes. This certainly is possible, and it sounds probable based on the experiences reported here. In addition, tolerance for low levels of dissolved oxygen (DO) varies among fish species.
One of the �toughest� fish when it comes to surviving low DO is actually the fathead minnow. They survive when there is almost no measurable oxygen in the water. Fishes such as northern pike and the yellow perch are also more tolerant of low DO than most other fishes. Fishes such as black crappies, largemouth bass, and bluegill will die before the yellow perch and northern pike as DO declines over winter.
Shallow, marshy, productive lakes in the north country often have pretty simple fish communities because they tend to have DO problems under ice cover. So, such waters often contain northern pike, yellow perch, fathead minnows, and black bullheads (low DO tolerant species) rather than walleye or black crappie that require more DO for winter survival.
Cecil made the point that a combination of factors likely stress fish and induce winterkill, and I�m sure he is right on target. The cooler temperatures likely add stress. The low DO obviously can add stress. Then, I wonder if some of the decomposition gasses that accumulate under the ice, such as hydrogen sulfide and methane, might add another stress. As I mentioned earlier, it may sound silly, but we don�t know everything that we should know about winterkill!
Dave
