The IDNR www.ifishillinois.org website has some useful information.
According to the IDNR: During winter, the oxygen supply under the ice depends upon the passage of light and the production of oxygen by tiny plants in the water. If snow covers the ice, sunlight cannot penetrate and the plants are unable to produce oxygen. The supply of oxygen is gradually used up by decay processes and by the respiration of fishes and other aquatic animals. If the snow remains on the ice long enough, oxygen is depleted and the fish suffocate. Dead fish are usually found in the spring after the ice melts. However, if the kill occurs early in the winter, there may be few, if any, dead fish observed when the pond opens in the spring. Winter kill is most likely to occur in fertile, shallow, weed-filled ponds.
Natural Mortality In the spring a few large fish may be found dead along the shoreline. Such mortalities are often the result of natural causes. The natural resistance of fish to disease is lower in the early spring than at any other time of the year. Larger fish often seem to be more susceptible to disease than smaller fish.
