(Fond du Lac) The Wisconsin Department of Natural Resources says it does not know what is causing a fish kill on Lake Winnebago. Samples of dead fish at Lakeside Park in Fond du Lac are being tested to see if the problem is viral or bacteria-based. Low oxygen levels have caused similar problems in the past. The state agency and Oshkosh fire officials have assured residents the fish were not killed by the runoff from an industrial fire Monday. The D-N-R says the results of the testing should be available in the next week.