Warmer temperatures are causing some of Iowa’s snow to melt off, but not fast enough for some lakes. Snow and ice cover is leading to low oxygen levels and fish kills on some lakes in Iowa. Iowa Department of Natural Resources fisheries biologist Jim Christianson says low water levels add to the problem. Christianson says the shallow water lakes in northern Iowa are having the most problems.He says the kills are a natural process that they have to deal with.Christianson says a winter fish kill isn’t all bad.as the kills help get rid of some of the less desirable fish and allow new fish stocked in the lakes to thrive. Christianson says the state has some aeration systems working in shallow lakes to add oxygen to the water and cut down on the fish kills.