My first question is does the pond water level change? If not you may have a spring. I say that because I have a similar situation and also being in Wisconsin I have never had a winter kill. I did have a mild fish kill in summer before I added aeration. Cloudy days without wind will do it, The more plants in the water the bigger the concern. I can tell you that in my swamp there are several areas that will not freeze because of spring water. I'm not talking water gushing out of the ground, just slow seeps. These areas have never frozen over in the 20 plus years I have owned it. Even if you have a spring the water may have very little oxygen in it. You can't go wrong with adding aeration. I have two ponds that are connected by acres of cattails, each has a wind aerator. I have enough water flow that water flows out my control structure year around, which is why the fish survive. I use the wind aeration to keep the water from stratifying which is a whole new Topic. I think you should stock some fathead minnows right away and think about what else to stock in spring. And then add electric aeration to be safe.

Are you sure there are no fish in now? You may want to put a minnow trap in and see what you get.

Every pond is different so watch your pond and learn by asking questions here.

I will take this opportunity to thank all the members of this site for their help. They are the reason I have been successful with my ponds. Thank you all!