I had this happen a few years back on a shallow-ish U.P. lake I love to fish. Great fishing in December and January, then around the beginning of February my minnows were dying. I had no idea what was going on. I thought I got a bunch of bad minnows or that I had them too close to the heat in my truck. But, in reality, the lake had very little oxygen. Talked with a guy that said the pike were swimming near the top trying to get oxygen. Noticed when I put the minnow down just a few feet they did ok. But, even with pike swimming higher, they weren't in the biting mood.
This winter seems like one where I would expect oxygen levels to be "ok". Ice is about normal and the snow on top isn't overly great....but, this does seem to be the time of year it happens. Need a few warmer days to get that melted snow going into the ice cracks. Just been so cold.