Scott,
O.K. here are all the even remotely possibly causes and my comments. Sometimes mortality is caused by a combination of factors but typically hypoxia is the highest probability.
1.) Ground water influx zip in D.O. and the already low water volume (4 feet?) caused a hypoxic condition due to the shear volume of the ground water influx.
Comment: It would have to be a significant influx of low D.O. water this time of year as this cold of water typically has lots of D.O.
2) A lot of snow blew and drifted over the ice you didn't see and had melted by the time you came out? That combined with low water levels could have been a factor.
3.) The fish were caught and released in cold water prior to ice cover by your client or someone trespassed and did so. Having recently been stocked the fish perhaps were already in a stressed condition. Some fish have problems with catch and release in cold water and develop water fungus problems.
4.) The fish went into the pond in less them optimum condition. Or the water chemistry is so different from what they are used to this along with other factors put them over the edge.
5.) The low volume water was subjected to a rapid temp change (dropped in temp quickly) along with a heavy snow fall in open water. We did have sudden cold event this fall although it obviously moderated later.
6.) A sudden influx of ground water or precip caused acid shock. (doubtful in our high alkalinity waters but it's not uncommon out east.)
7.) The ground water in his area is contaminated with something. I'm doubtful of this as you would have seen this earlier.
Did the dead fish show the classic appearance of environmental hypoxia? That is, the opercula flared and mouths agape in a desperate attempt to get oxygen?
It may have been one of these factors or a combination of them. You may never know.
