Theo, well water being discharged from the heat pump leaves a very small slightly warmer than normal area in the pond during the winter. There is usually a very small spot of open water next to the dock where it discharges even when the actual air temps are well below zero. This, along with the mild fall we had likley accounts for their survival through the winter. Personally I don't think these BG will ever grow much and most will likely end up as bass food before the end of the summer.