I can't tell by your registration information where you are located, but it sounds like you had a classic case of winter kill do to suffocation. What fish that did not die were stressed and now being attacked by bacteria as the water warms.The water has not warmed up enough for the optimum immune system response but the bacteria are lined up like hogs at a trough. The reddened area sounds like an aeromonas bacteria that is umbiqutous to most waters.When fish are healthy it does now harm. As with any fish disease it's when fish are stressed that they come down with problems. This will take it's course, and depending on how bad it is you may have to restock.
To prevent this next spring you need aeration in the winter. I aerate my ponds 24/7 365 days a year, but run a smaller compressor and separate airline to a diffuser in less than 6 feet of water to leave some water open, but not enough to mix the entire water column.
I came through this winter fine and probably feed up to 250 pounds of largemouths, another 100 pounds of bluegills, about 70 pounds of yellow perch, and another 70 lbs of smallmouth bass all in a .62 acre pond. (I raise to sell to taxidermists) See
http://www.ligtel.com/~jjbaird/bairdfish2.htm What size is your pond and what kind of average depth do you have?
I also snowblow the snow off my pond in winter.
