Look closeley at the gills. if they are swollen and look like hamburger meat then you may have what is called proliferative gill disease or sometimes called hamburger gill. It is thought to be caused by a sporazoan parasite that uses the channel cat as an intermideate host in one stage of its life cycle. it causes the gills to become large and inflamed to the point that the do not function properly. lack of oxygen is not likley this early in the year, but dont rule it out. if the cats were "bobbing" at the surface openening and closing their mouths but had no other signs of sickness hamburger gill might very well be it. it can be treated with potassium permangenate. but it will only kill the spores in the water not inside the fish so multiple treatments will be required. contact your local lake management rep. to get this done as it can get tricky.