I have a similar problem in my pond. (2 acres in south Alabama). I've noticed an average of 2 dead blue gill per day for the last 3 weeks. (5 this morning) There are heron and egret around but I don't notice any damage to the fish. Tails and eyes missing on some which I assume is turtles. No catfish, RES, or bass are dying. I have been feeding twice per day and stopped fertilizing this spring. Any ideas would be welcome.