I found 4 more large dead BG. That makes 12 in the past two weeks. I suspect it's from the 22 or so 3-4 year olds I put in the pond in December 2015 in order to get a rapid spawn the next spring. That would make them in excess of six, maybe seven years old now, and the stress of spawning maybe killed them. All appeared to be females, from what I could tell.