Whats happening to my fish?

Over the last week or so, here in PA warm weather has bestowed us. The ice melted off the ponds and dead fish started popping up. The small pond had 3 small bass floating and the large pond had a blue gill. If more fish are found floating I may have to take water samples to the local extension office for analysis. I hope its just mother natures way of thinning over population. Any suggestions?