My questions were designed to eliminate possibilities.

I was considering a mid-spring partial turnover and oxygen problem but now I don't think it is an oxygen problem because Carlander says bluegill and bass both quit feeding at about 1.0 ppm and death for both is around 0.8ppm at least for fingerlings. Although, for adult fish requirements for DO are usually higher than for fingerlings. IF it is a DO problem bass should also be dying. If you start losing other species of fish, esp larger bass then it is likely a DO problem.

I suggest that you cut open a couple to several dead bgill and check for body fat. If the fish are thin with no body fat I suggest the probability of starvation / crowding associated stressors. If you usually feed your fish during summer and the water is or has been in the mid-60-70F range then spring feeding should have begun already for crowded fish that are dependant on supplimental feed. Crowded fish and a lenghty, mild winter in GA could have caused extra body metabolism for fish compared to winter metabolism of other years.

Fish populations AND pond ecosystems are dynamic and always changing. These two complex variables almost always explains why "it" has not happened before.

What are your surface water temps?

aka Pond Doctor & Dr. Perca Read Pond Boss Magazine -
America's Journal of Pond Management