Where have the bullfrogs gone?

Where have the bullfrogs gone? Late in the spring I would hear bullfrogs plopping into our ponds as I walked the water's edge. Then summer came and poof - no bullfrogs. No deep throaty noises at night. Some people spotted the odd frog dead by the road. But what happened to the invasion? On our farm, something made them go away. And we didn't do anything to remove or kill them.