It really sounds like an annual hatch and an annual mopping up or feeding frenzy. Do you hear a bunch of burps coming from the water? It seems like you would have noticed this. I think it could be the old story of every prey has a predator and vice versa. 50 thousand frogs hitting the world from each water hole would be overwhelming.

Or could they have matured, grown legs and went walkabout? I guess you would have noticed that also.