TJ. I'm glad you brought this up.I've been thinking about this also.I have a small pond with no fish...and thousands of tadpoles.But no signs of turtles or any other natural predator. My main pond with CC, BG, and GSF(and now YP and WE).... many tadpoles but not like the other fishless pond. I also have a pond with mostly CC, and thousands of fingerlings, and many adult sized CC, I never see any tadpoles, or even frogs for that matter congregating the waters edge. The CC pond is void of vegetation, in the water and at the waters edge. The CC pond is highly turbid, and any sunfish caught from there look almost albino.
I have read and heard that many frogs, including the bullfrog, in their skin produces a toxin, that a predator will find disgusting and not eat the frog.I witnessed a guy a lot of years ago, trying to impress small children who had caught a bullfrog,he pretended to eat the frog, and put it in his mouth, and soon he was frothing like he was rabid.I have had people tell me that Leopard frogs were wonderful bait, but have never had even a bite while trying them. I have never cleaned any kind of fish that had anything resembling a frog or tadpole in their stomach.
I am personally skeptical about the truth of fish eating frogs or their tadpoles.But being from the "show-me-state", I have to see it to believe it.
I do know that large bullfrogs eat nice size BG. I helped my dad years ago clean bullfrogs. Their stomachs contained BG, and larger than you would think.I personally use them for target practice anymore.