I swear by Frogs. Especially American Bullfrogs. I have seen a balanced pond with a great LMB population and frogs also. Frogs are normally on the bottom of the food chain. Especially baby ones and polliwogs. Snakes and birds can really reduce thier numbers. Also lack of cover and poor water quality. I stock them wherever I can for forage. I even stock them in non forage areas so I can relocate them later. I think one of the most important factors is food and cover. If they have no cover they just don't make it to breeding size.