I had the same problem but after I planted a sustantial amount of female only yellow perch (Perca flavesens) in the pond and put up strands of line around the pond to keep the herons from wading in I can not find a single snail. Whether it's just keeping the birds out, or the fish or both I'm really don't know. I don't see anymore grubs in some of my fish either. Anther possible factor is my water did clear up drastically (seechi dish all the way to the bottom at 9 to 11 feet) last fall which may have made it easier for the fish to find the snails.

Could have been any of the previous factors or all of them together.

I used to have so many they covered everything and even had lot of them floating around.

If pigs could fly bacon would be harder to come by and there would be a lot of damaged trees.