place a big waterlily plant in the deep end of your pond and the bull frog should hopefully find it i tend to put 3 of these in the deep end it ususally does the trick .good luck

I'm kinda wondering where the bullfrog came from?
If it came from the neighborhood, it will find a place to winter over.
If you introduced it to the pond, you need to figure out how to over winter it (which you are doing by asking here)
We have little green frogs all over our property that find our fields that turn into bogs in the winter the best place to make noise and breed in the winter!
In the rainy season, when the rain stops, you can't hear yourself talk with all the froggies talking!
HTH Nancy