I also agree that purchasing gravel would not be necessary for adequate production of young bluegill. Bgill nests in MI and WI were found to have an average of 17,000-22,000 fry per nest. Plenty of fish for repopulating the pond even after the Hydra Ewest mentioned above get their share. Note that predation of fry by Hydra is dependant on 1. if Hydra are present in your pond and 2. dense weeds or some form of dense upright structure for Hydra growth need to be very close to the bgill nests for predation to be significant.
