I live 20 miles from the largest nesting area for Great Blue Herons in all of Western North America, in 2006 there were about 700 active nests. The only thing that worked for me is a deep pond with straight sides. Herron are wading birds and have a harder time fishing below the level of their feet, so if you can remove anything they can stand on in the water. The worst thing is if you have shelves(Buffet Seats) in the pond at a depth of 12" to 18", I have not had any issues since building my new pond.