I live in Oregon and have a swimming pool that I have turned into a pond. It is quite deep, 12 feet at the deepest end and about 4 ft in the shallowest area, other than the steps. Yesterday I saw a great blue heron standing on the edge of the deep section, waiting. My fish who come readily to the surface when I appear to feed them were nowhere to be seen. In fact, still this morning I see no sign of my 20 or so koi. My question is this: how big a threat are heron to koi in a large deep pool where they really can't get in and wade around? Are these diving birds? I would hate to lose any of my beautiful fish but also can't imagine trying to cover my 30' by 30' pool/pond. I have already determined from articles that decoy heron don't work reliably. Any thoughts would be most helpful!
Thank you! Laurie

Your koi are smart. Your heron is frustrated. This is good. Very good. You are 'way too deep for him. Good on ya, mate!

Herons and egrets are wading birds, and can neither swim nor dive. Kingfishers are divers, but are too small to trouble a good-sized koi, though they can take smaller fish. About the only bigger birds capable of causing you trouble would be cormorants, who have essentially taken over two of the heron rookeries in the far western 'burbs, and eagles. We in Chicago are too far inland for ospreys, though you probably are not, but we both have the possibility of suffering a surfeit of Bald Eagles.

Never give a wading bird a place to stand. Never give a diving bird unencumbered air space.

(Answer courtesy Bob Passovoy)