We had the same problem last year within a couple of months of building our first pond. I actually saw a heron land on our garage roof days after a comet came up missing...and I live on a busy street in the city! Forunately it did not get any of the 3 koi, but still :( We immediately went out and got a net to cover the pond and then bought a couple of "scarecrows" that are motion activated "water shooters". Although the net is not the most pleasant looking feature of the pond, we remove it when we have a day off and feel the fish are a lot safer while we are at work. The scarecrows had the added effect of detering stray cats from taking up residence in the yard!

Hi
When I first started my above ground pond in 82 I couldn't keep a single GF.. Mostly due to green herons , Switched to tropical fish. Though the water birds kept right on eating them they reproduced faster than the buffet lol
Anyway in 08 had a complete wipeout of the swordtails due to record cold and a breakdown of the heating system. Fearing mosquitoes ,put some 7 for a buck feeder GF in figuring the water birds would wipe them out . Went from 7 to well over a hundred now . haven't figured out why the water birds have left them alone. Wanted to restock with tropical fish but what to do with the GF. ??
Always something!!! gary

Three winters ago a heron cleaned out almost all of my forty goldfish, some were ten years old....

oh yeah...been there...last summer our little pond got discovered by a great blue heron. our pond is only about 15 ft from the house, just off our deck. he got my biggest, my prettiest and my oldest goldfish(s) (almost 16yr old) and we have dogs. Three dogs. this yr a heron flies over but so far we havent lost anymore fish.
i like nature but, darn, my best fish!

String, several high strength monofiliment lines (fishing line) at an elevation that doesn't look distracting (ugly) so that a heron would hit it when coming or going. Herons are that good at vertivle take offs and landings, there is glide and then they would hit a line. Smaller birds will just fly around it if they see it, but the small ones don't eat much.

Do a search (space at the top of the forum)for raccoon and you will see a huge number of posts about them, the damage they can do and the lengths ponders go to get rid of or at least discourage them. If nothing else you will get a kick out of some of the more humorous ones.

The scarecrow water cannon helped a lot until a new generation arrived and those particular delinquents required a direct hit from a hose held by me (at 3am)whenever they tried to approach. After a week of that I didn't see them anymore. I expect a new generation will be coming around this season but I haven't seen them yet. Of course I am an insomniac.

Oops, forgot to relate the post above to the herons. Sorry.

Similar treatment can work with the Herons if you don't want to put up nets or string. They may be water birds but they don't like being hit with a blast of it. Remember they are protected by law as a migrating species so don't hurt them.

Another trick that actually will work is putting out a pair of fake herons. I got a pair of aluminum ones from Harbor Freight that seem to work very well. A heron usually will not intrude on a nesting pair and respects the hunting area of other herons. This works well until they head south. Then the nets should be up.