Having had a koi pond for many years, and having been visited by egrets,
osprey, raccoons and more, I can say that I never lost a fish to a
predator,  first be sure the water is deep enough,at least a few feet so
the fish have a chance to go below the predators range. I used strawberry
pots on the bottom so they have hideouts, plants are big! pickeral rush for
one (pontideria cordata) makes great hiding spots, looks beautiful and
makes for a healthy pond, plus small birds will use the branches and leaves
for drinking and bathing from, use as many plants as you can, they also
keep the hot sun from shining directly on the water which can make
for.lethal water temps. If the.pond is in a wide open area, big birds will
have no issue flying in and picking off fish, so covering or surrounding
the pond be it trees fencing or whatever will hinder them flying into a now
smaller, tight space. Fake coyotes, foxes... are useless, think of all the
moving head horned owls that were put up on cell towers to keep ospreys
from nesting, those owls don't work on boats either, the birds will just
sit on the owls head and poop away or eat a fish.The last thing is that
fish get use to people hand feeding them and if trained will come right up
a eat out of you hand, I'm not so sure they know the difference between a
human silhouette to a herons. By the time they learn it probably to late.
Larry Flynn Mystic Ct

On Sat, Jun 22, 2019, 2:11 PM Lorrie via CTBirds <
ctbirds@lists.ctbirding.org> wrote:

That's a great idea.  I, too, have been "fished out" by a heron, which surprised me when it started two years ago since I'd had no problem for 13 years with large fish, and suddenly was "discovered". My pond is very small -- two pools, one 3'x3' cascading into one 4' x 7', which seemed way too small to be worth while. The fish are only in the larger pool. Fortunately, it is pretty deep (30" in center), so I constructed a hiding place in the middle and that worked well.  The sad part was that the fish were terrified and would no longer come to the surface to visit or feed, so I could not enjoy my beautiful fish.  Trying a number of things, I eventually got a blue heron decoy and it has worked like a charm (knock on wood). I move it around periodically.  Since your bird apparently is attacking your decoy, I'd suggested another such as a coyote or eagle.  I don't think a green heron decoy would work, but that would be smaller and more attractive...

And, just for a chuckle, until I needed the decoy I'd always scoffed when seeing them at garden centers -- I thought they were for people with bad taste in garden decor!!!!  Little did I know...

Sarah Faulkner
Collinsville