We are about 1 mile from a reservoir, and about 10 miles from a Great Blue Heron rookery, members of the egret family. We had a Great Blue Heron visit us in 1999. In a few visits he took about 120 of our best 4-8" Koi. There were only 100 left in that pond. He usually came at dawn and dusk. We saw him several times but he quickly, and very quietly took off. What a huge and magnificent looking bird. They stand about 3 feet tall, and weigh 5-8 pounds, with a 6 foot wingspan.
We tried a Great Blue Heron decoy that came with instructions to move it every day. The theory is they politely respect each others territory, and will not horn in on another Heron's meal. We saw him fly over the next morning at about 100 feet. That evening he flew over at about 50 feet. The next morning he flew over at 30 feet and we could see him checking out the decoy. The next evening he landed and had a sumptuous Koi dinner.
In the Denver, Colorado area people believe Great Blue Herons only feed heavily during May and June, my experience was mid-June to mid-July, when they brings fish back to feed the baby Herons in the nest. It was in July that he stopped landing. I did see him several times in July but he never landed.
It was easy to tell when he came if I didn't see him. The Koi would be badly spooked. They would all hide for at least a day. In fact one time I thought they were all gone. I couldn't see any. We use 12 little plastic stepping stools for plant stands, and they form a tunnel like area. They provide great hiding places for Koi, but they also make it very difficult for us to catch them. We saw a couple of Koi peek out from under the plant stands. Eventually the 100 survivors ventured out, but it took many hours. In fact there were a few we did not see for weeks.
I had trained them to come to me for meals by splashing my fingers in the water. I wondered if the Heron made a similar sound which would have led the Koi right to him.
Of course he took the most friendly and curious ones. Some of my favorites. It seems only the spooky and wary Koi survived. Afterwards, when I fed them if I moved my hand too quickly to shoo away a fly, they all darted for cover. It took a month for them to calm down.
One of the last times I saw the Great Blue Heron I thought he might have talked to me. As he flew over I heard a couple of crow-like "caws". I have never seen him again, and I never want to see him again..
Herons are a big Koi problem here. Some have tried putting netting over their pond. One put a sheet of Plexiglas just under the water's surface. That was a surprised Heron. In Japan they use fish line strung back and forth over the pond, a foot or two apart, which also threatens the well being of a Heron. They don't want to hit that line because it might hurt them.
Some have claimed success with a life size scarecrow, others with an Owl decoy. Some swear by having dogs in the area.
This year (2000) we strung a heavy green plastic construction fencing (Home Depot) over the pond. Although we did see the Blue Heron fly over our rooftop, he has never landed. Correction, he landed last night at 1:00 am standing on the edge of the pond, patiently fishing for Koi in a pie shaped opening. What a surprise, in the middle of the night. I scared him away, but he came back at 2:30 am, again patiently fishing for Koi in the small pie shaped openings. So now I have to cover the small pie shaped openings with more construction fencing.
I do not know how often he has landed at night, but now I understand why the Koi have been unusually spooky at feeding time.
It turns out that the males take care of the nest during the day while the female hunts, then the female stays with the nest at night while the male hunts. So if you see a great Blue Heron during the day it's probably a female, and at night it's probably a male.
If you have discovered a good anti-heron method we would love to hear what you did.