Say hello to the pond prime bird of prey, the Great Blue Heron.

The first time that you see this huge bird in your pond, you will admire the beautiful creature. When you watch him swallow one of your favorite fish, your opinion may change.
Maybe it's time for the migratory crowd to be here. Possibly, it's one of the Herons that stays around all year. Once the bird has found you, it WILL be back, of that much you can be certain, and it will probably come at. daybreak before people and dogs ae up and around.

The Herons are protected by the migratory bird act, so be careful how you react.

Herons won't land or wade in water deeper than where they can walk. Usually, they won't go in deep enough to get their wings wet. Remember that when planning to outwit them.

Herons are extremely patient, and will wait for hours for a fish to come close. They are smart and have even been seen regurgitating in the water to attract fish. Screening the pond is the only 100% effective way to keep them out. Moving scarecrows work if they are automated to move every 5-10 minutes.
We were quite successful using a five foot high human shaped silk blow up scarecrow that stood up, then deflated every ten minutes. We placed it on an upper level deck. It had a large fan and timer that inflated it. It wore out after two years and we plan to replace it.

Once a Heron finds your pond, he will never forget where it is. He will return daily, weekly, monthly, yearly, usually at day break.
The largest concentration is during early spring and again in the fall as they migrate through the area.

Fish are STUPID. Some say that their memories clear in five minutes.
I think it takes longer, but a hungry fish will want to continue browsing for food and won't stay deep very long. After you work around the pond, wait about five minutes and then throw in some food. It probably won't be long before they are trying to outswim each other to get it. They are EASY targets for Herons.
Personally, I prefer not to kill any creature except rats. I set a leg trap when I have a stubborn bird sneaking in repeatedly. The trap is smooth and does no damage to their legs. I set in where I see them hanging around in my pond. When a bird gets caught, I wrap up a newspaper and then smack the crap out of them to give them a really bad experience. Then I spray paint their beaks so I can identify repeat offenders. I have yet to see one of these marked birds return.

If you don't take up some form of pro-active response, they will eat you out of pet fish and budget. Their favorite tactic is to peck large Koi in the brain and wait for them to die. Then they drag them out and rip them into pieces that they can swallow. Hence the poop, blood and scales around your pond.
Netting is cheaper than Koi. It's somewhat of an eyesore, but better than losing your pets. You can find very lightweight pond netting and cover your pond entirely. Another tactic is building a 20 inch high fence around the pond. I've seen rebar pounded into the ground as stakes, then covered with fat bamboo. Pond netting can be strung around these stakes. From 15 feet back you can barely see the netting. The Herons can't reach over such a fence, and they won't land in the water. They will get frustrated and eventually leave your pond alone. Good Luck, you need it.