If you're not finding any heads or scales or other tell-tale signs, then you've probably got a heron or an egret visiting your pond.

Alligator heads MIGHT work if you lived in Florida or Louisiana, where the birds of prey become prey themselves to alligators, but your northern birds may not be aware of what they are, and therefore will have no fear of them. I"m not sure how far they migrate during the winter, though.

Unfortunately, once a heron or egret manage to obtain fish from your pond, they will return again and again until they have emptied your pond. At this point, the only defense against them is a sturdy net over the entire pond.

One more thought . . . Some people have reported that their outdoor dogs serve as excellent deterrents, but only if the dog is outside at all times.

Belted Kingfishers also swoop down and remove fish and leave no trace. I once saw one grab a 6-7" shubunkin from my pond and fly away without any effort though the bird isn't much bigger than a robin. Kingfishers and various herons are found over most of the US and are more likely than a hawk. Ospreys are so big most garden ponds aren't safe for them to dive into....

You are correct that opening up the tree canopy opened your pond up to aerial predation.

Raccoons would leave a hurricane of plant destruction in their wake and fins, scales or even headless corpses too.

You can add a floating island planter or just toss a piece of wood or styrofoam into the pond that the fish can hide under. You can also add more plants to provide cover. The wood or styrofoam is most helpful in winter when plant cover is low. A net is a sure thing but ugly.

I haven't had a problem since I put out a pair of life size aluminum herons.They are working for the birds but they don't work for the raccoons. I finally discouraged those by standing guard between midnight and two in the morning with a hose turned on to full force when they showed up. Until then they thought I was playing around. I think most of the birds and animals here would think the alligator head was just a big rock. One item that did some good was the water cannon but the small birds, squirrels and chippers loved to turn it on and the water bill got sky high.

I recently got an alligator head- will let you know if it works...

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I did order the alligator. Probably won't do any good. It's actually a 3-piece alligator that move "lifelike" in the pond. Kind of creepy actually. Will probably scare the grandkids more than the birds.

My pond is not really big. There is a very small upper pond with no fish that flows into a short stream and the lower pond is not much larger, and similar shape, to a grand piano. It's right off my deck and patio. We've had it for 15+ years and never had a problem until we took down those trees.

I am hoping that due to the small size of the pond the bird would not want to dive in with that floating object in there, even though it may not know what it is. A floating island probably would have served the purpose and would have been chepaer and not so scary.

It's too early here to get plants due to cold weather. I will leave the net on for a few weeks and see if more plants and the alligator do the trick.

I've had successful from string fishing line in a grid pattern across the pond. It's less visible than a net (especially after the plants grow. I live 10 miles from the Chesapeake Bay, so herons are a constant threat here. I also have a fake heron, but last year, my heron visit occured in May- I think he was looking for a mate. So fake heron goes into hiding in late April- June.

A single heron decoy probably won't do much good. A real heron would think it was an unmated bird and join it since there wouldn't be any territorial disputes. I had a heron visit on November 9 one year. I have no idea why it was there so late but since they are migratory it was likely just a brief stop. It still cost me two koi in a matter of minutes.